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I've just started taking a look at troubleshooting my Nephews 1996 Tweed Blues Deluxe. Works fine on the clean channel and also works fine when the drive is engaged unless I turn the drive up to about 8 - then it puts out a loud tone. Loud enough to even be heard with the master volume all the way down. Really loud if the master is up a bit. Playing with the tone controls will alter the pitch of the tone, especially the Mid control. Turn the drive down to about 7 and it's gone. Like flipping a switch. Haven't started tracing a test signal through the amp yet, or measuring all the test points but thought maybe someone has dealt with this before and can simply tell me where to focus. Maybe the filter cap feeding DC to V1? If so, why does it work fine at lower gains?
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I think that cap is a good first thing to try. Just tack another one in parallel.
In order to get a loop to oscillate two things must happen. One the total phase of the loop must to exactly 360 degrees and the gain must be >=1. So, changing the gain of any part of the loop changes the gain of the whole loop and that is what the gain matters.Experience is something you get, just after you really needed it. -
Thanks Nick for the quick reply. Just gave that a go and TaDA! it's gone! Put a new C46 in parallel with the existing one and that took care of it. All that's left now is to do the transplant surgery.
With only the bad one in there I also saw a jump in AC voltage across that cap when I turned up the drive. Very low until I got to about 8 then it jumped to 5 VAC
Still not sure I understand loops and oscillators, but my Nephew will be thrilled. Thanks againComment
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