customer amp. he said it works for a while then the volume drops to half. I found a very bad bent terminal on the speaker push on terminal from the amp to the first speaker. that would certainly cause issues. However I am unable to find any other issues. the bias was set kind of high when I checked it initially, like 85 or 90 mA vs the Carvin recommendation of 75mA across the stand by switch. I have it a tad low now, around 65-70mA, but it sounds good and the waveform looks good, sort of.
I hooked up my dummy load and a sig gen and sat the watching for something to happen. I ran it from 5W up to clipping at 34 watts for extended periods of time, with no troubles. Except for a brief period where I saw a little glitch develop right at the peak of the signal waveform. It got a bit bigger and then turned into a ringing event starting right at the peak of the sine wave. I could make it better or worse by adjusting the level and the tone controls a bit.
but then I touched the mid or bass control and it suddenly dissapeared. I tested for another 30 minutes and couldn't get it back. Here is a short video at the output across the dummy load. I could get it worse than this, but this was pretty much what was happening. I couldn't even get the notch to come back.
I suspect the tone controls or a solder junction near them. Especially one with high voltage across it, since it was happening at the peak of the waveform. Here is the video.
Oscillating Carvin Belair 212 - YouTube
I have attached the schematic.
I will check the solder around the tone pots.
I just hate to give an amp back to a customer, just to have them bring it back, or worse NOT bring it back, when it isn't fixed.
I hooked up my dummy load and a sig gen and sat the watching for something to happen. I ran it from 5W up to clipping at 34 watts for extended periods of time, with no troubles. Except for a brief period where I saw a little glitch develop right at the peak of the signal waveform. It got a bit bigger and then turned into a ringing event starting right at the peak of the sine wave. I could make it better or worse by adjusting the level and the tone controls a bit.
but then I touched the mid or bass control and it suddenly dissapeared. I tested for another 30 minutes and couldn't get it back. Here is a short video at the output across the dummy load. I could get it worse than this, but this was pretty much what was happening. I couldn't even get the notch to come back.
I suspect the tone controls or a solder junction near them. Especially one with high voltage across it, since it was happening at the peak of the waveform. Here is the video.
Oscillating Carvin Belair 212 - YouTube
I have attached the schematic.
I will check the solder around the tone pots.
I just hate to give an amp back to a customer, just to have them bring it back, or worse NOT bring it back, when it isn't fixed.
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