Everything to Know about Bose 901 Series I and Series II Active Equalizers & Speaker Systems (and the 800, too!)

Posted on June 12, 2026 by retrovoltage

The Bose 901 Direct/Reflecting Speaker System is as polarizing as it is interesting, a unique system from the golden age of hi-fi experimentation. It’s less about perfect sonic accuracy and more about the experience of listening to your music. And that’s one of the things I love about it!

While cleaning up some of my archives to save cloud space, I found that I’ve serviced nearly 500 Active Equalizers which are the important heart of these speaker systems. There’s been a few surprises along the way. There is a truly staggering amount of AI slop out on the Internet about these speakers, ranging from slightly wrong to utter nonsense. In contrast, this article was written by a human who has owned and listened to all of these systems, and serviced and studied hundreds of them over thousands of hours. I’ve distilled some of the knowledge from all of that experience here, to help anyone else who wants to keep these systems running, and get good info from someone who actually has any clue what’s going on.

This article covers the Bose 901 Series I and Series II Active Equalizers, along with their live-sound sibling, the Bose 800 Active Equalizer paired with the Bose 800 speaker system. Keep on reading for all of the details!

Bose 901 Series I and II

Introduced in 1968, the Bose 901 Series I was possibly the earliest forray into the brief era of active-equalized speakers which also included some offerings from DBX, EV, and McIntosh (and probably others I’m forgetting!) Designed by Dr. Amar Bose, these speakers prioritized ambience and presence over strict accuracy. Based on research suggesting that most of the sound you hear in a live performance arrives indirectly through reflections, Dr. Bose designed the cabinet with nine small, identical full-range drivers instead of a traditional woofer or tweeter. Eight on the back, one on the front. Everything the same size with no distinct woofers or tweeters.

To make the concept work, every 901 was paired with a dedicated active equalizer that dramatically reshaped the speaker’s frequency response. The equalizer really is necessary to get the best sound out of the system – and sadly, it gets lost or damaged a lot of the time. Without it, the midsized drivers don’t get the drive power they need on both the low and high ends of the frequency range to perform properly.

Before we get to the equalizer, a bit about the speakers themselves:

Bose 901 Direct/Reflecting Speaker System – Series I and II

A pair of Bose 901 Series I speakers stacked togther with the equalizer on top. The equalizer is turned on and has a glowing power switch; the speakers are facing in the correct orientation with the flat side carrying the single driver facing towards the listener.
A rear view showing the same two speakers from the previous image, facing away.

The Series I and II are basically the same. They’re both solid wood with fabric covers. The flat side is the front side, with a single driver towards the top-right. (Some variants have a solid wood panel and only a small strip of fabric over the driver.) The diamond side, with 8 drivers split into two banks of four on each side, face the rear. You’ll typically want to put these 12-18 inches away from a wall – the tulip stands really complete the look.

The great thing about the Series I and II is that these use a rubberized cloth surrounds, not foam surrounds as were used on the Series III and later models – so they’ll never need to be refoamed. There are also no crossover components inside, so no capacitors or anything to dry out and change the frequency response. All of that work is done in the equalizer.

You’ll want a pretty beefy amplifier for these to sound any good. While the specs say these 8 ohm speakers can be driven with a minimum of 25W, they’re rated for a maximum amplifier power of 270W RMS, and 400W peak power for under 5 seconds. And you definitely need it as the equalizer is applying +18 dB of gain to some frequencies, which comes out to about 63x the power. The midranges might get away with 2W, but with the equalizer’s gain would be asking for 126W at low frequncies which could push an underpowered amp into clipping, especially if you’re trying to listen at a high volume.

Bose 901 Series I Active Equalizer

The 901 Active Equalizer is an unassuming little box, and was sold worldwide (including for a great price at overseas military base exchange stores) so it turns up in both 120 and 240V versions, and there were a handful of versions as the design evolved over time.

A wood case Bose 901 Series I active equalizer sitting on the workbench showing the front with controls.

The earliest versions had a solid wood case, which was really nice. Later versions of the Series I and then the Series II used a particle board case with a contact paper veneer. These actually don’t look bad if they’re in nice shape, but the adhesive comes apart as they age and the paper can delaminate, or the entire case can come apart. It’s especially prone to doing this if there’s any rough handling in shipping. (You’d be amazed at the number of people who used to send me equalizers tossed into a box with no padding whatsoever and left to bounce around!)

A particle board case, which was destroyed in shipping and the sides have snapped off and folded inward, held together only by the shielding tape on the inside cover.

The earliest models had screwed components, and the serial number tag was on the bottom instead of the rear.

A rear view of an early production Series I equalizer, showing that the rear panel jacks are screwed in rather than riveted in.
The production label on the bottom of the equalizer from the previous image. The label reads: BOSE MODEL 901 Serial #230 Active Equalizer Pat. 3,038,964 Pat. Pending

This was the earliest one I ever saw through my shop, Serial #230, and the owner described being an Electrical Engineering student at MIT who toured the factory with a class, and the kid ended up with a 901 system given to him by Dr. Bose personally. The factory switched to rivets after a few hundred units, probably in the #700s.

A rear view of a later, but still Early Production, equalizer which has switched to rivets. Serial number 06805.

There were two versions of the Series I, an “Early Production” model, and the main mass-production model. The early version is quite chaotic.

An inside view of the Series I Early Production equalizer, showing the original unreplaced components in chaotic alignment.

It’s fairly straightforward as far as the schematic:

The electrical schematic for the Series I Early Production taken from the service manual.

Given the age, this one generally needs the most work:

  • All electrolytic capacitors need to be replaced. I usually go up a level in voltage to improve longevity, since caps today are so much smaller. Axial capacitors – one lead on each side, rather than both leads on the same side – are a bit harder to find these days, but they’re out there.
    • 500 uF x 2 (replace with 470 uF 35V) main power supply filter capacitors
    • 100 uF x 3 (replace with 100 uF 35V) small power supply filter capacitors
    • 5 uF x 4 (replace with 4.7 uF 35V) output & feedback loop capacitors
    • 3.3 uF x 4 (replace with 3.3 uF 25V) as part of a feedback network for the treble adjust
  • Transistors will almost all need to be replaced. The pencil-eraser packages don’t hold up well and nearly every one of this series found multiple bad transistors. Originally used 2N3393 transistors, I’d substitute those when I had them, or use 2N5088s. (As always, when replacing semiconductors, make sure you’re installing them in the correct orientation.)
  • Replace the two diodes with 1N4007s, these two fail pretty regularly.
  • Replace all of the film capacitors with identical values. These were often early paper-mylar caps and go leaky as well.
  • There are two 6.8V Zener diodes, which are in semiconductor packages where the third terminal is soldered to an anchor pad but is disconnected. These should also be replaced as well. You can use a 1/4W part here.
  • All of the resistors will have drifted, often outside of tolerance, but this isn’t the most critical thing. I replace them typically with Xicon 1% metal-film resistors (Mouser part, e.g., 273-10K-RC) but I’ll consider this step optional. Your channels may be slightly mismatched if not replaced, but it won’t damage anything.
  • The neon bulb is often dead, too. If you can find an NE-2 bulb with current-limiting resistor that’ll fit, go for it, or you can get a 20V LED assembly with dropping resistor and power it directly from the first filter capacitor.
The interior of a Series 1 Early Production equalizer, showing replaced componnets including capacitors and semiconductors.

