Trouble finding diode in Fender "The Twin"

[glebert]

Do you have 73V at C304? Is your R306 1.2K? Seems like D108 is the highest number diode on that board which means it was added last. Rev M or N is pretty high, so there were some others that preceded them (though perhaps some revs never had actual boards made). Since this board doesn't seem to have that part perhaps they were using a different mechanism to get to a voltage the LM339 could take in a previous version, but then found they had to add D108.

[g1]

I also don't understand the purpose of D109 connecting to pins 9 & 11 of the LM339.

Is this a typo? Which diode?
As implied by JM Fahey, the switching circuit runs on multiple DC voltages, that's how they can get away with a mono cable running a 2-button footswitch. What you can't overlook is the 30VAC coming into the footswitch jack via R132. This is integral to the circuit whether the footswitch is in use or not, via the switch contact of the footswitch jack. D201 rectifies that 30VAC.

[rascalrick]

Yes, a typo...should have read D108. Sorry for the confusion! I am currently away for a few days, but should be back on Tuesday to start in
working on the amp again.

[g1]

D108 does not connect to pins 9 & 11 of LM339, it is the zener you were looking for that goes from LM339 supply pin to ground.
Maybe you are wondering about D102 or D106? Those connect to LM339 pins 9 & 11.

[rascalrick]

Once again, I apologize. As I said I am not at home right now and don't have the paperwork or amp
in front of me. I just looked up the schematic online and I realize I meant D106, not D109 or D108.
So much for my memory these days! I got distracted by a major plumbing project at a relatives house
these past few days.

[g1]

D106 has to do with the reverb switching side of the LM339 (pins 8,9,13). So not part of your problem of ch.1 dead.
In the upper left of schematic is R1. When nothing is plugged into ch.1 inputs, DC will get routed through R1 and the ch.1 jack switches all the way to D106. Something to do with indicating parallel input mode perhaps. When either ch.1 jack is opened, that DC path to D106 is disconnected.

For loss of ch.1, it is not necessarily a switching issue, could be something else in ch.1 signal path. Unless you have verified ch.1 all working when in parallel mode.
For ch.1 operation, check if you have output at the pre-out jack. The only thing from the switching circuit that could prevent that is LDR1. Are voltages at IC101 pin 1 correct? If not, post DC voltages at pins 4,5, and 1 for ch.1 and ch.2 selections.

As far as the missing 30V zener, I know for myself, I could possibly remove it from being tacked across IC101 (during IC replacement) and forget about it.

[rascalrick]

Thank you for your suggestions. Actually channel 1 works just fine in parallel mode, but not when in switching or dual mode. I never get channel 1 indicator LED except when in parallel mode. I will hopefully get time tomorrow to work on this amp again. SO many projects
around my property this time of year!

[rascalrick]

I have the amp working correctly once again. Final analysis, here is what I found and replaced:

* R128 & R129 were both disintegrating so I replaced them.
* Replaced C110, C111, & C112. C110 had puked it's electolyte and the + lead was hanging loose.
* Replaced D104 & D107 as both were shorted, making the reference voltage at the LM339 wrong.
* Replaced LDR3 which seemed to work removed from board, but ended up with a shorted LED killing
the channel switching function.
* Added missing 30v zener D108 to stabilize "D" supply voltage.

In the end I didn't need to replace the LM339, but decided to leave the new one in. All input jacks
are operating correctly. The bias section was burned up at one time and someone "repaired" the
PCB, but did a pretty sloppy job. I cleaned up their work and got the tubes properly biased once
again. I did finally get my head around how the pedal circuitry works and I agree it is quite ingenious.
This is a great sounding amp, and I think the owner is going to be very happy to get it back in working order.

Thank you all for your insight. Sometimes you just need to bounce things off of other minds to help
look at things from a different perspective!

[g1]

Glad you were able to get it sorted out.

The bias section was burned up at one time and someone "repaired" the
PCB, but did a pretty sloppy job. I cleaned up their work and got the tubes properly biased once
again.

If it's a very old unit it will not have the protection diodes across R215 and R216 (D202 and D203). They were added because of the way that board kept burning up, and you should tack them across the resistors if they are not present.
These amps were biased quite cold from the factory, the 40mV setting shown on schematic works out to 20mA per tube. That's 30% dissipation and there is no objectionable crossover distortion, which tells you all you need to know about the mythical '70%' target.

[rascalrick]

Thank you. The customer is going out of state for a few days, and can't wait to get back and
play this amp once again.