Dynaco Stereo 120 Solid State Amplifier Repair Journal
by Chad Hauris, Retro Electronics and Audio Lab, Midland, TX. 9/12/07

This Dynaco dates from 1968 and was built as a kit. It had several problems and was a fairly challenging repair job. First of all, we could see that the speaker output capacitors were bulging at the top and appeared bad:
Old capacitors in Dynaco solid state amp

New capacitors in Dynaco 120 power supply.
To begin the repair, we replaced all of the capacitors on the power supply module. Some of the old can-type electrolytics were not available, so modern capacitors were mounted to terminal strips.
We found that the power supply regulation was not working properly...the voltage would increase from the normal value of 72 volts up to 90 volts. Using freeze spray, we found that a diode was thermally sensitive. Initially we thought it was the rectifier diodes, so a new bridge rectifier was installed. This did not cure the problem, and it was eventually found that a zener diode was bad in the power supply. As I remember, this was a 58 volt zener. We were not able to find this exact value today, but were able to order two diodes from the Mouser catalog, that put in series, came to exactly 58. (Zener diodes can be added in series to achieve the sum of their voltage values).

We then replaced the speaker output capacitors and tested the amp...One of the output modules, when connected, would cause the power supply to shut down. The other had thin, tinny sound.
Dynaco 120 circuit board repairs.
We replaced bad old capacitors on the circuit board, and found some more bad zener diodes (5.1 volt).

Testing the Dynaco amp.
Testing the Dynaco amp. We left the old speaker output capacitors physically in place to preserve the speaker choke coils wound around them. We also had to repair the power switch circuit as one side of the switch had gone open.

Testing finished Dynaco Amp.
Here is the completed amp, with all of the new capacitors installed.

Dynaco tuner and preamp.
Here is the matching tuner and preamp. We cleaned controls on both of them...checked capacitors, and all looked OK.

Inside the Dynaco tuner.
Inside the Dynaco tuner.

Stereo Equipment Page

Retro Electronics Home Page