Craigslist and eBay Antique Radios – Be Careful!

Posted on March 19, 2014 by retrovoltage

It’s not uncommon to run across antique or vintage radios on Craigslist, eBay and other sales site which are advertised as “working”, “repaired” or “restored”. Many times these are in fact professionally reconditioned items – just like you read about here – but occasionally, there are some critical hidden problems to look out for. Let’s take a look at an underside photo which was included in the for sale posting: Echophone-2

Not too bad! It looks like there’s some new capacitors carefully installed – but there’s a problem. Can you spot it?

Echophone-3

Again, with a hint this time! Several original paper capacitors look to be installed in the radio. It’s even believable they might work – by and large they’ll be well past their service lifetime, but through dumb luck you do occasionally find vintage capacitors which are still technically operational. Even if one is working now, though, they are universally all completely unsafe for operation for any length of time. Old vintage capacitors like these can spontaneously fail and short out, which can damage the radio or even start a fire! A repair like this is not in compliance with the best practices for antique radio repair, and I wouldn’t let one out of my shop in such a condition, that’s for sure!

If you’re buying an antique radio or vintage radio and have the opportunity, ask for photos of the underside of the chassis or ask to inspect it personally and check for old parts. If you’re having your old radio serviced, make sure to choose a reputable repair shop or service technician who will follow best practices and replace all components which are subject to spontaneous failure – not just the ones which are bad “right now”. Your radio – and maybe your home – depend on it!

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The Speaker Spotter – Speakers of Craigslist – March 14th, 2014

Posted on March 14, 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. Try searching for the keywords as sometimes they expire or are reposted.

It’s a good week for rare and unusual speakers in Seattle this week – several Electro-Voice vintage speakers, and the Altec Lansing Voice of the Theater!

Bose Speakers and Amplifier – $300 in Port Orchard

Bose Speakers Amp

This looks like a ready-to-go kit of Series IV or V speakers, Active Equalizer, and an unknown amplifier with two very large power meters. These also sound fantastic. Series III+ speakers used a design with resonant acoustic cavities as opposed to the acoustic suspension enclosure of the Series I/II and also sound fantastic. It’s worth double-checking the foam on Series III+ Bose drivers, though.

Bose 901 Speakers with Equalizer – $250 in Port Orchard

Bose 901 SpeakersEqualizer

This is a great price for a set of Bose 901 Series I/II speakers with a working equalizer, and of course Bose 901s are a very musical and life-like speaker to buy. Series I/II have cloth rubber surrounds on the drivers, as well, so you don’t have to worry about refoaming 18 drivers.

Electro-Voice Regenecys – $400 in East Wenatchee

Electro Voice Regencys

E-V speakers are pretty rare, they stopped making consumer gear in the ’70s. They made some of the finest speaker drivers and their finished speaker products are incredible as well. These feature 15″ woofers, T25 mid horns and T35 tweeter horns. Very similar driver compliment to a Klipsch Cornwall, for example. I bet these sound fantastic!

Electro-Voice Hi-Fi Speakers – $250 in Bellevue

Electrovoice Hi-FI speakers

Electrovoice Hi-FI speakers 2

I haven’t seen these before, but they look very interested. It looks like they have the SP12 extended response 12″ woofers, the T-25A midrange (same as on the Regency), but are missing what was likely a T-35 tweeter. You’d probably need to spend $100-200 on the missing drivers, but I bet these would sound very good complete.

Klipsch KG4 – $120 in SeaTac

Klipsch KG4

Despite some water damage, these speakers are a bargain – featuring an exponential horn, two 8″ active woofers and a 12″ passive radiator on the rear. They sound great and are very efficient – my pair sound fantastic. The water damage on these looks like it’d be pretty easy to refinish, too.

Klipsch Speakers – $600 in Granite Falls

Klipsch Speakers

Always a good find, these black Heresy II speakers look like they’re in great shape. They’re one of my favorite little speakers.

Pair of Vintage Wharfedale Speakers – $100

pair of vintage wharfedale speakers

These look like they’re from the 1960s, possible a little earlier. I’ve never heard any Wharfedale speakers, but being a high-end brand from the golden age of hi-fi, these are almost certainly worth listening to and the price seems pretty fair.

Sansui S-1117 Tower Speaker System – $100 in Tacoma

Sansui S-1117 The tower speaker system

These get a listing because they’re “interesting”, although they don’t have a reputation for good sound. With a pair of 12″s in each cabinet, though, they probably can deliver a decent amount of bass and they sure look very visually impressive. It’s unfortunate one has a replaced woofer.

