Wednesday, February 20, 2013
This gorgeous instrument would shut down unpredictably, or sometimes there would be excessive distortion on the outputs. My first suspicions were with broken solder joints, or perhaps failing electrolytic caps in the power section or on a circuit board. Sure enough, over time, I was able to detect low voltages from the power supply, as well as some AC on the DC rails. I replaced the filter capacitors, the rectifier, however the problem still persisted. This was an unregulated power supply,so there wasn't that much to replace or check.
I then disconnected, board by board, each of the various circuit boards from the power supply, to see which area was loading it down, but there seemed to be no particular issue with any of the different rails.
This was a real mystery, until I decided, finally, to check the resistance across the terminals of the power switch. Although the switch was working mechanically...opening and closing as it should, there was a 30 - 70 ohm resistance across the contacts of the switch even when the unit was turned on.
I was able to disassemble the switch, and spray it with some cleaner and used my new fiberglass cleaning brush to clean the contacts, and all was fine. It has not usually been my experience that AC power switches can fail in that manner, but after 30 years or so of service, I suppose anything can fail.