Trouble finding diode in Fender "The Twin"

[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.

[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]

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!

[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!

[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]

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]

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]

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]

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.

[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.

[rascalrick]

Sending the amp to a proper tech isn't my call, the owner is trying to get it done on a budget. I am not unskilled and have been
working with electronics my entire life, so please don't assume I don't know what I am doing. I have repaired amps for many
respected musicians in my area, and built several as well. This amp just is giving me more trouble than usual.

[J M Fahey]

Thisis acually a BRILLIANT design.

Any 4 button pedal needs *five* wires at least: ground + 4 "control" ones so a special multi wire cable, plus a 5 contact connector, that´s why 5 pin DIN sockets are popular.

OR you need a sophisticated microprocessor based system at each end, to code>send>receive>decode control commands.

GENIUS Fender achieves that using single conductor plain Guitar cable, available anywhere, what´s not to like?

Of course some complexity is needed, but it´s nothing compared to other options.

To boot, shielded Guitar cables are used, simple "because they are there" but I am quite certain plain unshoelded twisted wore or even zip cord will work just as well.

NO audio signal there, and voltage level is low impedance *30VAC*

Pure GENIUS.

PS: PLEASE don´t disassemble the whole amp just to test all parts "outside" one by one, this only leads to amp destruction (PCB destruction counts as such).

That is not a valid repair system, problem may very well NOT be a "bad part" at all and it´s an admission of "I have no clue" or "I have nothing to do here".

Send it to a proper Tech.

Whatever he charges will be way less than the replacement cost of a "The Twin" amp, even if second hand.

[rascalrick]

I totally agree about how convoluted their switching is in this model. I have done some measuring of the input jacks, and may
actually end up pulling them from the board and testing them out of the amp. This amp is definitely testing me!

[g1]

It's probably the most convoluted switching scheme Fender has ever come up with.
If it's not your amp, I would suggest first going through the owners manual to understand the normal/parallel setup of the channels. From there you will have to check out all the switch segments of the jacks not in use, as they are part of the switching scheme and those 9pin Fender jacks are common sources of switching issues.

[rascalrick]

I am having trouble understanding how channel switching is supposed to work when plugged into only
channel one. I can't see the path to ever turn off LDR2 & 3 which I presume is needed to kill the signal
going through channel 2. I can see how pulling the volume switch on channel 2 or the foot pedal causing
pins 1 & 2 on the LM339 to connect to ground would turn on LDR1 effectively killing the signal passing
through channel 1. I also don't see the path to kill the channel 1 LED either, when switching to channel 2.
I also don't understand the purpose of D109 connecting to pins 9 & 11 of the LM339. Can anyone enlighten
me?