Bose® 901 Series VI Active Equalizer #199732 Repaired

Cross-posted from the Rain City Audio Repair Blog:

A first for my shop, I recently worked on a Bose® 901 Series VI Active Equalizer! These are considerably newer than the Series I-IV that typically come through the shop, although they’re still getting to be 25+ years old at this point and that’s a long time for any piece of electronics gear, so I’d expect to start seeing more of these come through in the future.

It’s a wide, hefty black anodized aluminum case with mid-bass and mid-treble faders, a bass countour switch, LED power indicator, and tape monitor selector. Unfortunately interior shots aren’t available due to a camera mishap, but the design is fairly similar to the Series IV, except using slightly newer op-amp chips. Where the Series IV has four dual op-amp chips, the Series VI adds another active stage and uses a set of quad op-amps in addition to the duals, for a total of 6 op-amps in the circuit. It also features TO-92 package voltage regulators for the positive and negative rails, instead of just an unregulated power supply as earlier versions did.

As is standard practice, noisy old op-amps came out along with failing capacitors which were causing an intermittent signal in one channel. In went brand new Texas Instruments low-noise op-amps to replace, which will get this equalizer going as good as new.

After verification and burn-in testing, it’s ready to go home! With new chips and new Nichicon Fine Gold audio capacitors, this Active Equalizer should sound fantastic for a long time to come.

If your Active Equalizer isn’t working like it should, Rain City Audio can help!

Bose® 901 Series I Active Equalizer #55382 Refurbished [Repair Blog]

Cross-posted from the Rain City Audio Repair Blog:

Another Bose® 901 Series I Active Equalizer came through the shop recently for a full rebuild. These are all nearly 50 years old and are definitely showing their age, and this one was no exception – it came in with no output on one channel and definitely needed a full rebuild to get it going again. The Active Equalizer is responsible for shaping the audio signal to match the Direct/Reflecting driver’s frequency response, and without it, you’ll never get the good sound the 901 series is known for.

This one was pretty dirty, but nothing some rubbing alcohol and elbow grease couldn’t majorly improve. Inside, most of the electrolytic capacitors had been replaced at one point in the late 1970s, but were still 30-40 years old and well beyond their design lifetime. The rest of the components were all original, including the unreliable small signal capacitors in the center.

These original 2N5088s have color bands for their gain group on the back – all but one were identical. It looks like someone at the Bose® factory grabbed a wrong one during the original construction! Such a minor difference wouldn’t be noticeable to your ears, but might be able to be seen with sensitive test equipment, so it’s not a big deal. The transistors on this one came out and were replaced, since after the initial component replacement it still wasn’t quite right. One channel sounded great, but one channel had lower volume and suffered recessed mids which gave it an odd sound. Time for some troubleshooting!

Using my oscilloscope, I compared channels to see where the signal got lost, using the built-in math functionality to show the difference between the two channels. Ideally this difference should equal zero, a flat line, with both channels being identical.

After further probing and testing at different frequencies, the difference and phase shift were frequency dependent. That pointed me towards the crossover and feedback circuitry, where it turned out that channel’s 22 mH inductor had gone open and was no longer giving the right curve shapes. Replacing both inductors fixed it right up!

Problem solved! With that fixed up, I installed a new neon power lamp and boxed it up to go home, good as new.

Fully refurbished, this one is going to serve well for a long time to come!

If your Bose® Active Equalizer isn’t sounding like it should anymore, Rain City Audio can help.

Harman-Kardon Citation 17S Pre-Amplifier Refurbishing

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

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

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

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

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