Zoom in on this “after” photo where the semiconductors and capacitors were replaced for a view of how it might look when you’re done. The mass-production version is much cleaner:

An inside view of a Series I Mass Production model showing original unreplaced components.
The schematic for the Series I mass production model taken from the service manual.

One change is that to access the bottom side of the board on the mass-production version, you have to de-solder the two output jacks. Otherwise, it’s a much more mature design with improved power filtering, similified equalizer topology, and no Zener diodes. Starting with the Series I, you can generally get away with a more minimal service although I recommend doing the whole thing while you’re in there.

  • All electrolytic capacitors need to be replaced.
    • 500 uF x 2 (replace with 470 uF 35V) main power supply filter capacitors
    • 100 uF x 7 (replace with 100 uF 35V) small power supply filter capacitors
    • 5 uF x 4 (replace with 4.7 uF 35V) output & feedback loop capacitors

Now, you might get lucky and that’s it. Depending on the timeline for your particular model, you might have some more work to do. For starters, the dark red paper-mylar capacitors (like shown in the above photo) often go bad – you can even see how they start to discolor in the center. I recommend replacing all of the film capacitors.

A closeup of a mass production Series 1 circuit board, with a circle highlighting a set of three red paper-mylar capacitors which have visible discoloration in the center indicating that they have failed and need to be replaced.

Transistors go bad with less frequency (and on some of the earliest mass-produced ones, they’re still using the failure prone pencil-eraser packaging instead of the modern TO-92 packaging) but I do recommend replacing them in the Series I in general. Use 2N5088s here. The board is conveniently labeled with the pinout for easy servicing. If it works after a capacitor replacement and you’re satisfied, you can skip it, but I find it does improve channel matching. The diodes don’t generally need to be replcaed on this model, but you can replace them with 1N4007s if you want while you’re in there already.

The carbon resistors drift on this model as well, but as these are a few years newer, it’s not usually as bad. I recommend a full replacement in any case if you’re up for it.

An interior view of a Series 1 mass production equalizer with all components replaced, arranged neatly in rows.

If you have (or had) a 240V model, there will be a 10K 5W resistor on/near the power switch to do the voltage conversion. If it’s missing, or jumped over, it’s 120V. It’s a different shape sometimes.

A view of the rear of the power switch inside the equalizer, showing a 10K Ohm 5W resistor used to run the unit on 240V power.
A different view of the rear of the power switch inside the equalizer, showing a different 10K Ohm 5W resistor used to run the unit on 240V power.

A not-uncommon problem with these voltage-converted or overseas models is hooking up to the wrong voltage, especially if the voltage stickers aren’t swapped over.

A picture of the circuit board with the power transformer removed, showing large highly visible burn marks where the board was damaged and the transformer destroyed by running it on 240V power when it should have been run on 120V power.

If you find yourself in this situation, the Tamura 3FD-336 is a good transformer. You’ll need to drill a couple of new holes and use a couple of jumper wires – it uses a dual primary and dual secondary configuration. Make sure to read the datasheet. (If you’re handy with Ohm’s Law, you could use a different transformer and adjust the values of R20/R21 to get what you need, too.)

When you’re all done, the frequency response curve is pretty striking to see. It’s really doing a lot of work. It’s the same for both the Early and Mass-Production versions.

A frequency response curve of the Series I equalizer generated using an Audio Precision analyzer, with large peaks up to 20 dB centered about 30 Hz, and 14 dB at 20 kHz.

Bose 901 Series II Active Equalizer

The Bose 901 Series II Active Equalizer is substantially similar. The speakers are the same with only some minor visual changes; the equalizer itself removes a handful of components inside and removes the power switch (so you’ll want to connect it to a switched power strip or your system’s switched power outlet if there’s one available.) They have the same particle board and contact paper cases.

A front view of a Series 2 equalizer on the bench. Unlike a Series 1 equalizer, this one has no power switch and is always on when plugged in.
A rear view of a Series 2 equalizer on the bench showing a slightly different arrangement of input and output jacks.

Like the Series I, the Series II also comes in a 240V model, which uses the same 10K 5W resistor on the input power as described earlier in this article. Inside, there are a few components removed, and one change to the power supply: instead of 2 and 7, it’s 3 and 6 capacitors. A couple resistor values are swapped as well. Otherwise, it’s all the same stuff.

An interior view of the Series 2 equalizer showing original components.
An interior view of a Series 2 equalizer with all components replaced in neat rows.

Now, one unusual thing you’ll potentially come across is really different looking. Some of the earlier Series II models actually use a Series I circuit board with some jumper wires, with a few components ommitted:

An interior view of a Series 2 equalizer built from a modified Series 1 circuit board, with some components replaced, illustrating an unusual and undocumented factory revision they made to use up old boards before switching fully to new production.

The power supply segment is the same as the Series I, but even though the board layout is the same, some of the values are different, not populated, or are mounted directly to the contour switch. These aren’t super uncommon – I probably saw a couple dozen in this configuration – but are less common than the Series I Early Production version. Almost certainly in part due to the age difference.

In either case, the most basic repair on these is just the electrolytic capacitors. You can replace the film capacitors, semiconductors, and carbon resistors if you’d like to get the channel matching perfect but by this point we’re getting into the late 70s and manufacturing techniques and quality were much improved and more reliable.

Both versions of the Series II produce the same equalizer curve, which is almost identical to the Series I. You can use either interchangeably and it’s just fine. (Apologies for the weird scaling on this one.)

A frequency response curve of the Series 2 equalizer generated using an Audio Precision analyzer showing similar peaks as the Series 1, centered around 30 Hz and 15 kHz this time.

Bose 800 Professional PA System

The Bose 800 PA system is the live-sound version of the 901. As the number might imply, it uses only 8 drivers instead of 9 all on the angled side, and it’s designed to face into the crowd like a more traditional loudspeaker.

