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FIXED-----BC30 lost channel 2

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I havent played the bc30 in awhile so I decided to give it a go for a gig this weekend.


Clean channel worked fine, but when I switched to OD channel the volume dropped almost completely out, even though the red LED came on. All the valves light up.


Anyone have any ideas on what I might be probing with the multimeter?

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I switched the preamp tubes in V1 and V2 sockets, but the problem still persisted on the OD channel.


I'm going to check the cathode resistors R15 and R21 and cathode bypass cap C15 for loose connection or open.


It would be really cool if someone could provide some feedback...

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All the resistors checked out. I also checked for continuity between tube socket and appropriate board components. I cant check the caps without removing the board, which I'd rather not do. I will do it though if someone can convince me that it is necessary.


Also, I dont understand the channel switching part of the circuit. I'm at a loss for what to do next.


I'm going to reconnect the amp and rotate the drive and level knobs (VR2 and VR4) to make sure they function. Maybe that will give me some insight.


Any help or thoughts would be appreciated.

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It appears that I have fixed it accidentally...


I suspected that the signal was not making into the first preamp valve because pin 7 of V1 was always near ground potential no matter how the drive and level controls were set. The drive control (VR2) is a variable grid leak resistor that adjusts the gain into V1b, and it never could develop any resistance to ground.


I hooked the multimeter from pin 7 to ground using alligator clips and turned the amp on. To my surprise the OD channel was working. As soon as I disconnected the ground lead the volume immediately dropped out.


Next I clipped around both legs of C6 to bypass the cap. Still nothing. At that point I got confused and decided to clip farther back, on the leg of C2. Then I accidentally touched the other lead to the chassis and like magic it started working. I took all the leads off, and it kept working. Turned the knobs, switched channels, put it on standby, power cycled it, and its still working.


I'm pretty confused about what happened here, but my theory is that the coupling cap C2 had developed a short that was affecting the signal, but by providing a direct path to ground the cap heated up rapidly and the thermal energy was enough to clear the short.


Anyway I hope it keeps going. I did the Master Volume mod several months ago and I have been very pleased with the amp ever since.



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