A Dynaco Stereo 80 Upgrade

A few months ago, I refurbished a Dynaco Stereo 120 audio amplifier for my studio, replacing a Stereo 80 amp that had been in service for years. There wasn’t anything wrong with the 80, but I wanted the increased power and the sound improvements that the 120 upgrades would provide in the studio. So, the Stereo 80 went out to the shop to power a pair of Polk Audio T15 bookshelf speakers. So great to have a quality source of music in the shop!

A Fly in the Ointment —

Lately however, I noticed some distortion creeping into the right channel of the setup. I little testing revealed that one of the Dynaco channels was failing. At first, I thought it was probably the output capacitors, as they are likely to fail after this many years of service, but a quick test showed that although not up to spec, they were not the source of the audible distortion. More detailed troubleshooting was required.

Since the Dynaco amps were originally sold as kits, there is an excellent troubleshooting guide in the assembly manual, and I checked for each one of the potential problems. Supply voltages were close to correct, and voltage measurements  comparing the good channel to the failing channel did not reveal any obvious problems. It was time to remove the driver board and begin individual component testing. With a dozen external connections to each driver board, I took a photograph so I could remember the wire placement when the board was re-installed.

Round up the usual suspects —

The trouble shooting guide suggested that a transistor fault could cause problems. All the transistors seemed to be good, and a careful test of other easily-tested parts like resistors and diodes did not reveal the problem. That left the four electrolytic capacitors on the board. Output capacitors were known to fail in these amps after decades of use, why not the interstage caps as well? I removed and tested them. Each was out of spec, with the two 33 uF bipolar caps being nearly open. I was hopeful this was the problem, but my junk box did not contain any of the required values. Time to buy parts!

They don’t make those anymore  —

Sourcing parts turned out to be more difficult than I expected. 30 years ago, parts were easy to find and probably available from a local vendor. Not so today. Digikey and Mouser are good sources, but they stock mainly radial-lead caps (both wires come out of one end of the part) instead of the axial-leaded parts I needed. Worse than that, 33 uF bipolar caps seemed to be pure unobtainium. Finally, just to be thorough, I checked Amazon. To my surprise, they had the bipolar caps. They weren’t on Prime shipping,  so I would have to wait a few days (!), but that would have to be good enough. Axial lead caps were also hard to find or very expensive, but I found a radial electrolytic capacitor assortment that had all the required values (and hundreds more!) for under $20. Since I was re-capping the driver boards anyway, it was time to upgrade the power supply and output caps like I had done in the Dynaco 120. All were ordered in a few minutes.

Things started arriving in a couple of days, and I installed the UpgradeMyDynaco.com kit first. Like the Dynaco Stereo 120 upgrade kit, the instructions were excellent and I had the whole job done in a couple of hours. The capacitor assortment had arrived as well, so I installed a 50 uF and 500 uF values needed for the repair. I just needed the bipolar caps now.

I love it when a plan comes together!

They arrived the following week and I was back in the shop first chance I got. It took just a few minutes to install the two caps and replace the driver board into the chassis, and now it was time for testing. Ohmmeter tests did not reveal any problems, so I plugged the amp into my current-limiter/Variac power supply and applied power. The light bulb glowed pretty brightly for a second or two (gasp!) but then nearly extinguished. That must have been the giant power supply caps charging up for the first time. With the speakers connected and audio supplied by the headphone output of an iPhone, there was beautiful music. The distortion problem was gone! Voltage checks showed much better power supply regulation (once I disconnected the current limiter) and the repaired channel sounded brilliant. It sparkled — just like I remember it sounding years before.

Lessons learned.

As  final step in the repair, I removed the other driver board and replaced the same caps that had failed in the other channel. It didn’t take long, and now the amp was fully re-capped. Another substantial listening test showed the two channels were equal, and the amp sounded really good. This drives a couple of lessons home: decades-old electrolytic capacitors in vintage gear are almost certain to be failing and should be replaced before use. And, their failure is a very slow process, making it hard to notice the subtle degradation until something fails more catastrophically. Fortunately, there was no other damage from the failed caps, but another time it might be a different story. Given the difficulty of finding replacement parts, better to replace too soon than wait for an obvious failure.  Now, if you’ll excuse me, I have some to music to listen to!