A decorative arrangement of a set of Bose 800 Professional PA speakers, with road cases, and one grille removed to reveal the 8 drivers on the front with blue paper surrounds.

The speakers themselves use 1/4″ jacks with a passthru, and are fused for protection and coated with tolex like road gear often is. Like the 901s, due to the nature of the array of small drivers, they need to be used with the Bose 800 Active Equalizer to get the proper sound quality. The 800 offered Series I and Series II versions as well.

Bose 800 Active Equalizer

The 800 Active Equalizer came in a Series I and II version, based on the 901 chassis. I haven’t been able to find any service manuals or schematics for these, but based on the information above, you should be good enough.

A badly banged up earlier 800 Active Equalizer, similar to a 901 Series 1, covered in duct tape and with spray painted markings on top.
An 800 equalizer based on the Series 2 platform, in much nicer shape than the previous.

These often get pretty banged up following a life on the road. The Series I has a switch; the Series I lacks one.

A rear view, common to both types of the 800 equalizer, showing quarter inch input and output jacks common on professional audio systems.

Both have 1/4″ jack systems, and there’s no tape loop on these. They came in 120V and 240V versions which also used the same dropping resistor system as on the 901s. There were a few production variations; I’ve found Series I systems with Series II boards inside, and vice-versa.

An interior view of an 800 Series 1 equalizer showing original components.
An interior view of an 800 Series 2 equalizer showing original components.

I’ve also run into a few which were either modified or came from the factory with a 600 Ohm output impedance:

An 800 series 2 equalizer with all components replaced. This unit has two additional resistors connected to the output jacks.

The 800s came on the scene a bit later than the 901s. You can generally get away with replacing only the electrolytic capacitors and any of the red paper-mylar capacitors on these models. While I did often replace semiconductors by customer request at the time to improve channel matching, I don’t think I ever ran into an 800 with defective transistors or diodes.

The equalizer curve is much the same. You could get away with using 800 or 901 equalizers interchangeably.

A frequency response plot of the 800 equalizer produced using an Audio Precision analyzer showing peaks at 40 Hz and 12 kHz. Slightly different, but close enough if you only had a 901 and wanted to make it work.

Mods & Bodges

Now, with any popular system that’s been in service for a while, these things often had repairs of differing quality. Some came in with replaced electrolytic capacitors only, but film parts or semiconductors had failed since:

A 901 Series 1 early production equalizer with a handful of electrolytic capacitors only replaced but the rest of the original components, not following a best practice for repair.
A Series II equalizer with a very badly performed repair, with one electrolytic capacitor tacked in on top and large blobs of solder visible.

Or sometimes you’d get replaced jacks or a replaced power switch:

A Series 1 equalizer with a large red and black power switch in place of the original clear plastic switch, with an obviously different and clashing visual style.

I’m not entirely sure what this mod was trying to accomplish, and it looked pretty dodgy.

An interior view of a Series 2 equalizer with a modification to the power transformer; it's off-center (possibly from a Series 3) and held on by one screw, with mystery diodes connected to an unidentified wire and the unit's ground plane.

I saw a few that had gone through another well-known shop back at that time, with a ton of expensive upgrades, but the electrolytic capacitors in the power supply had failed. Could be bad luck, the workmanship was good. This shop had a different perspective than I did which was always interesting to see. Lots of room for different opinions in the vintage audio world.

A Series 1 equalizer with significant work done previously, including an all new power transformer, electrolytic capacitors, and expensive film capacitors installed.

Finally, I saw one absolutely bonkers homebrew clone of the Early Production Series I:

Front panel of a homebrew equalizer with all of the same controls as a Series 1, but of a pretty rough looking design.
The rough rear panel of the homebrew Series 1 equalizer showing input and output jacks and a few miscellaneous screws.
The interior of the homebrew Series 1 equalizer showing a partial view of two out of three circuit boards on their original components.

At least I think it’s homebrew, although those are some nice looking circuit boards. I did fix it, and it did work!

No, I am not accepting repairs currently, but I hope this will help you with yours.

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The Speaker Spotter — June 10, 2026

Posted on June 10, 2026 by retrovoltage

A recurring feature curating interesting speakers for sale on Craigslist from around the Pacific Northwest. Links to Craigslist postings might go down at any time if they’re sold or pulled from sale. This one was delayed while I was traveling, so we might skip the Friday 6/12 issue to give folks time to list new and interesting things for sale.

IMF TLS 50-II speakers loudspeakers; pair

$775 · SE Portland (Portland)

IMF TLS 50-II speakers loudspeakers; pair

British-made from the late 1970s/early 80s, the IMF TLS 50-II is a transmission line loudspeaker — a relatively rare bass-loading design that uses a long, damped internal duct to absorb and release woofer back-wave energy rather than trapping it in a sealed box. These used KEF drivers and sold for over $1,500 new. Really interesting design – I’m not super familiar with transmission line speakers, but I’m a fan of KEF.

Paradigm 40 Active

$1,500 · Hillsboro (Portland)

Paradigm 40 Active

The Paradigm Active 40 is a self-powered studio-reference speaker from Paradigm’s acclaimed Active series of the late 1990s — each cabinet contains its own tri-amplified electronics with individual amplifiers for tweeter, midrange, and woofer, eliminating passive crossover losses entirely. Original boxes included is a bonus for safe transport; at $1,500 these are a significant but worthwhile ask for powered monitors of this caliber.

McIntosh ML-1C speakers with stands

$350 · Albany (Corvallis)

McIntosh ML-1C speakers with stands

McIntosh is better known for its amplifiers and preamplifiers, but the ML-1C is a solid example of the company’s speaker division — a 3-way acoustic suspension design from the 1970s built with the same attention to build quality the brand is famous for. Re-foamed with stands included at $350, these are a collectible piece of McIntosh history at a very approachable price. Pair them with an MQ-101 equalizer and you’ve got a fantastic system!

Acoustic Research 3a Speakers

$600 · Boise

Acoustic Research 3a Speakers

The AR-3a was one of the most important loudspeakers ever made — introduced in 1967, it helped establish acoustic suspension (sealed box) design as an audiophile standard and remained a benchmark for bass accuracy and imaging for over a decade. Fully functional at $600 in Boise, this is a rare opportunity to own a foundational piece of hi-fi history in a cute and unassuming package.

