The Speaker Spotter – Speakers of Craigslist – March 7th, 2014

Posted on March 7, 2014 by retrovoltage

There’s always something interesting for sale on Craigslist, especially in Seattle. Here’s a selection of especially notable vintage speakers and other audio products for sale on the local Craigslist. I’m not affiliated with any of these sales, but if you’re interested, you should contact the seller through the respective Craigslist page! Do click through each of the titles – the original postings contain a lot more information and many more photos, along with the seller’s contact information to request more information. If the ads are removed, the speaker probably sold, so don’t be alarmed if some links don’t work.

This issue of the Speaker Spotter covers both interesting finds in Seattle and Portland since last week. And so:

Vintage Allison One Speakers

Allison OneAllison One (2)

I bet these have a very wide sound stage from the bipolar angled design.

Bose 601 Series III Speakers – $700

Bose 601 Series III (2)

Bose 601 Direct/Reflecting speakers produce a very wide, warm, rich sound that’s very life-like. The 601 speakers feature a pair of 8″ woofers and 4 tweeters arranged in a reflecting pattern. When properly set up in a room they sound excellent.

Used Speakerlab K-Horn DIYs – $250

DIY K-Horns

These speakers started out as the flat-pack kit Speakerlab K-Horns, but the builder took some liberty with the designs. Instead of the same driver compliment usually found on the Klipschorns, with the pair of Electro-Voice mid and high horn drivers, they seem to have elected to build the bass cabinets and use alternate mid and high frequency drivers in separate enclosures. This would make the base of an interesting frankenhorn system, perhaps paired with an Altec or JBL multicell horn on top?

Vintage Fisher XP95B Speakers – $50

Fisher XP95B

From before Fisher went a bit downmarket, these look pretty interesting and might perform well. The seller reports the woofers (12″ or 15″) have been refoamed, and the phenolic ring tweeters typically appeared on higher end equipment, including speakers made by Altec Lansing and Marantz.

Infinity Kappa 8 Speakers – $300

Infinity Kappa 8 

Vintage Infinity speakers aren’t especially common in my experience. These are shallow 4-way designs featuring a pair of Emit planar tweeters (one in front, one on the rear) with dome midrange drivers. The Emit tweeters are bright and crisp, I bet these have a lot of definition in the high end.

Vintage Infinity RSe – $140

Infinity RSe

With what looks to be 8″ woofers and horizontally mounted Emit planar tweeters, these vintage Infinity bookshelf speakers probably sound great for near-field listening.

Two Pair Vintage Kenwood Speakers – $180

Kenwood Family

Kenwood Family (2)

I rather like the vintage wood lattice grille Kenwood speakers. In this case, this is a pair of KL-777As, 4-way/6-driver, and a pair of 333D 3-way/3-driver speakers. These are known for being very lively and musical and would make an excellent, affordable vintage quad system.

Marantz Speakers LS-20 – $300

Marantz LS-20 (2)

I don’t run into Marantz speakers very often and haven’t heard a set myself, but I know they have a reputation for being able to thump and in general sounding excellent. These look like they’re from the ’80s, a bit flashy, but worth checking out!

Sonab OA-14 Speakers Made in Sweden – $850

Sonab OA-14

Sonab OA-14 (2)

These are pretty rare vintage hi-fi speakers from Sweden. They have an interesting reflecting design that reminds me a bit of the Bose 601, with angled upward-firing woofers and four tweeters. It’s a very interesting design, I’m curious how they sound!

Speakerlab Corner Horns – $1600

Speakerlab Corner Horns

These have been on Craigslist a few times in the local area in the last year. It looks like they’ve changed hands since the last time, and the price is considerably higher. These have the 15″ woofer inside the folded horn and an Electro-Voice tweeter, but feature a 6.5″ midrange driver instead of a midrange horn. I believe this was the last year Speakerlab sold the corner horn design, which had moved away from the Klipsch-inspired design of the Speakerlab K.

Speakerlab Six

Speakerlab SIX

Speakerlab SIX (2)

You don’t see these very often, the Speakerlab Six is one size down from the Super 7s. The main difference is the Six lacks the 10″ passive radiator found on the Seven, and accordingly different driver placement.