Speakerlab SKO Speakers – $750 in South Seattle

Speakerlab speakers (SKO)

I think this is the late-generation Speakerlab Corner Horn after they moved away from the Klipschorn-inspired design. These have been mounted angled slightly back on speaker stands. I’m not sure what to make of the whole thing, personally, but given the 15″ woofer inside the folded horn and the T35 tweeter, I’m sure they can deliver.

Electro-Voice “The Duchess” Speakers (1962) – $900 in Lynnwood

Speakers The Duchess Speakers 1962

Very rare speakers, but these are missing some parts, and I’m unclear from the description whether they are functional with the modification. Might make a good, if somewhat expensive, project to have a very desirable vintage speaker.

Vintage Akai SW 177 Speakers – $40

Vintage Akai SW 177 speakers

I’m sure these would make a great garage speaker – a 15″ woofer, mid, dual tweeters, and durable butyl rubber surrounds on the drivers.

Vintage Sansui SP-100 Stereo Speakers – $55 in Edmonds

Vintage Sansui SP-100 Stereo Speakers

I just love the look of Sansui vintage speakers with the scalloped wood grills and solid wood cabinets, often with heavy dampening inside. These are a very early series with a woofer, mid-range, and a horn tweeter. The crossover will certainly need to be rebuilt for best performance, but you’d have an inexpensive vintage hi-fi setup with warm and rich sound.

Vintage Wharfedale W-35 Speakers – $400 in West Bremerton

Vintage Wharfdale W-35 speakers VG condition 2

Vintage Wharfdale W-35 speakers VG condition

A bit smaller than the other Wharfedale speakers, and a bit more expensive, I’d love to know more about the features and differences of those two models.

Altec Lansing Voice of the Theater Speakers – $50,000 in Redmond

Vintage Altec Voice-of-the-Theater Speakers

Vintage Altec Voice-of-the-Theater Speakers 2

“A better investment than a Porsche” these holy grail speakers out of Redmond, home of an unlimited amount of tech money, aren’t for everyone but they might be right for you. Originally these were high-end PA Hi-Fi speakers for movie theaters, etc. they feature large an powerful woofers mounted in flared horn enclosures and enormous multi-cell horns coupled to compression drivers. They’re known as being incredibly efficient, absurdly powerful speakers and might actually be a better investment than that sportscar.

If you’re interested in any of these speakers, click on the title to visit the original Craigslist posting to contact the seller!

Do you know anything about those Wharfedales? Or own a pair of the speakers listed here? Leave a comment!

More Speaker Spotter

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1939 RCA K-60 Console Radio Repairs

Posted on March 10, 2014 by retrovoltage

This beautifully preserved RCA K-60 radio came to me after a Golden Retriever puppy got a little too up close and personal.

Otherwise, it’s been beautifully refinished in the past, and was fully overhauled in the late ’90s according to a stamp on the rear of the chassis and the replacement speaker.

Replacement grille cloth is somewhat hard to find these days, but the pattern was almost a perfect match. It’s much cleaner, too! The speaker must have failed at one point, because it had been replaced. I’d have picked a different substitute driver myself, but this one seemed to work well enough.

On the rear, the output transformer and speaker lead connections were relocated to the basket.

Unfortunately, during transport, the field coil (which had been retained but relocated) wasn’t securely mounted and ripped from its mounting point, damaging the speaker voice coil leads in the process.

I carefully peeled back the paper and soldered new, stronger leads; then reinforced the structure with Gorilla tape. The new coil checks out – about 1K Ohms resistance, and 2.3 Henry of inductance. That’s a lot! Any further back of a break would have been much more complicated to successfully fix.

Before this repair, the field coil was just hanging from a peg with exposed metal clips into the coil – a major shock hazard, with several hundred volts present on an externally exposed, uncovered piece of metal!

‘

And, all set!

After a short power-up to make sure the field coil repair was sound, she’s ready to go home! Hopefully the puppy has learned a few lessons about the proper handling of antiques in the past few weeks so we can avoid a repeat performance.

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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!

 

Posted in Audio, Hi-Fi, Projects, Speakers, Stereo, Vintage | Tagged , , , , , , , , , , , , , , | 1 Comment

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

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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.

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.

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Coming Soon: KN0CK and KF7LZE RTLSDR Receiver Rev. 5!

Posted on January 6, 2014 by retrovoltage

Edited 6/10/2014: Now for sale directly at KF7LZE.net!

I’m proud to announce that in just a few days, Easy-Kits.com in cooperation with KN0CK will be releasing Rev. 5 of the popular RTLSDR Integrated Upconverter with a brand new design and smaller form factor!

This new design will feature a lower price-point and is designed for operation in direct sampling mode for improved flexibility across the entire frequency range. It contains the familiar Mini-Circuits MAR 8 wideband amplifier, to deliver unprecedented weak signal performance when paired with a quality receiving antenna setup.

Available NOW!

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