artto Posted November 16, 2014 Share Posted November 16, 2014 I have a 1968 Standel musical instrument amplifier. It is solid state. I haven't turned it on in a few years. It worked fine the last time I did. Today when I turned it on it made a somewhat loud buzzing noise (60Hz or 60Hz harmonic?). It sounds like when the cord from the guitar jack or cord at the amplifier input is not plugged in all the way or the cord is bad. I tried different cords and guitars but always the same result. I tried bringing up the voltage slowly with a variac. At about 50% the amp starts making a buzzing sound - like a bad ground. As the voltage approaches 100% it gets much worse, like a really bad grounding problem. Reversing polarity doesn't make any difference either. Any ideas what's wrong? Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted November 16, 2014 Share Posted November 16, 2014 A coupling cap gone bad? Quote Link to comment Share on other sites More sharing options...
artto Posted November 16, 2014 Author Share Posted November 16, 2014 A coupling cap gone bad? Maybe. At first I was thinking caps too. I can't see anything visibly wrong with any of the caps. In fact they all look pretty good considering the amp is 46 years old. I can play a guitar through it at 50% voltage on the variac but is sounds like a lot of harmonic distortion with some vibrato or oscillation. When the voltage is turned up to 100% there isn't much more volume but the buzz gets considerably louder. After thinking about for a while, since it sounds just like a grounding problem, especially the kind experienced with a bad guitar cord or phone/RCA connector I'm going to pursue that after trying to reform the caps. Also, the power cord doesn't look too good where it connects to the amp. I'm going to replace it. Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted November 16, 2014 Share Posted November 16, 2014 Leave in on for a few hours. It might revive itself while you mess around with another project. Quote Link to comment Share on other sites More sharing options...
WMcD Posted November 16, 2014 Share Posted November 16, 2014 (edited) I would say the power supply filter cap(s) are the issue. When you turn up the Variac to a certain level the dielectric breaks down in the caps. Maybe if you leave the Variac somewhat below when the buzz starts, the film will reform. Then try stepping it up little by little. But I would replace the supply cap(s). WMcD I had to edit some bad grammar. Edited November 18, 2014 by William F. Gil McDermott Quote Link to comment Share on other sites More sharing options...
mikebse2a3 Posted November 17, 2014 Share Posted November 17, 2014 But I would replace the supply cap(s). +1 At 46 years old if I wanted to keep this amplifier going and sounding it's best I would replace all the electrolytic caps. miketn 1 Quote Link to comment Share on other sites More sharing options...
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