If one of these speakers looks interesting, click through to Craigslist and contact the seller!

Previous issues of the Speaker Spotter.

Posted in Audio, Collections, Photos, Speakers, Stereo, Vintage | Tagged , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , | 1 Comment

Ohm Acoustics Model D Speaker Refurbishment

Posted on February 21, 2014 by retrovoltage

Classic speakers are always a delight to work on, and speakers have capacitors and other parts which require service, too. Over the years those components can fail and result in poor sound and even potential speaker damage.

In this case, the speakers on the bench are the Ohm Acoustics Model D. Ohm is more well-known for the Walsh speakers which produce an interesting omnidirectional sound from a vertically oriented cone; these Model D speakers were a more economical version that still offers warm and rich sound, especially known as being great for rock and roll music.

Ohm is still in business making boutique hi-fi speakers, and these speakers which originally cost $400 a pair in the early 1970s are still worth a $400 trade-in credit towards a new set today. They feature a 10″ woofer and a phenolic ring tweeter packaged in a solid hardwood cabinet with a resistive port. A resistive port is half-way between an acoustic suspension enclosure and a reflex enclosure, it’s not the most common design. Inside, there’s a 2 uF and a 20 uF crossover capacitor, an inductor, and a rheostat to adjust the tweeter level.

These speakers were used for only a few years in the ’70s, picking up only the tiniest of nicks and dings, before they were put back in their original boxes and stored for the next 40 years. Even with such careful storage, though, the woofer foam had degraded and was due to be replaced – so the first order of business was new foam surrounds.

Then it was to address the crossovers.

Bad capacitors mean bad sound! And this one certainly was bad. It’s supposed to measure 20 uF, but instead is measuring 0.0013 uF. That’s no good at all.

And this one’s showing more ESR than I’d like:

Beautiful classic speakers like this deserve nothing but the best, so I used brand new audiophile grade film crossover capacitors which are exceptionally stable and have a low dissipation factor for unmatched performance.

It was rather interesting – these were purchased as a pair, but inside, used slightly different capacitors. Both of the same value, but one used a single 20 uF bipolar capacitor and the other used a pair of 10 uF in parallel.

Now, it’s time to put the woofers back into the cabinets after cleaning up the gaskets:

Finally, time to package it all back up!

 

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1945 Westinghouse Little Jewel “Refrigerator” H-126 Radio Repair

Posted on February 18, 2014 by retrovoltage

A local client brought me a radio which was in the family for many years, a cute little “fridge radio” made by Westinghouse from 1945-1947. The same year Westinghouse released this new model of home refrigerator, they also released this Little Jewel radio in the same style as a companion. Many examples of this radio found their way into homes and kitchens over the years and are a great little collectible from the post-war era.

This little radio is a 6-tube AC/DC radio with a series-string tube line-up, 12SK7 12SA7 12SF7 12SJ7 35L6 35Z5. It has an integral tuned loop antenna and an RF Amplifier for great performance.

They sure packed it into here, too. The radio is held in place by two chassis bolts, one in each side and a rod which stabilizes it in place.

The dial scale has badly warped with age, but the grille cloth is nicely intact.

This radio’s owner mentioned that it had been playing but recently cut out. No surprise why – the original parts under the chassis looked to be in pretty bad shape!

Melted, leaking and blown capacitors will cause all sorts of issues – no wonder it wasn’t running like it used to! This radio was pretty cramped underneath the chassis, but there was enough room to install both electrolytic filter capacitors and the cathode bypass capacitor on the output tube under the chassis instead of needing to re-stuff the can on top.

I had a reproduction dial scale made up to replace the warped original, and replaced some tubes which were testing weak.

Finally, it was time to test it out and tweak the alignment trimmers, then back together! It sounds a lot better than I’d expect with such a small radio – the case makes an interesting vented enclosure. On 880 KIXI, you can really hear the low notes, and the vent on the top seems to serve as a port for the speaker.

After being refurbished electrically, this radio is going to perform beautifully for many years. It’s got a rich tone and great audio clarity even more than you might think from such a small speaker.

