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
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.
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.
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...
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.
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
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?
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
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.
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?
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