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:
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.
We replaced bad old capacitors on the circuit board, and found some more bad zener diodes (5.1 volt).
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.
Here is the completed amp, with all of the new capacitors installed.
Here is the matching tuner and preamp. We cleaned controls on both of them...checked capacitors, and all looked OK.
Inside the Dynaco tuner.
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