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1962 Zenith MK2670 Classic Tube Hi-Fi Console Stereo Repaired

Posted on February 15, 2014 by retrovoltage

A version of this article appears in the October 2014 issue of Nuts and Volts magazine.

I recently got to work on something a little bit different – a tube hi-fi stereo console! This particular one, a 1962 Zenith MK2670, was a very high-end unit back in its day. It’s a dual-chassis unit with 19 tubes total, using the 12K25 stereophonic FM tuner and 7K31 stereo amplifier chassis with EL84 outputs. All housed in a beautiful mid-century modern cabinet with built-in stereo speakers featuring 12″ woofers and mid- and high-end compression drivers and horns.

With 19 total tubes, this model features hi-fi integrated speakers with high-efficiency 12″ woofers and mid and tweeter horns, push-pull EL84 output tubes for each channel, and built-in FM Stereo Multiplex decoding for true stereo hi-fi reproduction.

This hi-fi’s owner reported it was working well for several years but started to go downhill shortly before she got in touch to have it fixed. It was taking longer and longer to warm up and sounding more and more distorted, no longer delivering the rich warm sound of a classic hi-fi console. It was good she got in touch – waiting any longer could have led to catastrophic consequences such as component failure or even a fire. I visited her home to test the tubes and pull the chassis, then it was back to my shop for repairs. And what a job it was!

This unit had been serviced a couple of times in its life – there were some ’70s era film capacitors installed, and some of the output tubes had been replaced. Most of the tubes were original Zenith fittings and tested strong, though, so very little needed to be replaced. Since it came into the shop in working condition, too, it made the diagnostic process much easier!

The amplifier circuit is a bit different than most I’ve seen. The negative phase of the output transformers was connected to chassis, and there were two positive phased taps each connected to half of the speakers. It’s an odd arrangement, certainly, which would have let Zenith use woofers of different impedance then the mid/tweeter network without an expensive and complicated impedance matching network. Underneath, though, it’s pretty easy to work on:

Right away there’s some visible damage. This molded ceramic capacitor blew a piece of the ceramic clean off from overheating. It’s unlikely it was doing much of its job at this point. Despite a nice ceramic body and epoxy sealed ends, it’s still an acid-paper/foil capacitor inside subject to failure, and fail it did.

With the amplifier chassis sorted, it was time to move onto the tuner. It’s nicely shielded on the bottom.

And there’s a lot going on inside.

Now  it was time for a power-up test. This particular radio uses an odd multi-tapped output transformer arrangement with drivers hooked up to both taps in the cabinet, so I ended up hooking up 4 distinct speakers for testing.

It fired right up and sounded great! There were a few minor issues to resolve, though. For one, there was a bad volume-invariant hum on the AM band only. This was due to a small short which was corrected. The volume controls were behaving pretty erratically, though, which took some investigation.

The unexpected behavior turned out to be due to a control scheme I hadn’t run into before. The “balance” control isn’t an actual fader; instead, the volume controls for the left and right channel are ganged together with a friction clutch. Turning the outer ring turns both together – but turning the inner ring adjusts the friction clutch allowing one to be turned independently to achieve the left-right effect. At the new set point, then, the outer ring will turn both volume controls together to adjust the loudness equally after the fade is applied. It wasn’t a popular control scheme, being replaced after a couple of years, because many consumers found it to be annoying and counter-intuitive.

After understanding how it worked, however, it turns out that it wasn’t gummed up, it was actually working properly. So, on to the next steps! Adjusting the bias on the channels:

RF and IF Alignment:

Lots of parts came out of this one!

This radio is going to continue to serve faithfully for many years, pumping out a warm and rich hi-fi sound and be a beautiful family heirloom to pass on. They just don’t build them like they used to!

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For Sale: The KN0CK and KF7LZE RTLSDR for HF Rev. 5!

Posted on January 28, 2014 by retrovoltage

I’m happy to announce that Rev 5 of the KN0CK and KF7LZE RTLSDR for HF is now available for sale in its 5th iteration – now featuring Direct Sampling, while still keeping the familiar wide-band pre-amplifier! On account of the reduced parts count, we’re able to offer it at a discount over the previous version too. Yours for only $85.00 with free shipping and a plug adapter!

