V4B - Standby switch doesn't work / cut power

[Roman Ships]

A while the standby switch stopped functioning. The amp was just "on" with the power switch.

I replaced the switch, didn't fix it.

I have opened it up and thought I had found a bad D5 diode but I tested it in circuit again later and it tested fine. Probably just tarnished / corroded surface and my DMM wasn't making a good connection.

I know the standby lifts ground on the filter caps after the first Cap. I haven't found any obvious grounding issues. I did find a screen resistor that was loose, pretty much just sitting in the socket, not connected. I assume it got very hot and un-soldered it self. According to the schematic, it looks like I should have plate voltage on standby, I do... is this correct?


Before I tore it apart I think I tested voltage drop across plate resistors and V5 adn V6 were pulling waaay to much current. They look like they have been very hot, the JJ text looks burnt.


I am in the process of updating screen resistors, grid stoppers, one original filter cap ( C17 ), the rest are JJ filter caps.


There was some charr / gunk on the back underneath the screen resistors, was I getting some conduction over that perhaps?


Thanks,
eli

[Hangman]

what has likely happened is that when someone replaced the filter caps, they just hooked the negatives all directly to ground. shorting out the standby switch and D5.

Take a look at all your main reservoir filter caps and the screen supply filter caps
and make sure that they are not grounded directly to the chassis. they should go to the standby switch which then goes to D5, that goes to ground.

A while the standby switch stopped functioning. The amp was just "on" with the power switch.

I replaced the switch, didn't fix it.

I have opened it up and thought I had found a bad D5 diode but I tested it in circuit again later and it tested fine. Probably just tarnished / corroded surface and my DMM wasn't making a good connection.

I know the standby lifts ground on the filter caps after the first Cap. I haven't found any obvious grounding issues. I did find a screen resistor that was loose, pretty much just sitting in the socket, not connected. I assume it got very hot and un-soldered it self. According to the schematic, it looks like I should have plate voltage on standby, I do... is this correct?


Before I tore it apart I think I tested voltage drop across plate resistors and V5 adn V6 were pulling waaay to much current. They look like they have been very hot, the JJ text looks burnt.


I am in the process of updating screen resistors, grid stoppers, one original filter cap ( C17 ), the rest are JJ filter caps.


There was some charr / gunk on the back underneath the screen resistors, was I getting some conduction over that perhaps?


Thanks,
eli

[Roman Ships]

what has likely happened is that when someone replaced the filter caps, they just hooked the negatives all directly to ground. shorting out the standby switch and D5.

Take a look at all your main reservoir filter caps and the screen supply filter caps
and make sure that they are not grounded directly to the chassis. they should go to the standby switch which then goes to D5, that goes to ground.

To clarify, the amp just started doing this. It has been several years since I had it in a shop. So something happened recently within the amp.

C18 and C20 are for sure tied to the D5 Ground, I'll have to check C17, but I think it is all proper.
There is a ground from the speaker output tied into the ground on C17, perhaps it is grounding into the chassis there.... I'll investigate.

thanks,

[Roman Ships]

It also might be worth noting that I had checked the bias voltage on at each control grid pin and they were all -64 V.

So I assume the bias circuit is not to blame. The plate resistors are original but test pretty close to 4.7 Ohm which is what they state on them.

[Hangman]

The only way that the amp could be on with the standby switch off, is if those caps are somehow grounded.

c17 should be chassis ground, but all of the other filter caps should be tied to the standby switch ground.