Sherwood made some excellent hi-fi FM tuners back in the day, and this S2000-III is no exception. Unfortunately, there’s not a lot of documentation about this model available, but that didn’t stop it from getting an overhaul and alignment.
The Sherwood is marked like it’s a stereo tuner, but the stereo de-multiplexer is an add-on module on an optional separate sub-chassis which was either removed, or never fitted, to this particular model. As a result, most of the front-panel options for various stereo and blend settings don’t operate.
Most of the capacitors were ceramic but several bad paper-mylar capacitors were installed, contributing to its poor performance. The large metal plate is where the sub-chassis for MPX reception would be fitted, plugging into an octal socket above the chassis – but it’s missing, so this tuner will only receive mono.
In addition, the 6BZ7 tube, used as a cascode RF amplifier, was defective with an intermittent heater; it needed to be replaced. After sorting the tubes and capacitors which were keeping it from running at all, I proceeded on to the alignment, coupling my signal generator to the distortion analyzer and using the 300 Ohm antenna connection. It was definitely out of spec:
Alignment was fairly straightforward. First, I tuned up the oscillator to bring the dial into alignment, then adjusted the RF amplifier for strongest signal and lowest distortion. Re-tuning the RF amplifier significantly improved performance, but it had a ways to go still:
Finally, adjusting the IF chain from back to front for the point of lowest distortion at each stage really cleaned it up.
There we go! Much better. Most alignments can be completed, even without factory instructions, by aligning for lowest distortion with a distortion analyzer.
Fresh tubes, fresh capacitors and an alignment have this tuner performing very well. It’s going to be great for a long time, too.
I recently got to work on a Harman-Kardon Citation 17S pre-amplifier. Vintage HK gear is some of my favorite to work on: it’s robust, reliable, and built to be easy to service. The Citation 17 pre-amplifier is the companion for the Citation 12 power amplifier, and has a ton of features.
This one features a total of 6 inputs with 2 phono options, two tape loop outputs, and two pre-amp outputs which could drive a pair of power amplifiers.
Inside, it’s extremely open, and all of the circuit boards are mounted on edge connectors that can be easily removed for service outside the chassis.
There are 4 circuit cards with a variety of capacitors on them. These caps were starting to show their age, but weren’t quite dead yet, but several had high leakage and high ESR and were definitely not doing their jobs. This one was the worst, where internal leakage and heat was causing the skin of the capacitor to shrink and pull back.
Lots of replaced parts from this one! Testing showed it was performing perfectly after the service. I also cleaned the balance pot as it was a bit scratchy.
This pre-amp will continue to serve faithfully with its companion power amplifier for a long time to come, and deliver the detailed, accurate sound HK is known for.
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!
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.
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.
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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