Uncased Direct Sampling Receiver

Get yours today!

Previously seen at RTL-SDR.com

Posted in Electronics, Gadgets, Hardware, Radio | Tagged , , , , , , , , , , | 2 Comments

1977 Yamaha CR-2020 Vintage Stereo Receiver Repair

Posted on January 26, 2014 by retrovoltage

The Yamaha CR-2020 drives 120W/channel into an 8 ohm load and supports 3 pairs of speakers (but only plays 1 or 2 pairs a time), dual phono inputs supporting both moving magnet (MM) and moving coil (MC) pick-ups, tape-copy functionality with a separate output selector, and a variety of tone control and FM Stereo adjustments. This was one of the biggest flagship receivers from that era and it really shows!

No, I will not repair one of these for you. It is absolutely not worth my time and sanity, no matter how much you offer to pay. To properly service one of these, you basically have to completely disassemble it including removing the dial string – which is going to set you up for a real hassle getting tracking and tuning correct after you’re done. Sorry!

The owner reported the unit was playing, then suddenly went silent and wouldn’t play anymore. Telltale signs of smoke rising from the inside can be seen on the vent slats.

Let’s take a look inside…

An absolutely mammoth transformer, and the two final amplifier modules with enormous heat-sinks. Above the transformer is the power supply section; the far left moving out of the photo is the tuner.

The power supply has had three capacitors replaced but most are original. The lights have been modified at some point as well, although somewhat sloppily.

Yamaha used a drive-shaft type arrangement for the MM/MC switch with two flexible linkages to turn a switch all the way at the rear of the unit from the front panel. The dial arrangement is one long string that wraps around about a half-dozen pulleys. Not only is this a very complex, powerful electrical design it has a lot of physical components as well.

Overall, there’s a lot going on under the hood. It’s time to investigate further.

Here’s a previous repair with a sloppy solder joint visible.

Old and new caps:

It’s generally not recommended to leave old caps in place if some have failed…the others are the same and experienced the conditions, they’re going to go eventually – which is what landed this one back in the shop.

On the underside, the board was badly damaged during the capacitor replacement. It’s tough to repair these old boards without some damage, although that’s a pretty good chunk of foil missing. If a de-soldering iron at too low of a temperature had been used – or a piece of braid – that could have happened pretty easily. All of the joints are pretty cold and looked poorly flowed, though. I re-flowed the ones that looked like it wouldn’t further damage the board with a dollop of new solder.

Underneath the board with the regulator transistors is the rectifier board and massive filter caps.

I see some leakage around the bottom. And there’s evidence of heat from below:

At this point, all the old capacitors in the power supply are candidates for replacement. It’s clear the power supply boards have suffered several failures and need a complete overhaul. There’s quite a few transistors:

More damage. Lifted traces.

The rectifier board with the large filters has quite a few large wires going to it. These are the B+, B- and ground wires for the final boards attached to the rear.

New, computer-grade filter capacitors rated a tiny bit higher than original – all while being slightly smaller. I didn’t get a photo of how I attached it to the spade terminal originals – I used asmall piece of a copper bonding strap, screwed to the old capacitor and then bent at a 90 degree angle through the original slot in the board. This was easy because of the extra 1/4″ of space available with the new caps, and actually has a larger contact area than the original terminals so it should be more robust for high current handling.

Nichicon capacitors, some of the highest quality available, were used in this replacement.

Before:

After:

On the left, capacitors which definitely failed – either very high ESR, out of specification, or open circuit. On the right, capacitors which were “technically ok” for now.

There’s a handful of caps on the final boards, too.

It’s time to pull the finals.

From right to left to the power resistor: signal common, signal input, b-, b+,

From left to right: TP1 bias meter point (no wire), amplifier output, ground/CT, B+, B-.

Ah yes, I seem to have found the problem:

Looks like something had a real bad time and let the magic smoke out. It’s destroyed a pair of resistors, a small-signal diode, the HW-21468 fuse resistor, a ceramic disc cap, and a driver transistor – that we know of.