Klipsch KLF-30 Tower Speakers

$1,900 · Boise

Klipsch KLF-30 Tower Speakers

The KLF-30 is one of the largest and most powerful speakers from Klipsch’s KLF (Klipsch Legend Forte) line of the mid-1990s, featuring a 12-inch woofer and Klipsch’s signature horn-loaded midrange and tweeter for high efficiency and room-filling sound. All-original with no replaced drivers and tight cabinets — a known weak point on some production runs — this pair in Boise at $1,900 is a compelling find for Heritage-adjacent Klipsch collectors. I used to own a set of these some years ago, and they’re excellent.

Vintage Nelson Reed 8-04/BI Tower Speakers (1 Working, 1 For Repair)

$1,350 · Bellingham

Vintage Nelson Reed 8-04/BI Tower Speakers (1 Working, 1 For Repair)

Nelson Reed was a small, boutique American speaker manufacturer from the 1970s-80s that built large, highly efficient horn-assisted tower speakers largely unknown outside of audiophile circles — making any clean pair a genuine rarity. At 47 inches tall, these imposing towers are listed as one working and one needing fuse/repair work; at $1,350 there is a project element, but for the right restorer these could be a worthwhile score.

Sansui SP L700 Speakers

$350 · Port Townsend (Olympic)

Sansui SP L700 Speakers

The Sansui SP-L700 (sold as the SP-G200 in Japan) represents one of Sansui’s most ambitious home speaker designs — a large, furniture-grade enclosure with distinctive Mid Century Modern styling and a reputation as one of the best speakers the brand produced. This refinished pair in Port Townsend has had its foam replaced and grilles are intact; the missing acoustic lens covers are a minor cosmetic issue on an otherwise beautiful and hard-to-find speaker at $350.

KLIPSCH KSF 8.5 Floor Speakers

$250 · Tacoma (Seattle)

KLIPSCH KSF 8.5 Floor Speakers

The Klipsch KSF 8.5 is a floorstanding speaker from Klipsch’s mid-1990s KSF series, featuring a horn-loaded tweeter and an 8-inch woofer — a step down from the Heritage line but still delivering Klipsch’s characteristic high efficiency and dynamic punch. At $250 for a pair in working condition in Tacoma, these represent solid value for a vintage Klipsch floor-stander. Sounds great for mid/high, but you’ll want a sub for the low end. I’d say these are better as a part of a home theater system than a necessarily a listening system.

JBL 2600 Bookshelf Speakers

$80 · Cully (Portland)

JBL 2600 Bookshelf Speakers

The JBL 2600 is a compact two-way bookshelf speaker from JBL’s consumer lineup of the 1980s, notable for using the same titanium-dome tweeter technology found in JBL’s professional studio monitors of the era. Well cared for and fully working in Portland at $80, these are an affordable entry point into the JBL vintage ecosystem.

NHT Audio VT-1A Tower Speakers, Gloss Black – Excellent Cond

$375 · Meridian (Boise)

NHT Audio VT-1A Tower Speakers, Gloss Black - Excellent Cond

NHT (Now Hear This) was a California-based audiophile brand of the 1990s known for its sealed, time-aligned designs and glossy lacquer finishes — the VT-1A is a slim tower that punches above its weight in imaging and soundstage width. USA-made and in excellent condition with the original gloss black finish intact, this Boise pair at $375 is a solid buy for fans of 1990s American high-end audio. “High end speakers from companies that are less focused on audio” is always an interesting genre and sometimes you get a real gem. Not sure about these, but they’re certainly different.

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Upcoming Projects for 2026

Posted on June 2, 2026 by retrovoltage

Having had a chance to unpack some of my shop from storage and settle in to the new space, I’ve got an idea of what projects I’ll be working on myself in between projects with my shop partner.

Bose Active Equalizers

Of course, since I used to be a top service provider for these, I have a whole bunch in stock which I’ll need to get through. I picked these up as parts units whenever I could find one for a steal, which was often due to cosmetics. The earliest 901 Series I cases were made of solid wood and hold up pretty well, but past the first ~10,000 units produced switched to particle board with a contact veneer until the Series III was released. Those get to be in pretty bad shape. I wonder if my partner can reproduce them on the wood shop side of the house.

With four Series I, a Series II, two Series IV, and a pair of 800 Series I equalizers (the same as a 901 Series I except with 1/4″ connetcors and none of the tone/tape controls) this should keep me busy for a little bit.

A 901-I/II has about 90 components to replace for a full makeover and takes about 4 hours from start to finish. The 800 equalizers have about 60 components and take maybe 2 and a half hours. Bose switched to a different design for Series III, and the Series IV only has a handful of capacitors in the power supply and signal chain. Some models used LF353N op-amp chips which were subject to a service bulletin and need to be replaced with TL072s. (Interestingly enough, some of these seem to be fitted with chip sockets from the factory and some have soldered chips, so sometimes it’s a really easy swap.)

This will keep me for maybe 10 sessions in the shop. I don’t make it in every week, and when I do, it’s usually for 3-5 hours.

1934 HMV 444

I’ve also got a His Master’s Voice Model 444 radio which I added to my collection more than 15 years ago. (I have the blog post to prove it!) At the time, I paid $60 (plus driving two hours each way) for the radio which had been restored by a long-retired Master Carpenter who found stuff locally at garage sales and gave it away for the same price he paid after re-doing the wood.

It’s not a big radio – maybe up to my waist – and it was only a 5 tube radio so nothing especially interesting electrically. But this is a 220V UK radio, and the truly striking art deco styling sets it apart. The schematic is drawn very differently from U.S. conventions but knowing the general theory, it’s pretty easy to follow. Should be a fun project, although it’ll take a few sessions of detailed measurement and tedious component replacement to get it across the finish line.

Sony TA-5650 VFET Stereo Receiver

I ended up with this Sony TA-5650 VFET receiver tin 2019 or 2020. I’ve done a good handful of these in the past, including several for multiple Grammy-nominated record producer Kavi Alexander of Water Lily Acoustics. This one is from an original owner, even. They’re really special, and deceptively simple, except for one detail.

These are from an era of heavy experimentation in semiconductor technology, the early-mid 1970s, and ended up using output transistors (FETs) which are a drastically different design and topoogy than anything else in common use before or after. They all need to be perfectly matched within the same rank as each other, and some traditional service best-practices (like using a variac to slowly bring an amp online for the first time after service) are guaranteed to destroy the output finals if used with this particilar piece of kit.

If you end up in that situation…good luck finding those semiconductors anywhere else ever again. There are some mods floating around to let you use a more traditional output device, but it’s a heavy rework job, and you lose some of the character that way. (These also use a small-signal VFET in the preamp chain for some reason.)

Adding to the mystery, when initially inspecting this one on intake, I found that it looks like there’s a factory revision to the final circuit board which wasn’t documented in the service manual. There are a few pieces of service errata – a couple of component swaps, changed bias values – but this one looked to have a few extra components and some different values installed. I didn’t look into it further to know whether it’s an early production or a late prodution model. Should be interesting.

