Pekka Väänänen must have been bored over the holidays, as he came up with a fantastic and fascinating new use for an oscilloscope and a laptop: playing a round of Quake on his Hitachi V-422 scope in X-Y mode.
After some pre-processing, apparently it’s a playable vectorized Quake game with a very unique display. Never seen anything like this before! Lots more details over on his site. Check it out!
Season’s Greetings!
Cross-posted from the Rain City Audio Repair Blog:
I recently got to work on a very interesting piece of vintage stereo gear from the golden age of hi-fi in the ’70s, the Sony TA-5650 VFET Integrated Stereo Amplifier.
It’s a little rough, and while the power amp section works great the pre-amp doesn’t produce any output, this is a rare and interesting amplifier. In the output stage, the finals are Sony VFETs – a new and experimental type of vertically oriented FET which was being pioneered around this time. Sony used them in a small handful of receivers from the same year and never again in any other models or years; Yamaha produced a couple of models which used them as well, and oddly enough they turned up in a handful of 1990s MTX car audio power amplifiers – but overwhelmingly, it’s a rare and esoteric output device.
It’s also missing a knob.
It’s a well laid out amplifier inside: Towards the front, the final module with large bottom-mounted heat sinks and chimneys to the top-side vents for good airflow. The pre-amp controls, power supply, and rectifier are along the front and there’s a large power transformer.
A shot of the regulated power supply. As a part of the process, all the electrolytic capacitors get replaced. At this point in the process, the underlying cause of the pre-amp failure isn’t known, but that doesn’t change the procedure. The most likely cause of failures is often a failed electrolytic capacitor which leaks and damages near-by components. With the front face and knobs removed, the boards can be removed.
Overwhelmingly the capacitors were all replaced with Nichicon Fine Gold capacitors, although a handful in power supply circuits in high-ripple locations were replaced with other models with a better current capacity to ensure reliable operation. Unfortunately, however, this didn’t fix the problem: it turns out the 2SK76 small-signal VFETs were defective. That does mean this integrated amplifier will never have a functional pre-amp again, but it’s still a fantastic power amp stage.
The chimneys clip into the board supports, with the VFETs along the bottom. Removing it exposes the board to view.
One major reliability problem with these Sony VFET amplifiers, which has sent many of them to an early grave, is the varactor diodes used in the bias circuitry. They’re used to provide a stable, temperature-invariant voltage reference but unfortunately over age (aided by leaky capacitors) they tend to start to avalanche and fail to prove bias, instantly destroying the VFET output devices. They’re unobtanium, so if this happens, really the only source of new parts is another one that’s died for some other reason.
It’s not pretty, but it works: these VD-1221 varactor diodes can be replaced with a pair of 1N4148 in series.
One other issue with the Sony VFET series of amplifiers is the rectifier board. Sony used screw-in capacitors with a 10mm lead spacing; these are no longer manufactured. It was necessary to extend the leads and mount the board slightly on an offset. Again – not pretty, but completely functional.
After mounting everything back together, adjusting the power supply’s voltage reference, and adjusting the bias on both channels it was time for a burn-in test. This one plays very well with exceptionally low distortion, crisp and clear highs and a very triode-like midrange owing to the VFETs.
Quite a few parts were replaced during this repair – and I found a “new” chickenhead knob as requested by the owner to replace the missing knob (on the far right.) Fully reconditioned like this, it’s going to sound fantastic for a long time! These are pretty uncommon to find these days, so even with a bad pre-amp section it’s definitely worth the effort to repair – and with the low distortion and unique VFET sound it’s great for an audio enthusiast. This particular one belongs to a Grammy® Award-winning record producer if that gives you any idea of the quality and performance.
(Appended years later: this same exact unit came across my bench a second time from an owner who picked it up at an estate sale and wanted the preamp repaired — unfortunately that wasn’t going to be possible for him, but he put it to good use paired with the companion amplifier anyway.)
This Philco 42-365 came to the shop in need of major repairs. It was missing its dial string, lamps, and the power transformer had melted down due to a failure in the main electrolytic capacitors.
It’s melted and run everywhere, and left quite a bit of smoke residue. Fortunately, the power transformer on this model was also used on a few models spanning several years, so it was fairly readily available.
Transformer leads back then weren’t always labeled with standard conventions, so it’s always important to test them.
The replacement has a bit of surface rust but otherwise works perfectly.
The dial cord on this model is two separate strings wrapped around 5 pulleys, but it went back together okay.
It all went back together! The dial scale on this model is attached to the cabinet, and the owner brought the chassis and speaker alone, so I aligned the IFs but wasn’t able to align the dial tracking.
Back in its cabinet, though, it looks great!
If you’re looking for a great visual primer on how speakers convert electricity into the music you hear, check out this fantastic animated infographic by Jacob O’Neil at Animagraffs
If you’re a subscriber to Nuts and Volts Magazine, check out my article in the December 2014 issue where I build, test and review the Boxed Kit Amps Gobo Stereo Amplifier, a great little desktop reference amp for any skill level in a beautiful laser cut blue acrylic case.
We’re lucky that these days you don’t need to stock very many diodes in the parts drawers for refurbishing most vintage gear – pretty much everything except for the main power supply rectifiers can usually be replaced with some combination of 1N4007 and 1N4148.