Final output transistors. The amplifier board is held on by the base connections which are screw terminals through these.

The failure was so violent it scorched the board and blew one of the legs off the transistor’s case.

That’s a fair amount of dead parts. Time to hook it all back together and test some more.

Powering back up….nothing. The lights come on, voltages appeared, and nothing caught on fire – but there’s no output. Time to do some probing. Initially, I wasn’t even getting a signal out of the pre-amp stage. There are several places along the way to check for the presence of a signal on the volume gang:

After toggling several switches I did get audio to the inputs, finally. The un-failed channel was doing fine, but the other one, not so much.

The speaker protection relay is checking for proper voltages before connecting the contacts.

Something must be dragging down one of the rails. It turns out one of the sense lines was being shorted to ground; moving some wiring around corrected this problem. The relay clicked and engaged, and perfect sound started coming out – from one channel only. It was pretty clear there were some dead transistors which needed replacing also.

This one played great as well. I tested them on my bench speakers for quite a while with the guts spread across my workbench; the bench speakers are very inefficient and this allowed the amp to get a decent work-out. Finally, it was time to get everything back together. I fixed up a bad connection to the dial lights where a resistor lead had broken. Then put it all back together.

Time to adjust the bias! Using the oscilloscope, I measured the voltage between TP0 and the speaker output terminal for a 10mV +/- 1 mV voltage difference.

Finally back in the case, and hooked up to my K-Horns for some real live testing.

The tuner section could stand to be refurbished later on to bring broadcast reception up to standard, but other than that, it sounds fantastic. Warm and rich, it consumes the FLAC audio I use for testing easily and pumps out incredibly crisp, accurate sound with a ton of power and headroom to match. It’s great to have it back in operation. I’m probably going to get myself one of these at some point, it would make a great center for my own vintage hi-fi use.

Posted in Audio, Electronics, Hi-Fi, Music, Stereo, Vintage | Tagged , , , , , , , , , , , , , , | 34 Comments

1948 Motorola 99-FM21 Console Radio Back to Life

Posted on December 23, 2013 by retrovoltage

I recently had the chance to work on this beautiful Motorola 99-FM21 from 1948, currently owned by the granddaughter of its original owner. It’s been in the family since its purchase new in 1948 and was lovingly cared for the entire time, even after it was no longer in active operation. It really showed, too.

This is a post-WW2 radio when very few radios contained any shortwave bands. This Motorola has the AM Broadcast Band, and the newly developed FM Broadcast Band from 88-108 MHz. That’s right – you can pick up modern radio stations with this tuner. It has a pair of 6V6 tubes for the output (although its bass response is limited due to an undersized output transformer) and a treble roll-off tone control.

The radio looks like it had been serviced one or two times in the past. Most parts were original, but two capacitors looked to have been replaced in the ’50s.

Here’s that small output transformer. To give you an idea of size, it’s a little bigger than two stacked boxes of matches, like a restaurant might give out. A similar hi-fi amp, also driven by a similar pair of 6V6 output tubes, uses a transformer about the size my fist in the same circuit position. The amount of iron in a transformer is directly correlated with its frequency response, so Motorola seems to have cut a corner by fitting this particular transformer. It sounds good, but doesn’t have quite the bass response that’s possible.

This radio’s owner requested the ’50s capacitors be replaced with period units, and the can capacitors restuffed and the shells retained. Not many elect for that additional service as it’s quite time-consuming to perform, but in this case it’s a great choice with the radio being in the same family for so many years. I started with labeled clip-leads in place of each replaced unit to keep the circuit straight. With the schematic to double-check, this ensures error-free service.

Restuffing capacitors involved heating them to melt off the wax, then pulling on the leads until the body of the capacitor was removed. I’d then ream the cylinder, add a new capacitor, seal with polymer clay and dip in wax, then replace in the radio. I found two period-correct capacitors to replace the two ’50s units, as well.

Capacitor re-installation was pretty straightforward:

I also used similar sized electrolytic replacement capacitors which look similar to how a period repair would have been performed. It’s useful to have a bag of donor parts around to provide shells for restuffing.