I also need to adjust one of my two Audio Precision System One analyzers (and probably replace a few components in the power supply), so realistically, this set of projects will take me the rest of the year. I’ll post about it here as I go.

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The Speaker Spotter — May 29, 2026

Posted on May 29, 2026 by retrovoltage

A recurring feature curating interesting speakers for sale on Craigslist from around the Pacific Northwest. Links to Craigslist postings might go down at any time if they’re sold or pulled from sale. This week’s report is KEF heavy, with a dash of Sansui, and a few randoms thrown in. Happy listening!

Revel Performa3 M106 (Bookshelf Speakers)

$1,099 · Lynnwood (Seattle)

Revel Performa3 M106 (Bookshelf Speakers)

The Revel Performa3 M106 is a serious audiophile bookshelf from Harman’s flagship Revel line, featuring a 1″ ceramic dome tweeter and 6.5″ aluminum cone woofer tuned for reference-level accuracy. These would be the base of a solid desktop system, or even potentially as surrounds for a home theater.

Klipsch Quartet Speakers Oak Color

$700 · Tacoma (Seattle)

Klipsch Quartet Speakers Oak Color

The Klipsch Quartet is a full-size Heritage-adjacent floor stander from the late 1980s, built on Klipsch’s classic horn-loaded philosophy for high efficiency and dynamic punch. At $700 in Tacoma these show normal cosmetic wear but are described as working well. I’d love to own a set of these myself.

Sonus Faber Venere 1.5 Speakers and Stands

$700 · Tacoma (Seattle)

Sonus Faber Venere 1.5 Speakers and Stands

Sonus Faber’s Venere 1.5 is a slim two-way standmount from the Italian maker’s mid-tier Venere line, prized for its warm, musical presentation and refined cabinet work. This Tacoma pair comes with matching stands in excellent condition, they’re often beat up or missing for some reason.

Magnepan Speakers

$75 · Vancouver (Portland)

Magnepan Speakers

The Magnepan SMGA is a quasi-ribbon planar speaker from the early 1990s, offering the wide, seamless soundstage that Magnepan is famous for at a size that fits most rooms. This Vancouver pair is sold as a project — one panel has the common delamination issue — but for a capable restorer the price is right. A local stereo consignment shop near me has 3 or 4 pairs of these at any given time and they sound pretty good (if not with a ton of bass) but need a really big power amplifier to drive them properly.

KEF Floorstanding Speakers (103/4)

$350 · Lynnwood (Seattle)

KEF Floorstanding Speakers (103/4)

The KEF 103/4 is a respected floorstanding entry from KEF’s Reference Series of the mid-1980s, equipped with KEF’s B200 bass unit and T27 tweeter in a carefully tuned cabinet for smooth, wide-dispersion sound. This Lynnwood pair is a strong pickup for anyone wanting classic British hi-fi engineering at a fraction of its original price. I personally like KEF quite a bit and use their speakers if I do a resto-mod upgrade for a vintage radio because of the coaxial drivers. (You’re not doing that with a floorstasnding unit, though!)

KEF C40 Floorstanding Speakers

$150 · Lynnwood (Seattle)

KEF C40 Floorstanding Speakers

More KEF, these KEF C40 are a compact two-way bookshelf from KEF’s C Series, delivering the brand’s characteristically accurate midrange in a package suited to smaller rooms or desktop listening. Priced at $150 in Lynnwood, these represent genuine KEF quality at an accessible entry-level price.

Dahlquist M905 Speakers

$150 · Mountlake Terrace (Seattle)

Dahlquist M905 Speakers

Dahlquist is a celebrated American speaker brand best known for the DQ-10 time-coherent design that defined audiophile listening in the 1970s; the M905 is a later bookshelf that carries that legacy of careful voicing and imaging. The seller in Mountlake Terrace is offering demos, which is always a good sign. This isn’t the big, flat, open-baffle one you usually see for sale but it looks interesting, especially with the metal? metalized? woofers.

Vintage Kef Speakers

$250 · Albany (Corvallis)

Vintage Kef Speakers

Lots of KEF today. The KEF Model 103/3 Reference is a sealed three-way loudspeaker from 1986 that originally retailed for $1,500 a pair — a genuinely serious box from KEF’s peak Reference period. This Albany pair is in unspecified condition but that’s a solid price at $250 and they look intact and set up to listen, so it’s probably worth it.

Vintage Sansui Stereo Collection – AU-717, TU-919, AU-317, Turntable, Speakers

$2,500 · Bonners ferry (Spokane)

Vintage Sansui Stereo Collection – AU-717, TU-919, AU-317, Turntable, Speakers

This is a rare single-owner collection of vintage Sansui gear: two integrated amplifiers (AU-717 and AU-317), the highly regarded TU-919 tuner, an FR-D3 turntable, an SE-5 equalizer, and speakers — all from a builder that defined the “Golden Age” of Japanese hi-fi. At $2,500 for the lot from Bonners Ferry, it’s the kind of collection that rarely surfaces intact. (Everything is certainly going to need restored, but this is a whole collection. I’d try and snag it for $1750-2000, personally.)

Infinity SM-85 Studio Monitor Bookshelf Speakers

$275 · Bothell West (Seattle)

Infinity SM-85 Studio Monitor Bookshelf Speakers

The Infinity SM-85 is a studio-monitor-grade bookshelf from the 1980s, built around Infinity’s Poly-Cell™ tweeter and featuring a notably high 98 dB sensitivity that makes it easy to drive. This Bothell West pair tests out well and at $275 is a sensible buy for anyone wanting analytical, high-efficiency monitoring. Overall, a good little speaker.

Micro Acoustics MA Pro-1 Speakers

$250 · Happy Valley (Portland)

Micro Acoustics MA Pro-1 Speakers

Micro Acoustics was a short-lived but innovative American speaker company whose electret-based tweeter array drew a patent lawsuit from Bose — making these speakers a genuine piece of hi-fi history. The MA Pro-1 is the top of the Pro Series line, and this Happy Valley pair are described as very rare and hard to find. Really unusual looking.

Sansui SP L700 Speakers

$450 · Port Townsend (Olympic)

Sansui SP L700 Speakers

The Sansui SP-L700 (sold in Japan as the SP-G200) is considered one of Sansui’s finest speaker achievements, featuring furniture-quality cabinetry with a teak veneer and a four-driver array tuned for deep, musical bass. This Port Townsend pair is described as solidly built and in excellent shape — a genuinely uncommon find in the Pacific Northwest. If only it had the beautiful lattice grillework to go with it you find on the more recognizable Sansui models!

