Hi there:
A friend asked me to change out a pot on his Fender M-80. I thought I was being careful but—since replacing the Contour pot, I've got a hum in the preamp, with no signal passing to the power section.
Using a meter on the power supply, the node for the power amp section reads about spec—+48 DC on one side; -48 on the other. The preamp node, however, reads spec -16 but only +3. The feed is from 2 330R resistors and 16v Zeners. I replaced the + side's Zener and am getting the same issue.
1) If I plug into the "power amp in", the power sections functions
2) If I tap the output of the clean channel at the at the output of the second stage (the B side of a TL072), I get a a decent signal.
3) The overdrive channel seems dead.
Question: does this look like a power supply issue? or could it be a bad op amp causing the problem?
I've attached a screen shot of the schematic (Fender was kind enough to send a PDF, but it's a scan of a photocopy).
Thanks in advance!
Emile
A friend asked me to change out a pot on his Fender M-80. I thought I was being careful but—since replacing the Contour pot, I've got a hum in the preamp, with no signal passing to the power section.
Using a meter on the power supply, the node for the power amp section reads about spec—+48 DC on one side; -48 on the other. The preamp node, however, reads spec -16 but only +3. The feed is from 2 330R resistors and 16v Zeners. I replaced the + side's Zener and am getting the same issue.
1) If I plug into the "power amp in", the power sections functions
2) If I tap the output of the clean channel at the at the output of the second stage (the B side of a TL072), I get a a decent signal.
3) The overdrive channel seems dead.
Question: does this look like a power supply issue? or could it be a bad op amp causing the problem?
I've attached a screen shot of the schematic (Fender was kind enough to send a PDF, but it's a scan of a photocopy).
Thanks in advance!
Emile
Attachments
The 330r 5W resistors run HOT. Either they fail, or they burn the solder off their legs, or they melt adjacent parts like the 16V Zener....
*Did it work before you touched it??* The contour pot is not involved in power troubles. Never trust the customer to know what needs fixing.
*Did it work before you touched it??* The contour pot is not involved in power troubles. Never trust the customer to know what needs fixing.
You "fixed" something, and now the amp has a new problem not related to the old problem. It is a good bet that you knocked something astray in the process. It might be an actual part, or it might be you knocked a wire loose nearby. You may have left a bead of solder touching two adjacent traces. And so on.
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