Posts Tagged ‘capacitor replacement’

Harman-Kardon Citation 17S Pre-Amplifier Refurbishing

January 2, 2015 Leave a comment

Cross-posted from the Rain City Audio Repair Blog:

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.

Rain City Audio Vintage Stereo Repair

Bose 901 Series 1 Active Equalizer #46453 Repair

December 22, 2014 Leave a comment

Cross-posted from the Rain City Audio Repair Blog:

Another Bose® 901 Series I Active Equalizer came through the ship, Serial #46453. It came in with a variety of issues, just not sounding like it used to, and the owner requested a set of new audiophile-grade film output capacitors as an upgrade as well.

Based on the different ages of capacitors, it looks like this one was serviced some time in the late ’70s or possibly early ’80s. Several of the black capacitors had failed and were causing quality issues.

This particular one received Vishay and Panasonic signal capacitors in the intermediate stages.

After repair, this equalizer sounds perfect and the upgraded capacitors really bring out a lot of clarity and detail in the mid-range and upper frequencies. Paired with the 901 speakers, it makes a fantastic, room-filling, life-like sound and will last for a long time.

Rain City Audio Bose® Equalizer Repair Service

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!

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


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