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DBX SFC-10 Soundfield Imaging Controller Repair

Posted on May 23, 2026 by retrovoltage

I’ve had this DBX SFC-10 Soundfield Imaging Controller in my stash for a while, and figured I’d ease into repair work again with a relatively easy project. I’ve always been interested in speakers which use an active equalizer – the McIntosh MQ101, the Bose 800/901 series, there’s a rare Electro-Voice, and there’s the DBX SFX-10 speakers.

These were nice speakers back in the day, but for whatever reason aren’t especially collectible and must not have sold very well because they’re pretty uncommon to come across. They use an omnidirectional design. They’ve got 2 x 10″ woofers, a mid, and a tweeter array each and are designed to throw sound in all directions while preserving imaging. Somewhat similar in that regard to the Bose 801 Direct/Reflecting system. You can often find these for a pretty low price, a few hundred dollars, but as with most other systems of this type the controller is needed for them to sound any good, and it tends to go missing.

Front view of the DBX SFC-10 Soundfield Imaging Controller on the work bench.
DBX equalizer rear view showing connections; input/output on the left and tape loop connections on the right
A top-down interior view of the upper side of the circuit board inside the EQ

It’s a surprisingly complex little device in there, with a date code showing it was manufactured in mid-1985. There’s 8 op-amps, a handful of transistors, and a hefty regulated bipolar power supply. The board is 2 layers, with plated via – some engineer must have had a big budget when designing it.

A top-down view of the bottom side of the board inside the equalizer

There was evidence of a previous repair at one point. Not entirely surprising.

A close-up view of two electrolytic capacitors on the circuit board, one is newer than the other showing the evidence of repair.

I did an initial power-up test just to see the current condition, and unsurprisingly, it wasn’t working properly. Bad capacitors, per usual.

A close up view of the power supply section of the equalizer, with the two main capacitors removed, revealing a ring of fluid leakage beneath one confirming evidence of the failure.
A view of the bottom side of the removed capacitor from the previous image, showing that the plastic sheath is crinkled slightly - evidence of leakage producing heat - and there is a crust of leaked fluid around the edges.

Replacing them all with nice new Nichicon caps from my existing stock took about 20 minutes. While I had the board apart, I also cleaned the equalizer controls.

An angled view of the equalizer circuit board, showing several new Nichicon capacitors have been installed.

Okay, great. Time to see what it does! My AP ATS-2 died the final death some time ago, but when I came back to the hobby I scored a great deal on a System One, which is the one I started with many years ago so it’s like an old friend, and it’s exactly what I needed to check out what this controller does.

The equalizer hooked up to the Audio Precision System One analyzer, showing a complex graph of overlapping frequency response curves on the attached PC display.

Let’s take a closer look:

A close-up of the PC screen readout showing six curves plotted on a single display, with labels below to differentiate them. The equalizer has a large bass peak around 30 Hz, a dip at 50Hz, a slight peak at about 100 Hz, then declines to a low point around 1000 Hz, before steadily climbing up to around 20 kHz.

One thing that took me some time to figure out was the “ambience” control on the front. It never seemed to do anything that I could see with the analyzer. No matter the control’s position, there were no meaningful changes in the frequency response, phase, or distortion. It took a trip to the manual to figure out what was going on:

Well, that explains it. With the equalizer hooked up to the AP, both channels were getting an identical signal, so L-R = 0, and nothing at all is modified in the signal. You can download the full manual here, originally from HifiEngine, but if you don’t already have an account over there it can be difficult to get one.

All in all, this was a quick and easy project, and a great way to ease back into electronics repair.

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The Speaker Spotter — May 22, 2026

Posted on May 22, 2026 by retrovoltage

A recurring feature curating interesting speakers for sale on Craigslist from around the Pacific Northwest. Links to Craigslist postings might go down at any time if they’re sold or pulled from sale.

Pair of Vintage Bose StudioCraft Tower Speakers

$200 · Seattle (Seattle)

Pair of Vintage Bose StudioCraft Tower Speakers

Bose’s StudioCraft towers are a late-1980s footnote in Bose history, employing the brand’s multi-driver psychoacoustic approach in a floor-standing format that is far less common than the famous 901. This Lacey pair needs some work (two drivers may require reconing) but offers a fun restoration project or parts source at $200. I’m almost positive I’ve seen these listed for literally years, coming way down in price, and they’re getting to be worth it for some unusual Bose. I’d probably take 601s or even 501s if I wanted floor-standing Bose, though.

Bowers and Wilkins B&W 805 Nautilus Stereo Speakers

$1,475 · Spokane (Spokane)

Bowers and Wilkins B&W 805 Nautilus Stereo Speakers

The B&W 805 Nautilus is one of the most celebrated standmount monitors ever made, featuring B&W’s iconic Nautilus tube tweeter loaded from behind to virtually eliminate rear reflection distortion. This Spokane pair includes upgraded aluminum phase plugs and is in excellent condition at $1,475 — a significant step below the original ~$4,000 MSRP.

JBL 3 MKII Powered Studio Monitors + Focusrite Scarlett Solo

$180 · Portland (Portland)

JBL 3 MKII Powered Studio Monitors + Focusrite Scarlett Solo

The JBL 305P MkII borrows boundary EQ and image control waveguide technology from JBL’s high-end studio gear, making it one of the best values in powered nearfield monitors. This Portland listing bundles a pair with a Focusrite Scarlett Solo audio interface — everything needed for a desktop studio or nearfield listening setup — at $180 for the lot.

Klipsch KLF retro tower speakers

$750 · Boise (Boise)

Klipsch KLF retro tower speakers

The Klipsch KLF series are horn-loaded towers from the 1990s that bridge the gap between the classic Heritage line and modern designs, rated at a room-filling 101 dB/W/m sensitivity. This black-satin pair near Boise is listed at $750 — dynamic, punchy, and easy to drive with even modest amplification.

Bose 901 speakers

$225 · Seattle (Seattle)

Bose 901 speakers

The Bose 901 is arguably the most audacious speaker design of the 20th century, using eight rear-firing drivers to bounce 89% of the sound off the walls in Amar Bose’s recreation of a live concert environment. This Tacoma pair includes the required equalizer and all 18 drivers are confirmed working — a complete, functional system at $225. The 901 Series 1 and 2, like these, use rubber surrounds which don’t degrade so these should last a very long time.

