Reviving a Dynaco Stereo 120 for the Studio

I am listening to a classic solid-state amp from the 70’s. Just a couple of weeks ago, it arrived from eBay in rough and partially-working condition. I intended to bring it back to working and reasonably-clean condition. The last two weeks reminded me of my teenage lust for the wunderkind of the era, direct-coupled solid state amps, and I was not disappointed.

Back in the Day . . .

I should explain. At the distance of some 40 years, my interest is supremely ironic. You see, I cut my electronic teeth on tube equipment. It’s all I could afford. In fact, while still in High School, I built — from scratch — a complete vacuum-tube-based stereo amplifier. It had high-quality output transformers and two 6L6’s in a push-pull configuration – Just like the tube amps that are considered desirable and exotic today.

But it’s not what I wanted. Like all my friends at the time, I wanted a killer stereo. Transistors were the future and I desperately wanted a solid-state (aka audiophile) amp. I became aware of such things through magazines like Popular Electronics and they regularly featured projects that were available as kits from Southwest Technical Products Corporation (SWTPC). They were most famous for their early microcomputer products, but also had quite a few high-quality audio projects. But $115 for a two-channel, 180-watt amplifier  was just too expensive for my meager resources. So I built a tube amp instead.

Years later, while managing a recording studio, I learned of another audio kit manufacturer, Dynaco. A few of their amps were in service around the studio, and knowledgeable audio friends thought they were excellent. Later, I bought a Dynaco Stereo 80 on eBay and used it to power the rear channels in my home studio. I was especially glad to have it when the front channel amp failed earlier this year and I could substitute it to power the front channels.

A few studio upgrades . . .

Fast-forward to today, and the repair of my JBL 4408 studio monitors . Given the current interest in all things tube, I thought about finding a high-quality tube amp, but I wanted it be as trouble-free as possible. Studio amps tend to sit in an equipment rack or closet and run for years with no attention. Perhaps a solid-state amp was more practical. An eBay search led me to a second, larger, solid-state Dynaco. The Stereo 120.

The eBay vendor had taken care with the packing, but it was clear this amp had seen better days. A greasy, sticky film covered everything and patches of rust had formed on both the chromed chassis and perforated metal top. A look inside revealed a less-than-careful build with loose circuit boards indicating an abortive attempt at repair. Making sure there were no obvious problems, I slowly powered it through an isolation transformer and variac and it came back to life. Well, partially. There was DC power, so thankfully the power transformer wasn’t fried, and one channel seemed to work. Just as the vendor had promised.

There are several large electrolytic capacitors in the Stereo 120, and I checked them with an ESR (equivalent series resistance) meter. One of the output caps was open, and likely the reason for the dead channel. Now the hard part: where to find replacements. This style of “computer grade” capacitors hasn’t been made in decades, and most components have gotten much smaller since the late 60’s. It wouldn’t be a problem to find workable parts, but the mounting strategies would have to change due to the smaller size of the new components.

Let’s clean this mess up.

Fortunately, information, including a complete assembly manual, was readily available on the Internet. In fact, there was a surprising amount of interest in these old amps, including a vendor, updatemydynaco.com who specialize in parts, repairs and even upgrades for them. They also provide schematics, a manual and troubleshooting guide. A quick check revealed a replacement output capacitor kit as well as new larger caps for the power supply. Both promised to substantially improve the sound of the amp. I ordered both kits.

Meanwhile, the amp needed a thorough cleaning. You could practically smell the french fries from its former life in a restaurant or bar. It wasn’t hard to remove everything from the chassis, and after a quick scrub with Simple Green, it and the metal top both went into a tank of Evapo-Rust. That stuff is magic. A few hours later, most of the rust had dissolved. I scraped the loose paint off the top and then sanded it with 220 and 400 grit. A few coats of satin-finish black paint and it would be good as new. Don’t forget to mask the Dynaco logo attached to the cover!

Although most of the rust was removed, the chrome chassis still looked pretty dingy. A couple of applications of Mother’s Chrome Polish  worked wonders. As a final step, I polished all the chrome with white jeweler’s polishing compound on a drill-mounted buffing wheel. Not perfect, but the best it would be without re-chroming. Time for assembly.

and add a few new parts.

First in, the power supply. Transformer, switch and fuse holder are easy, but according to the updatemydynaco.com assembly instructions, a modification was also needed to the power supply board.

This means detaching it from the heatsink, removing a small electrolytic and replacing it with a new part. It was tricky to get everything reattached, but otherwise went well. Three new caps replace the old one, and triple the filter capacity. Best of all, they fit into the existing mounting ring. It’s important to use thermal grease when attaching the “L” shaped heatsink to the chassis. These amps get warm, and every thermal advantage is needed.

The two new output capacitors use a similar mounting strategy — fitting three, smaller capacitors into the mounting ring from the old cap — but it’s a tighter fit and requires a little bending of the mounting ring. Like the filter cap, the output caps end up having three times the capacity. Unlike the filter cap, the output capacitors are wound with an inductor. A portion of the old wire is used and is wired the same way as the old inductor. The excellent assembly instructions from updatemydynaco.com are easy to follow and show these details well.

Last of all, the two output channels are mounted after an application of thermal grease like the power supply board. Two wires are reconnected to each of the new output capacitors and two power connections are replaced on each board — just as they were before modification. The remaining connections to speaker outputs and channels inputs are completed and it’s time for testing. I applied power through the isolation transformer and variac, and measured supply voltages in several places. All was well.

So how did we do?

A couple of interesting items surfaced as I researched the amp. They tend to run a little hot, and the biasing on the output stage changes as they warm. When cold, they may suffer from slightly increased distortion at low levels. As the amp warms and the biasing changes, the low-level distortion is reduced and the amp sweetens with more 2nd than third-order distortion — resulting in a warmer sound. Judging by the apparent longevity of this particular amp, and since I didn’t change anything that would effect output stage biasing, I think we’re good to go. The refurbished Dynaco Stereo 120 sounded good at power-up, and did improve slightly as it came up to temperature. I’m going to call this a win!