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Hello all,
Working on this amp that at 1st was completely unaffected when turning tone and master volume controls. Could reach full volume, but only controlled by the pres in both channels. I first thought perhaps the eq/post controls had lost a common shared ground. That checked out okay though.
What baffled me, is when I inserted signal into effects loop, I got no signal at all. Nor did the jack inserted into the FX loop break the signal coming from the input of amp. Looking at schematic, it doesn't seem possible. I then thought maybe the effects loop buffer/return tube could be funky. Sure enough, the tubes top was coated with the white powder of death.
After replacing that 12AX7, I have eq back, (I think) and some control of the post controls, but very odd behavior. Basically, POST volume signal is on 10 when the knob pointer is on "0", and only drops some when knob is at 11 o'clock. Again, this is on both channels. Reverb, Resonance, and Presence controls are okay.
Really trying to avoid taking the laced circuit boards out, so I did what measuring I could on v3 (4th tube over from right). I confirmed that the follower resistor was indeed 100kish by measuring from pin 8 to ground (from female side of socket), and measured 68K from pins 1 to 6 for other triode's plate resistor. So I think the loop follower/return section is okay.
I'm stumped. For the life of me, I can't figure out how signal is making it to the output. I'm getting the specified 50 watts into 16 ohms. Also, quite a bit of signal is getting through to output when I insert a dummy jack into the effects return jack. The schematic tells me this can't happen. So, is there something leaking?
Help is greatly appreciated.
Stan
Working on this amp that at 1st was completely unaffected when turning tone and master volume controls. Could reach full volume, but only controlled by the pres in both channels. I first thought perhaps the eq/post controls had lost a common shared ground. That checked out okay though.
What baffled me, is when I inserted signal into effects loop, I got no signal at all. Nor did the jack inserted into the FX loop break the signal coming from the input of amp. Looking at schematic, it doesn't seem possible. I then thought maybe the effects loop buffer/return tube could be funky. Sure enough, the tubes top was coated with the white powder of death.
After replacing that 12AX7, I have eq back, (I think) and some control of the post controls, but very odd behavior. Basically, POST volume signal is on 10 when the knob pointer is on "0", and only drops some when knob is at 11 o'clock. Again, this is on both channels. Reverb, Resonance, and Presence controls are okay.
Really trying to avoid taking the laced circuit boards out, so I did what measuring I could on v3 (4th tube over from right). I confirmed that the follower resistor was indeed 100kish by measuring from pin 8 to ground (from female side of socket), and measured 68K from pins 1 to 6 for other triode's plate resistor. So I think the loop follower/return section is okay.
I'm stumped. For the life of me, I can't figure out how signal is making it to the output. I'm getting the specified 50 watts into 16 ohms. Also, quite a bit of signal is getting through to output when I insert a dummy jack into the effects return jack. The schematic tells me this can't happen. So, is there something leaking?
Help is greatly appreciated.
Stan
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