Vintage KLH Model 17 Speakers

$175 · Pullman (Pullman)

Vintage KLH Model 17 Speakers

KLH was co-founded by Henry Kloss, the acoustic genius behind the Large Advent and the AR bookshelf speaker, and the Model 17 is a compact two-way that showcases his talent for natural, musical sound in a small cabinet. This carefully refreshed pair in Moscow, ID has new capacitors and treated surrounds, and the seller will demonstrate on a receiver before purchase at $175.

Sansui vintage speakers

$200 · Spokane (Spokane)

Sansui vintage speakers

The Sansui SP-1500 is a large 3-way floor stander from Sansui’s golden era of the early 1970s, featuring a 12-inch woofer and the characteristically warm, full sound that pairs beautifully with vintage receivers of the period. A solid opportunity in Spokane at $200 OBO for anyone building a vintage Japanese system.

Wharfedale Obsidian Tower Speakers

$600 · Olympic (Olympic)

Wharfedale Obsidian Tower Speakers

Wharfedale, founded in Yorkshire in 1932, is one of the oldest hi-fi brands in continuous production, and the Obsidian Tower is a 3-way reflex design with a 1-inch silk dome tweeter, twin 5.25-inch mids, and an 8-inch side-firing woofer that delivers classically voiced British hi-fi. Available near Sequim, WA for $600.

Cerwin Vega AT-12 speaker

$60 · Portland (Portland)

Cerwin Vega AT-12 speaker

The Cerwin-Vega AT-12 is a 3-way tower known for high sensitivity and the ability to play genuinely loud without straining — a CV hallmark. This single cabinet came from an estate sale in Gladstone, OR and is offered with a money-back guarantee at $60, ideal for someone hunting a matching partner or looking for a robust bass cabinet.

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The Speaker Spotter — May 15, 2026

Posted on May 15, 2026 by retrovoltage

A recurring feature curating interesting speakers for sale on Craigslist from around the Pacific Northwest. Links to Craigslist postings might go down at any time if they’re sold or pulled from sale.

JBL L65 “Jubal” — $1,800 (Kennewick, WA)

JBL L65 Jubal

JBL’s legendary L65 “Jubal” from the mid-1970s is one of the company’s most coveted vintage offerings, pairing a 12-inch woofer with a midrange and titanium tweeter. This all-original pair is in good condition with minor cosmetic damage; the seller notes they’re best suited for larger rooms with acoustic treatment — a hallmark of these high-sensitivity, expressive three-way speakers.

Dahlquist DQ20i — $700 (College Place, WA)

Dahlquist DQ20i

The Dahlquist DQ20i is the final design from Jon Dahlquist, featuring an innovative open-baffle midrange/tweeter array that produces a famously wide, airy soundstage. This very-good-condition pair originally sold for ~$2,000; buyers should note the woofer foam surrounds may need re-foaming after 30+ years, but at $700 the price-to-performance ratio is exceptional for a speaker of this pedigree.

Altec 416A Woofers (Pair) — $600 (Kennewick, WA)

Altec 416a Woofers

The Altec 416A is one of the most respected 15-inch woofers ever made, prized for its legendary low-frequency extension and efficiency in horn-loaded and vented enclosures — the same driver found in countless professional PA systems and high-end DIY builds. This pair is in very good condition with one professionally re-coned in Portland, making them an excellent foundation for a serious speaker project.

1978 Klipsch La Scala (Single) — $1,000 (Shelton, WA)

1978 Klipsch La Scala

A 1978 Klipsch La Scala in birch — one of Paul Klipsch’s cornerstone Heritage horn-loaded designs — offered here as a single speaker in excellent condition with original box and paperwork. At 104 dB/W/m sensitivity, these folded-horn three-way cabinets are extraordinarily efficient and instantly recognizable by their large, boxy footprint and live-sounding presentation. A rare piece of PNW audio history.

Speakerlab Three’s (Pair) — $160 (Edmonds, WA)

Speakerlab Three Floor Speakers

Speakerlab was a beloved Seattle-based loudspeaker company that built a devoted following in the 1970s–80s offering audiophile-grade kits and finished speakers direct to consumers. These floor-standing Threes are a working pair in excellent condition — all drivers and cones intact — originally retailing for $1,200–$1,500. A genuine piece of Pacific Northwest audio heritage at a very friendly price.

ADS B7 Bookshelf Speakers (Pair) — $150 (Sequim, WA)

ADS B7 Bookshelf Speakers

ADS (Analog and Digital Systems) earned a strong reputation in the 1980s for exceptionally accurate, neutral-sounding monitors favored by home audiophiles and recording engineers alike. This pair of B7 bookshelf speakers is in excellent condition with all drivers intact; the seller will throw in a third B7 for just $50 more — handy for a spare or parts. Delivery is also available.

Vintage ESS AMT 1b Heil Air-Motion Transformer Speakers — $580 (Tigard, OR)

ESS AMT 1b Heil Air-Motion Transformer Speakers

The ESS AMT 1b is one of the most technically fascinating speakers of its era, featuring Dr. Oskar Heil’s Air-Motion Transformer — a folded pleated diaphragm that squeezes air sideways rather than pistoning it forward, producing exceptionally fast and detailed high-frequency reproduction. These large floor-standers (16.5″ × 16.5″ × 35.5″) are in good shape and are a genuinely rare find for collectors of unconventional transducer technology.

Polk Audio SDA 1 Stereo Dimensional Speakers — $975 (Vancouver, WA)

Polk Audio SDA 1 Speakers

Polk’s SDA 1 pioneered Stereo Dimensional Array technology, using a proprietary interconnect cable between the two towers to cancel inter-aural crosstalk and create a dramatically expanded, three-dimensional soundstage. This mint-condition pair includes the interconnect cable (which sells for $120 alone on eBay) and is rated for 50–500W amplifiers — an audiophile experience at a fair price for the technology involved.

Klipsch Cornwall Speakers — $3,000 (Vancouver, WA)

Klipsch Cornwall Speakers

Early-1980s Klipsch Cornwalls from the original owner’s estate — these Heritage series three-way speakers use a direct-radiating woofer combined with mid-range and tweeter horns for the combination of efficiency and dynamic authority that made Klipsch legendary. Both speakers work perfectly and have been in the same family since new. The Cornwall remains one of the most sought-after Heritage designs for its ability to fill large rooms at whisper-quiet amplifier levels.

JBL 4430 Studio Monitors — $3,000 OBO (Spokane, WA)

JBL 4430 Studio Monitors

The JBL 4430 is a professional Bi-Radial studio monitor used in major recording control rooms throughout the 1980s and 90s, featuring the legendary 2235H 15″ woofer, 2344 Bi-Radial horn, and original compression drivers — all intact and functional. The cabinets have been professionally re-veneered and left unfinished so the buyer can apply their preferred oil or stain. These are not typical consumer speakers; they are a serious find for a serious room.

