Marshall TSL 122 NOT bias drift.

[The Dude]

Yes I agree, but F.P.R. probably wouldn't sell as well as V.P.R.

[Danglin' Fury]

I've noticed something now. When I turn the standby off and measure the AC before the rectifier, it starts out at around 100 Volts AC and climbs to ~ 204 volts. Con19 goes to con 1 on the back panel. It sends the signal to the Emulated Line Out. R71 reads 274 ohms. The voltages on either side of it reads 259.4 Volts DC

[Helmholtz]

Seems there's something wrong with the Standby switch.
Looks as it neither opens nor closes completely.
What happens if you jumper it with a wire?

[Danglin' Fury]

I will certainly do that in a moment. And there is a board that the mains switch and standby switch connect to. With a couple of capacitors on it. I at first thought it could be those caps so I changed them. No joy. I will hardwire across the standby and see whqt I get them report back.

[Danglin' Fury]

So I jumpered the yellow HT wires on the standby switch.... Aaaaaand I now have ~530 volts DC on pins 3 and 4 of the power tubes. R4 and R5 in the PI are still shorts in circuit, so I think there is still a problem with the V.P.R./Output Mute circuit though. But thank you for pointing me toward the Standby switch.

[Helmholtz]

SW3 on page 2 shorts R4 and R5 for full power.

[Danglin' Fury]

Gotchya. It never changes, no matter the switch position. Also, I took this Standby switch out of the circuit, and the only thing it measures on both sets of pins, is Open (OL) in the "OFF" position, and a direct short in the "ON" position...

[Danglin' Fury]

Ok. I measured across the pins, bottom left and top right.... Found the resistance. Makes sense as that is how the dude had the wires connected to said standby switch... Now I just have to wait for my parts to get here. We used to have a really sweet local parts house... NOT Radio Shack....lol but the dude was done I guess so he sold everything to someone. Now there are no more local parts houses. Lame. Lol. Anyway, I will report back once I have changed the parts and tested the amp.

[Danglin' Fury]

Well... The switch and the driver for the relay came in and I changed them both and voltages looked good. Nice and solid... Put the tubes in it and powered up the amp... It sounded pretty good for a couple of seconds, and then the smell. Looking at the back of the amp, the 2 right power tubes were red plating. Dammit... Lol! Now I gotsta repair the bias circuit. Lol. This amp is a gigantic pain in the ass. I think I would rather repair a Mesa....lolololol

[Danglin' Fury]

Ok, so now I'm noticing that the bias voltage on pin 5 is reading around -50 volts DC....aaaaand when I adjust the bias pots in the back, the voltage does not change at all... I can't get it to go lower. This amp truly is a pain in the ass. The connectors from the main board to the bias board, all pins have continuity. Any ideas?

[Valvehead]

check everything in the bias supply and the 4, 5.6ks . i have seen those bias pots go bad. also check C6/ C7 after the PI tube . Have you pulled the Pcb's out ? 90% of these ive repaired had a lot of cracked solder joints

[Danglin' Fury]

Hey man. Thanks for the reply. I checked C6 and C7... They read open. I'm gonna lift a leg on them Tomorrow and check them with a cap tester. This amp has had so damn many things wrong with it. I did check and reflow all cold solder joints. The foot switch area was most egregious. Nothing looks like a cold joint in the bias circuit. I reflowed it anyway. Now I have (looking at the back of the amp) -39 volts DC on the left pair and -50 volts on the right. The left side will adjust, but not the right. I replaced C36 and C37 in the bias circuit. All the 5.6k resistors measure good. The 220k resistor (R67 and R69) are reading a little low. (150k and 117k).... I'm gonna measure the pot on the right and see if that is it.

[Danglin' Fury]

Ok, got the bias sorted.... apparently the left pot adjusts the tubes on the right??? Weird... Also, when the Standby switch is engaged (sound off), I'm reading ~259 volts DC at pins 4 and 5. When I disengage the Standby (sound on) I'm reading ~ 530 Volts DC.... Do these voltages seem a bit high?

[Danglin' Fury]

Not pin 5. Sorry. Been up since 4:30... Definitely pin 3 and 4 I have ~ 259 off and ~530 on. Stupid being tired. Well....time for a 5 hour energy. Lol

[g1]

That does seem a bit high, but what did you set the idle currents at?