Dead channel on marshall vs100h HELP!!!

[charlie13]

I have a Marshall vs100h amp that suddenly lost the 2nd overdrive channel. No light and no apparent dammage to any of the pc boards. If i crank the gain and volume for the channel i get what sounds like a dry clean signal from the guitar at a very very low volume. The clean channel/overdrive 1 work perfectly. I have the marshall schematics for the amp and have been trying to find the fault to no avail if anyone has had this problem or knows how the fix it please let me know. I have checked all the in and output jacks, all the pc boards for loose connections,burned traces and burnt componets.

[Enzo]

I would suspect a bad op amp. Look at each one in that channel for unwanted DC on an output pin.

[charlie13]

Thank you Ill look at that tonight and let you know what i find

[charlie13]

Still at it

After some more probing I found a weird loop in the od 2 circut. It seems to stay closed all the time. I removed the switch an checked it, works fine. Then I checked the connections on the board an the circut is still closed. I checked the switch on clean/od channel and found when the switch is off (in clean mode) the circut is closed, when the switch is pressed in the circur opens (no tones from the meter). So this leads me to believe that the od 2 circut is acting like its off (switch out). On another note IC6 (op-amp) shows on post 1 (switch) about 11 millivolts DC and about 10 millivolts DC on the output pin (#5). I have rechecked all the resistors and caps along the circuts path and all seem good. I'm not sure what it could be. The reason I'm trying to do this myself is that it will cost more to ship it out to get fixed then what I paid for it and I want to get some experiance in this type of repair. I have worked on guitars for a long time but never amps. I anyone has any info that might help please let me know.

Thanks

[Enzo]

So when you try to switch channels, the one channel that w9orks goes off, but the other channel won;t come on, right? And the light for that channel won;t come on, right? Sounds like maybe a switching issue then. Follow the switch control circuit. There is some transistor logic, and perhaps some of that is stuck.

I don't worry about a few millivolts offset in the op amp- circuits.

[charlie13]

If its not one thing its another

Well I found out a bad zener diode in the Z1 position on the preamp board was causing the switch loop now the resistors at R20 and R21 are heating up. I traced the problem to the power board both LT+ and LT- coming out ofo con 5 to the preamp board is at about 43v DC but on LT+ I have 53milliamps and on LT- I have 13milliamps so my outputs are unbalanced.
and suggestions.

Thanks

[Amp Kat]

I think you have a bad output transistor or driver transistor. Check the rails on the output trannys and see what they are. Also on the preamp board go to the tube holder clip and see if it dug itself into the board. If it did take it off and repair the trace as it may be bridgeing that trace to ground. Those resistors get quite hot as they are the 16 volt rail power droppers and to be honest that whole regulator circuit(79/78) series runs hotter than it should in these amps but your problem is on that power board with the heat sinks.

[charlie13]

Weird Science

Well I traced the milliamp issue back to the bridge rectifier. on the AC side I have a balance input volts and amps but on the DC side my volts are the same but on the DC plus side I have 56 milliamps and on the negitive side I have 13 milliamps. I then removed the rectifier and did a bench test on just it and found it to be the same way as above. Also tested another rectifier to confirm my results it had the same results as the one from the amp. Am I wrong to think this is to be a balanced signal or am I missing something. My thought lead me to believe this is why on the preamp board R20 the DC+ (LT+) side works normally and R21 on the DC-(LT-) get so hot it melts the solder and shows little to on output to REG 2 Because in the step down to -15v dc it is also losing the 13 milliamps in the process. Now the voltagegoing to the preamp board stays consistant but my thinking is that there either has to be a capasitor the step the amps up to balance + and - DC or that in the loop around the the board it finds a balance. But this does not expain why R21 get that hot. I am planning on putting heat sinks on REG 1 and REG 2 to cool them down a little bit.
On the power board the scemaitc shows a caps C24,C23,C22,C21 that run the circut on the DC side just after the rectifier but I have searched high and low and con not locate them? Weird , any way , any one with some advice or words of wisdom I would greatly appericate it.

Thanks

[charlie13]

Learning More

Well I learned that the millamp offset on the bridge rectifier is correct so my thinking was wrong, and I found on a small Marshall DFX 30 I have that the millamps go through a set of + and - step down resistor the same as on the vs100 the only differences being voltage in and also on the DFX 30 the resistors drop the voltage but the amps (milli) stay the same. This is not the case on the vs100 the amps change going through the resistors I think my next step is to replace both resistors and see if that helps. Is there a upgraded resistor or heavy duty one that I could replace the preamp board R20 and R21 with that mat run a little cooler?

Thanks again for any info anyone has.
Charlie

[charlie13]

Its coming around

Well I think its starting to go my way. I found out the reg1 and reg2 on the preamp board were zapped (thanks amp cat) and almost have that resolved. I am playing around with various resistors (step down) but I susspect a bad acap in the circut. Im starting a new Thread for a new problem though.