Bowers & Wilkins DM601 Series 2 — $450 (South Hill/Spokane, WA)

Bowers and Wilkins DM601 Series 2 Speakers

The B&W 601 Series 2 is a well-regarded late-1990s bookshelf monitor featuring a 7-inch Kevlar bass/midrange driver and a Nautilus tube-loaded 1-inch alloy dome tweeter — technology trickled down from B&W’s flagship 800 series. This pair is in excellent condition with original grilles, Zu Audio jumpers, and original B&W boxes included, making it one of the more complete and ready-to-enjoy packages in this week’s roundup.

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Returning from Hiatus – 2026

Posted on May 14, 2026 by retrovoltage

It’s been a few years since I stepped away from electronics repair, but I’m excited to announce that Retrovoltage is once again back in action and working on projects. I don’t expect to take direct customer work any time soon but am teaming up with a local furniture restorer to extend her business to include vintage speakers and console stereo equipment. I also have a handful of projects which I’ve kept since I last posted and am finally ready to tackle!

A photo of the new workbench showing two large studio monitors on either side, with multiple stacks of test equipment in the center showing various analog and digital displays.

I kept a few pieces of gear from the old shop, including the variable power supplies and isolation transformers, the soldering equipment, stereo dummy load, AM/FM alignment generator, and GPSDO. The counter, oscilloscopes, signal generator, multimeters, and Audio Precision stack with external variable filtre are all new.

With the center vents, I can technically have two projects going at once on the left and right side of the bench. I’m looking forward to getting to work!

My first project was a repair and evaluation of a DBX SFC-10 Soundfield Imaging Controller, which I’ll post about later. Currently working on a KLH Model 25 which had some dubious quality work done in the past that I’m sorting through.

In any case, look for the next installment in days or weeks, not years!

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HH Scott HHS-20 “Scotsman” Receiver Overhaul

Posted on January 27, 2017 by retrovoltage

A local repeat customer recently brought in his old HH Scott HHS-20 receiver for an overhaul. It worked a few years ago when he put it away, although not without a few issues of its own, and when he dug it out it was right to the shop for an overhaul.

The HHS-20 was a very entry-level receiver, and not much information turned during research other than some speculation about it sharing an FM section with a bigger sibling. Inside, it used construction that would have been at home in a late-’60s early solid state receiver with a couple of odd exceptions, there’s a single PDIP-14 op-amp chip, and an assortment of TO-39-style op-amp chips in the FM IF strip.

It’s a cute little receiver with an FM MPX tuner, a tape loop, a single aux and phono input. I’d more accurately describe it as a self-propelled FM radio, more or less, because the -20 in the model number “HHS-20” represents the total power output: a maximum of 10W per channel, as measured after the repair was complete. Sensitivity measured at 150 mV LINE and 4 mV PHONO for maximum output.

Inside, it’s built on a pretty simple chassis, with separate boards and jumpers connecting everything.

Component replacement was entirely uneventful. There were several styles of electrolytic capacitors, but no real challenges.

Time for a power-up! It did great, given the low power, on the AUX input and Phono settings, but the FM tuner was dead. The Germanium output transistors gave this one a very warm, tube-like sound. Time to investigate the FM IF strip.

This radio checks out much like any other. Starting at the detector and working my way back, I injected a modulated 10.7 MHz signal into the circuit and listened for the tone.

Good injecting into the discriminator, but injecting into the input of the 4th IF Amplifier IC gave no output. However, when bypassed with a cap, the tone came in loud and clear. A bad IC!

According to Internet research, these UA703 IC amplifiers are a common failure item. I obtained some new Fairchild UA703HC chips in a more reliable metal case (date code 7603!) and replaced the defective amplifier.

Better! The new IC passed a signal, but the IF chain was still broken. Additional tracing revealed that the problem was the 2nd IF transformer, between the 2nd and 3rd IF Amplifier ICs – so, this receiver had both a dead transformer and a dead chip in the IF chain. Another jumper fixed the problem. Unfortunately these IF transformers aren’t exactly easy to obtain, however, it was easy to bypass entirely with a small capacitor out of the way and no real significant change in operation.

With the jumper in place on the bottom of the IF board, the receiver picked up stations immediately, and indeed the dial tracking was very close to correct. Time for an IF alignment. Received FM distortion started out about 3.1%, but adjusted to <1%.

With that adjustment, the FM sounded very good over the air, but there was a lingering issue with the FM MPX circuit failing to fire the Stereo lamp even with a good bulb. Unfortunately, the service manual provided no instructions for an MPX alignment and the MPX design in this receiver was an unfamiliar one, and since stereo decoding appeared to be working even without the light firing, so other than a quick adjustment of the stereo separation no additional work was done on the MPX decoder.

The factory service manual came with quite a few hand-written notations from a previous shop or tech, including a hand-written FM IF alignment procedure (involving a no-modulation test signal, 100K resistor, and DC voltmeter) but the distortion alignment was an even more precise adjustment, and none of the extra notes provided any insight to the MPX, unfortunately.

Like many budget receivers of the era, this one used RCA jacks for speaker connections.

Time to put the amp through its paces!

All told, this amplifier delivered 10W per channel into an 8 Ohm load, all channels driven, 50 Hz – 20 kHz +0 / -3 dB, with THD < 0.5% / THD+N < 2.2% at 1 kHz. The channels are ever so slightly imbalanced, about 0.5 dB – not perfect but not enough to worry about.

Not too bad, considering! There’s a few tricks which could bring those distortion figures down a little bit, including replacing a number of extra resistors and some of the coupling capacitors, but the labor on such extra work quickly becomes uneconomical.

Quite a few replaced parts! Caps, and one IC.

This receiver had its share of issues, but they were easy to track down and resolve. With new, top quality parts installed, this little receiver should keep singing for a long time to come!

Posted in Audio, Electronics, Projects, Stereo, Vintage | 3 Comments

A Surprise in a Re-Stuffed Capacitor

Posted on October 18, 2016 by retrovoltage

I was recently re-stuffing some capacitors for a radio. It’s not something I do often, but I’ve accumulated a pile of pulled vintage caps over the years so I have some on hand if a customer requests the most detailed service possible. I found a surprise inside of one!

restuffcap

Interesting! Someone must have purchased a radio, seen original parts and clipped them out and replaced them, without knowing that someone had re-stuffed the components already. I’ve only found this one with a new cap hiding inside, but we’ll see if any more turn up in a future project.

Posted in DIY, Electronics, Radios and Tubes, Vintage | Tagged , , | 3 Comments