At this point, I hooked up my phone as a test source and it played loud and clear through the phono input. AM reception was okay, and FM was quite poor, however. It was time to do some further testing.

This radio is one of the first to use integrated couplets, blocks of several components in a single package. In this case, the component in question is a 47K 1/2W resistor paired with two 50pF capacitors. The resistor is reading 73.9K Ohms, which is well outside of its tolerance. This one was replaced with a similar visual appearance carbon composition resistor from IRC.

I replaced quite a few more resistors, and attempted an alignment. First, I tried using the triggered oscilloscope and sweep generator, which even required building some of my own test leads:

Unfortunately, my sweep generator’s FM options are geared towards more modern equipment which deals in whole-number modulation percentages; the Motorola wanted deviations around 2.2% and 1.4% which I couldn’t provide. So I went with the FM Alignment with AM Generator option, using my laboratory signal generator the Leader 3216. The procedure was a rather lengthy one, involving de-tuning the discriminator coil secondary to cause it to respond to AM signals, then tweaking other adjustments for peaks.

After adjusting the IF transformers in order and the RF trimmer for dial tracking, I re-peaked the discriminator primary using an insulated driver to eliminate the AM noise. The FM discriminator is supposed to be only sensitive to frequency deviation; a no-deviation signal (like an AM signal) should produce no sound at all. It was a very touchy alignment, taking about a half an hour of the tiniest of adjustments to null the signal. The curve had a very slope and it was incredibly difficult to peak – similar to the discriminator in the GE F-135, but even more precise because of the much higher frequency (10.7 MHz vs. 455 KHz).

Finally, however, it did peak up and I attached a 300 Ohm twin-lead dipole. The FM antenna has a known impedance, while the AM is a flat-board loop with a terminal for an optional external longwire antenna.

After this alignment I was able to pick up stations across the dial loud and clear on AM and FM, and the phono input performed perfectly. At this point I let it play for several hours hooked to a bench speaker as a burn-in test, then arranged with the owner to return the radio and test it out. Sadly, her home is on the declining edge of a valley which offers exceptionally poor radio reception and there’s nearly nothing to be heard. Where I received nearly all AM stations and many FM stations on the same equipment, there was only a few faint broadcast stations to be heard. We ended up attaching an amplified TV/FM antenna with a 75 ohm output to a 75:300 balun and attaching that to the radio, which vastly improved the FM band – although on some of the strongest local FM broadcasts at that point, now we were driving the front-end of the radio into distortion. So clearly, this wasn’t a great location for radio. However it sounds phenomenal on period music and is going to serve for many years to come.

This is a beautiful cabinet in original condition and with reconditioned electrics is going to be a great conversation piece, keepsake and music player for many years to come.

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JVC XV-S60 DVD Player Repair

Posted on December 17, 2013 by retrovoltage

A local gentleman brought in his JVC DVD player, model XV-S60, for repairs. It’s a high end unit from around 2001 with some sentimental significance and despite it being a fair bit outside my normal interest, I decided to take this one in on a best-effort basis and see what I can come up with.

It either had a capacitor leak, or some water damage, or possibly both which caused some corrosion inside, and was refusing to play DVDs – only CDs.

With this board being suspect, I located a replacement board which was known to be operational and tested it to verify, then proceeded to perform a complete capacitor replacement to ensure it will have a long lifetime for its owner.

Although not really made to be serviced, the silkscreen nicely indicates polarity for the electrolytic capacitors. It’s only a single-sided board; some ICs and SMD components are on the bottom, and the top side is populated with through-hole components, with jumpers up as needed to cross traces.

After component replacement, I powered it up again to verify operation – DVDs are now coming in great!

I’m not really sure what kind of life to expect out of the refurbished DVD player. The capacitors themselves will be good for a long time, but I don’t know the state of any of the other components or their estimated longevity – laser diodes, controller circuitry, really anything else. The player sat in a damp environment for a little while and has some quirks from being over 12 years old and well-loved at that, but now it should be good for a while.

