Marshall TSL 122 NOT bias drift.

[Danglin' Fury]

Hey all.... I got a doozie. Dude brought me a 2006 Marshall JCM 2000 Triple Super Lead that comes on, but no sound. Ok. I crack it open and someone had already replaced R6 and R9 (1 ohm in power section) as I assume they burned up. Well, they were open again. And C46 looked like a piece of charcoal. R76 (1k 5watt screen resistor) was burned open. Only, it looked fine... I was only able to tell it was open by meter reading. Bias is at pin 5 on the EL-34s like it should be, and it is solid..what has me buggered is, on pins 3 and 4, im only reading approximately 225-250 Volts DC. I was pretty sure it was supposed to be close to 490.... All measurements were taken with the tubes out. Pre and power. Another weird thing, R4 and R5 are shorted....until I remove them from the circuit. That tells me that the V.P.R. switch is either being held open, or the relay isn't working. Something probably took out the TL071 that drives it... I'm buggered..that V.P.R. doesn't half the plate voltage, does it? If anyone has seen any of this before, I would really appreciate any direction. Thank you. Oh and when the HT is connected to the board, I'm reading ~ 225-250 Volts AC across the spade connectors but it starts out around 160 Volts and climbs so im wondering if the caps in the rectifier are toast (C40 & C41). I read ~360-380 Volts AC when I pull the HT wires off the board.... Something is seriously loading it down...

[Helmholtz]

Would help to see the schematic here.

[Danglin' Fury]

Shoot....sorry. lemme see if I can post it..

[Danglin' Fury]

Here it is. Let me know if it posts please.

[Helmholtz]

Works, thanks.
Does the amp use the correct mains fuse F1 (T4A)?
What is the HT winding voltage?

[The Dude]

C46 is a common failure for these amps. Replace with higher voltage cap. It's to prevent oscillation. Some people just clip it out. I replace it. I figure they put it there for a reason.
R76 burning up might indicate a bad V8.
Bad, shorted, arcing, etc. tubes might cause excessive current and burn R6 and R9 cathode resistors.
I agree that leaky caps COULD be loading your B+. You have lots of AC on the B+ line, and with tubes out, there isn't much else there.

[g1]

Are you sure this is the right schematic? Where is the VPR stuff?

[Danglin' Fury]

It is... The VPR stuff is on the 2nd page from the top. "Overdrive front panel" page, up at the top, right corner. Related to "CON 20". You will see 2 switches. SW3 is the V.P.R. switch, "SW4" is "Output Mute

[Danglin' Fury]

Helmholtz, it is using the correct fuse. And the HT windings, not connected to the board, is approximately 360-380 volts AC. Funny thing, neither the mains fuse, nor the HT fuse blew when this amp went out. Dude said he turned it on, took it off standby, set his guitar down for a second and he heard a "Pop" then a "fzzzzt". And then no sound.

[Helmholtz]

Seems that regarding power amp operation SW3 just reduces power tube drive like a fixed PPIMV.
Where does CON19 connect?
(Is there any logic behind the Marshall connector numbering?)
What does V.P.R. stand for ?

What are the voltages at each end of R71? Does it measure good?

[Helmholtz]

Oh and when the HT is connected to the board, I'm reading ~ 225-250 Volts AC across the spade connectors but it starts out around 160 Volts and climbs so im wondering if the caps in the rectifier are toast (C40 & C41). I read ~360-380 Volts AC when I pull the HT wires off the board.... Something is seriously loading it down...

Had missed this. Very strange.
So loaded HT voltage is 225-250Vrms and unloaded HT is 360-380Vrms?
If this was a loading effect only, the main fuse must blow.
Is this before or after the standby switch?
What is the voltage drop across the switch when connected to board?

[The Dude]

......Is there any logic behind the Marshall connector numbering?......

That could easily be interpreted as facetiousness. It's one of the things I hate about Marshall schematics. Jump from here to there with crappy connector labeling. Their documentation is a mess.

Edit: Oh, and V.P.R. is virtual power reduction.

[Helmholtz]

Oh, and V.P.R. is virtual power reduction.

Thanks.
So no real power reduction .

[The Dude]

LOL. I imagine it's a sales ploy. People seem to like new fancy terminology, even if it's old tech.

[Helmholtz]

LOL. I imagine it's a sales ploy. People seem to like new fancy terminology, even if it's old tech.

Yeah, but as they only seem to limit the PI output this might actually be seen as fake power reduction.