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Posts Tagged ‘capacitor replacement’

Sony TA-5650 VFET Integrated Stereo Amplifier Overhaul

December 18, 2014 Leave a comment

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.

If you have a Sony VFET amplifier that needs a preventive overhaul to ensure it doesn’t meet an early demise, Rain City Audio can help!

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Bose® 901 Series IV Active Equalizer #116907 Overhaul

December 12, 2014 Leave a comment

Cross-posted from the Rain City Audio Repair Blog:

Another Bose® 901 Series IV Active Equalizer came through the shop recently. It’s owner shipped it in without the wooden cover for security. This is a bad idea on the Series I/II, but for the III and later which have a rigid metal shell it’s a fine way to ensure the wood case isn’t damaged.

This one was sounding muddy and distorted, and it was time for an overhaul to keep it sounding like new. It has plenty of original electrolytic capacitors, along with old style vintage op-amp chips.  The bridge rectifier is also made up of discrete diodes in this model; later used an integrated bridge package.

Interestingly, this is an earlier board revision with U101 in a different orientation than on later boards. It also shows evidence of having been replaced before – the stock filter capacitors were both 470 uF units; at some point in the past they’d been replaced with a 330 uF and a 1000 uF.

The op-amps had been replaced once before as well. This was a common factory repair on the earliest Series IV models, the chips occasionally suffered reliability problems. I doubt these were repaired by the factory, though, as Bose® factory service back in the day would never have used the wrong parts and damaged the PCB this much while soldering.

Pads were badly damaged, but I was able to repair them to make a good physical and electrical connection with the new chips.

With new electrolytic capacitors, new op-amp chips, and repaired traces on the bottom of the board this equalizer is back up to full performance and it should stay that way for a long time to come.

Rain City Audio Bose® Equalizer Repair Service

Bose® 901 Series I Active Equalizer #33133 Overhaul and Upgrade by Rain City Audio

December 8, 2014 Leave a comment

Cross-posted from the Rain City Audio Repair Blog:

Another Bose® 901 Series I Active Equalizer was through the shop lately, serial #33133. It came in working well enough on its original components for an overhaul and upgrade to ensure it lasts for a long time. It’s pretty unusual for these to still be running on original components so this was a bit of a surprise.

This one got all new parts. The electrolytic capacitors are all Nichicon Fine Gold audiophile electrolytic caps, with an even bigger upgrade on the output capacitors to high-end film capacitors with an ultra-low dissipation factor. It also got a new set of gold-plated RCA jacks to accommodate today’s thicker and more heavily shielded audio cables.

This equalizer is going to be a great focal point of a vintage stereo system for many years to come.

Rain City Audio Bose® Equalizer Repair Service

Bose 901 Series IV #244005 Repair by Rain City Audio

December 4, 2014 Leave a comment

Cross-posted from the Rain City Audio Repair Blog:

Bose® 901 Series IV equalizers are coming through the shop more regularly in the past year. It’s expected as these are definitely getting up there in years. This one was sounding muddy and not like it used to, so it came in for a full overhaul.

Manufacturing technology has reached the point by the Series IV that most of the film capacitors are reliable and don’t need to be replaced, but there are plenty of electrolytic capacitors, plus the original op-amps get noisy over the years and should be replaced as a matter of course.

This one received all new Nichicon Fine Gold electrolytic capacitors, and new TI TL072P op-amps.

If your Bose® Series IV Active Equalizer isn’t performing like it used to, Rain City Audio can help. Bose® Equalizer Repair Service

EICO Stereophonic Integrated Amplifier HF-81 Repair

November 7, 2014 1 comment

From the late ’50s, I had this nice example of an EICO HF-81 amplifier come through my shop. These are nice little stereo amplifiers with a built-in phono pre-amp and RIAA equalization and feature about 14W of power output per channel from a pair of push-pull EL84s each at about 1% distortion.

EICO was a popular brand of electronics kit maker back in the day. They made a lot of hobbyist or entry level shop test equipment (oscilloscopes, generators and testers) but had a handful of hi-fi stereo kits as hi-fi listening grew more and more popular. The fact they were assembled as kits makes them pretty straightforward to service, too – the layout is clean and there’s a lot of room on the chassis. It had been worked on a few times in the past – several coupling capacitors were replaced with early film caps in what looks to be the late ’60s/early ’70s. Just recently, the owner reported it was giving noise from one channel and took it to another local shop where the technician replaced a 50 uF capacitor (orange, bottom left) with a 100 uF cap that looks to me like it was probably an old-stock capacitor (blue, bottom left.) They’d clipped the old cap out of circuit but left its shell intact; not exactly what I’d call a best practice. More problems started cropping up shortly thereafter as the rest of the original capacitors degraded, so it came to me for an overhaul. After replacing the parts, there was still noise,  distortion, and lower volume on one channel seen on the oscilloscope screen in yellow. Tracing through the circuit, it turns out the CRL couplets in the tone network were failing.

I fabricated new PCBs with the same values to replace the damaged parts and mounted them to the shield.

That fixed the problem right up! After some final tweaks and a burn-in test, she’s ready to go home. Lots of parts out of this one:

It’s going to sound great for years to come!

Rain City Audio Tube Audio Repair

Phase Linear 2000 Pre-Amp Overhaul

October 6, 2014 Leave a comment

Cross-posted from Rain City Audio:

I had a Phase Linear 2000 through my shop lately. It was running okay, but had some pops and clicks and the controls weren’t all that smooth. Time for an overhaul before anything worse cropped up!

This is a pretty fancy pre-amp made to be used with the Phase Linear 4000 power amplifier. It has a variable level output to be paired with a fixed input amplifier.

After fixing it up, I found this one to have a neutral, tight, almost laid-back sound without any extra embellishment. It was very nice – and part of that is owing to the very simple design.

The underside of the single-sided board Quality construction for the era. The bank of switches is an open design which gets gummed up easily but control cleaner easily solved that problem.

Of course, a pre-amp like this deserves nothing but the best: Nichicon Fine Gold series electrolytic capacitors.

The matching power amplifier will come up for service soon, so stay tuned! This Phase Linear is good as new and another piece of gear from the height of hi-fi is saved.

Rain City Audio Stereo Repair

 

Bose 901 Series II Active Equalizer #67402 Repair

August 25, 2014 Leave a comment

This equalizer got a full overhaul with new 1% precision resistors, electrolytic capacitors, audiophile-grade output capacitors, transistors, gold-plated RCA jacks, 4% silver solder for all connections, a thorough switch cleaning, and computerized frequency response testing and a listening burn-in to verify proper operation. It sounds just fantastic with a very dynamic and life-like presence that really draws you into the experience of the source material, just like the Bose 901 should.

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Check it out at Rain City Audio

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