 

 

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1936 Zenith 5-S-29 Repair with Dial Face Replacement

Posted on December 12, 2013 by retrovoltage

I recently had the privilege of working on this beautiful 1936 Zenith 5-S-29 tabletop radio. It’s a beautiful 5-tube table radio with a 6-inch speaker and the iconic black Zenith dial and lightning bolt Z pointer. This one is special, too, because it has a swept second hand to enable fine tuning.

This radio came to me locally from its owner who had purchased it on eBay a short time ago in “restored” condition. Unfortunately due to a memory issue with my camera the first set of photos was lost. The radio had definitely had some work, but this is definitely a case of “buyer beware” on eBay: it looked like the previous technician got bored half-way through and left most of the original capacitors intact. About 3 had been replaced with film capacitors, and the electrolytic capacitors had been replaced, but otherwise it was all original. The dial had some coloration wiped off the back, too, and a dried out rubber band instead of a proper dial belt. The zipties were there as well, although they appear to be serving their purpose so I left them alone.

The curved glass dial is held in place by a metal clip ring around the outside. Inside between the dial face itself and the glass was a ring of cork as a spacer.

In order to replace the dial belt it’s necessary to remove the dial ring.

The dials, side by side:

New dial belt slipped over both pulleys, and dial holder replaced:

I carefully re-glued the cork spacer (with its original gap in the ring) to the perimeter of the new dial face.

Dial pointers reinstalled on the dial:

I performed an RF and IF alignment to peak up the signal and really bring out the rich tone. Afterwards, 880 KIXI is coming in nearly spot-on. Prior to the alignment, it was coming in about 910. It tuned very well through all 3 bands – even bringing in 3 shortwave stations on Band C with the shop antenna! It’s that kind of reception that made Zenith famous with their “Long Distance” radios. Even an entry level set like this one was capable of excellent performance.

Reinstalled in the cabinet! Fully serviced and aligned, this radio will play beautifully in its owner’s home with a wonderful rich tone. Just in time for Christmas, too!

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Bose 901 Series I Active Equalizer #50911 Repaired

Posted on December 3, 2013 by retrovoltage

I recently had this Bose 901 Series I Active Equalizer, serial number 50911, in the shop for repair. These are a lot of fun to work on and very rewarding for the great sound they produce when they’re working properly. This particular one looks to me like it had seen some service in the past, and the left corner of the front cabinet looks like it had cracked. The Early Production versions of these cabinets were made from hardwood, but the later ones merely MDF with a veneer applique on top.

Inside, it looks like there was a series of repairs which replaced most of the electrolytic capacitors and a few of the signal capacitors in the middle. Either that, or it was a production stock changeover – but I find that unlikely. On all of the original components, the lead ends are slightly crimped in addition to being soldered – you can’t pull a desoldered bare lead which is sticking through the board back out the top side. On every replacement part I’ve seen, the wire end is not crimped. It’s not a foolproof method, but it seems to be pretty accurate.

You can see the capacitors starting to leak:

This is the first time I’ve seen this, although it’s a familiar technique: the transformer’s electrostatic shield, which is connected to the outer mounting lugs of the transformer on the core cover, is joined to circuit common. Tube radios used this technique as well to reduce interference. These Series I Active Equalizers are very resistant to interference in the first place – especially compared with the Series II and Series III – but this will theoretically help even more.

This equalizer’s owner requested I make a small modification to the circuit. These were originally fitted with electrolytic output capacitors – and that’s what I usually use as an OEM-level replacement. Electrolytic capacitors both then and now do have a fairly high dissipation factor compared with other capacitor materials. Hi-fi audio makes use of a wide array of film capacitors which offer a much lower dissipation factor – about 100x lower in many cases – to improve the output quality and lower distortion. This is one area where materials science has advanced significantly and we can make a real performance upgrade.

The specific audiophile capacitors I use vary depending on what I have in stock, but all of them offer the same advantages. In this case, I’m using a 5.1 uF capacitor to replace the factory 5 uF capacitor. This is well within the +80/-20% tolerance rating of the original capacitors which would consider any capacitor 4-9 uF correct.

Looks great all finished up! This one will run beautifully for many years to come, and the upgraded output capacitors will really help bring out the dynamics in the music.  After cleaning the switches and a burn-in period it’s all boxed up and ready to go home.

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