Some special diodes do still turn up, though, and they’re a bit harder to replace. Varactor diodes frequently turned up in the bias circuits of ’70s amplifiers where they were used to keep a very stable reference voltage for bias under changing temperature and current conditions. One commonly encountered varactor is the VD-1221, which turns up in the Sony TA-4650 VFET amplifier (3 diodes), the Sony TA-5650 VFET amplifier (4 diodes), and the rest of that VFET line. They also happen to not be the most reliable, and a failed VD-1221 will destroy all the VFETS in that channel. Those are unobtanium these days, so if you’re servicing any VFET amp it’s practically mandatory to replace those diodes as well.
Fortunately, in this application, they can be replaced with a pair of 1N4148s in series. Observe the correct polarity, but it’s as simple as that! It’s not the prettiest fix, but it’s cheap and effective.
2 x 1N4148 in series will also work to replace the common VD-1212, VD-1220, VD-1222, and likely several other similar series diodes.
Every once in a while, you run into places where a drafting tech made some mistakes on the schematics. This one was a Philco 42-360 radio, which has the wrong tube called out in the tube line-up vs. the schematic.
The actual radio uses an 84/6Z4 rectifier tube, but the tube line-up shows a 5Y4 in its place. These would definitely not work if substituted.
It’s pretty common to find drafting mistakes. Another Philco, wired per the schematic, had no B+ on the detector/1st AF tube. The Sam’s Photofact of the EICO HF-81 has the tubes all numbered wrong, versus the EICO schematic where they’re correct. And so on.
Keep a close eye out when working on vintage gear! You can’t always trust the schematics.
This little Harman/Kardon Control Amplifier, model A-401, just came through the shop for a full overhaul. It was in a pretty sorry state to begin with – one channel distorted, one barely gasping for air, and a world of scratchy controls in general need of attention. Despite the electrical shape, though, it was in great cosmetic condition and definitely worth a full rebuild.
Physically, it’s a very unassuming piece of gear with elegantly understated styling, the height of ’70s simplicity. Featuring up to 20W output per channel with low distortion, it’s a great small desktop amp or perfect for pairing with a vintage hi-fi system and will fit in with nearly any decor. No wonder the owner wanted it fixed up!
The inside was untouched, which is always nice to see. It’s a very open, accessible construction design. All of the PCBs could be accessed by removing the top and bottom covers without removing any boards from the chassis itself, and every component was easily accessible. After 40+ years, the original 2SC1030 transistors had tarnished pretty significantly but otherwise everything was in decent cosmetic shape.
For whatever reason, this amp had two tantalum electrolytic capacitors – the blue components on the right of the vertical board – but the rest were all standard aluminum electrolytics. I set out for the component replacement which was very straightforward. As expected, the caps were failing and had started to leave some residue on the boards.
All the electrolytic and tantalum capacitors in this amp were replaced with Nichicon Fine Gold audiophile grade electrolytic capacitors. These are much better components than anything that was available at the time and really help to bring out the best in these vintage amps.
Finding drop-in replacement capacitors is pretty tough, but I got lucky with these. Left, the original 6800 uF 35V capacitors, and right, a pin-compatible drop-in replacement of 6800 uF at 100V. Easy! Running massively de-rated like it is, and with the advantages of modern construction techniques, these main filters will almost certainly last decades. It’s always possible to make modifications to use whatever capacitors are available, and sometimes it’s unavoidable (10 mm-spacing screw terminal radial capacitors come to mind) but it’s much easier, and cleaner, when there’s a drop-in replacement available. This amp could take up to about a 10000 uF capacitor in that position and still be within tolerances, so there’s some flexibility.
Even after all these repairs, it sounded fantastic – from one channel. The other channel was cutting out intermittently and had hum and distortion. While probing, the amp blew both its power supply fuses. It turns out that one of the mica insulators under the output transistors was damaged and wasn’t insulating very well; there was a conductive path to the heat sink. These transistors were pretty worn, anyway, so the owner approved a minor upgrade to a modern, new manufacture TO-3 output transistor with much improved ratings – the MJ15003.
During the troubleshooting process, I did identify the defective 2SC945 transistor in that channel’s driver circuit which was causing some distortion. It was a part of a push-pull pair, but only delivered about 1/10th the gain as its compliment on the other side, thus causing the distortion. Replacing it cleaned up the worst of it, but the channel was still popping and snapping a bit for several minutes after powering on. A quick check of the driver transistors in that channel found a few that were weak, intermittently conducting, and poorly matched – so they got replaced, too.
That took care of the problem!
After replacing the parts, everything checked out! It was time to adjust the bias to ensure it’d be a safe operation, then on to performance characterization.
I was very impressed with the results of the tests. This amplifier was flat +/- 0.3 dB through my distortion analyzer’s signal generator range, 10 Hz – 20 kHz. The MJ15003 is also much more capable than this amplifier is asking of it so there’s quite a lot of headroom, and the more efficient signal path due to the up-rated components plus more powerful output transistors resulted in 50% more measured power output at very low distortion.
At 1 kHz, THD was below 0.1% through most of its original power rating of 20%, beating its specification by 67%. This one, however, was able to deliver 30W with a 150 mV input at maximum volume, still with a THD below 0.5%. That’s audible to many ears but still listenable for the few times you might really need the extra power.
This was a fun project, and it’s going to be a great little amp for a desk or an apartment for a long time.
RETROVOLTAGE 
Watch a McIntosh Tube Amp Come to Life
Discovery’s How It’s Made shows us how a McIntosh vacuum tube audio amplifier comes to life:
http://www.youtube.com/watch?v=2HgS6gvokEI
Definitely worth watching!
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