Burnt filament virtual tap resistors

[Randall]

I've got a Rivera era 1982 Fender Concert amp on the bench with a lot of preamp noise. I notice right away that the 100 ohm resistors on the pilot lamp are burnt up. They got real hot because there is evidence of smoke. I find no path to ground from either one. The amp was working, just making a whole lot of hum, most of which was effected by the volume control. Also, I have a similarly charred screen resistor. Tested all the tubes, and none were shorted, and all look strong in conductance.

I could use some advice as to what might flame out the filament virtual tap resistors like that. Both of them charred to death. The tubes light up and make sound, so the winding seems like it would be OK.

Same question bout the single screen resistor. I realize there may have been other tubes installed. It is an amp a guy just bought on Reverb, so who knows?

I have yet to find the correct print.

[g1]

Burnt heater resistors sometimes related to power tube failure. Screen resistor almost always.
Could have been a fault in the installed tubes that only shows up intermittently under high voltage, or bad tubes that were since replaced.

[DrGonz78]

Like in those Ampeg V4’s where you find a fried hum balance pot. In those amp I think it was break down of the tube socket and HV arc’d over the pins on the socket. So I would always check those socket’s just to rule it out.

[garytoosweet]

Perhaps the amp had a catastrophic power tube failure, then the owner replaced the tubes to sell it (without having the amp checked out).

[Helmholtz]

This one https://schematicheaven.net/fenderam...cert_ii_83.pdf used a humdinger pot.

[Randall]

Yes, the above print is the closest I could find, but not totally accurate, as the amp I have has no hum balance pot.

[mozz]

Well you are not gonna find a hum balance pot and dual 100 ohm resistors, 1 or the other or a 6.3vac center tap to ground. Replace the 100 ohms and see if your hum goes away. Then measure each leg to ground VAC and see how balanced you are.

[Randall]

Also, this amp has one of the fancy dual switching input jacks replaced with a cheap Marshall style jack. Looking at the print, I'm not sure just exactly what the isolated normally connected switch is doing. Something to do with channel switching, I think. What I do know is I am so far having no luck sourcing it. It is tip switched like a 12A, but also has an isolated normally closed switch activated by the plug tip such as in a 13E.

https://www.mouser.com/datasheet/2/3...cd-2943192.pdf

[g1]

Also, this amp has one of the fancy dual switching input jacks replaced with a cheap Marshall style jack. Looking at the print, I'm not sure just exactly what the isolated normally connected switch is doing. Something to do with channel switching, I think. What I do know is I am so far having no luck sourcing it. It is tip switched like a 12A, but also has an isolated normally closed switch activated by the plug tip such as in a 13E.

Are the 2 inputs not activating different channels? Looks like only jack 2 has the isolated switch fully floating, on jack 1 it is grounded.

[pdf64]

I think that switching circuit works to mute (later in the signal path) both normal and overdrive modes if nothing is plugged into either input socket.
Jack sockets incorporating DPDT switches are used in many Fender amps of the past 3 decades, so may be available as spares, and could be used to replace the input sockets of this design.
But I think regular shorting jack sockets could be used provided the switching circuit is kept isolated, isn’t connected to 0V circuit common. The downside being that hiss from the overdrive circuit will be present in the normal mode. Or if nothing is plugged in.

[g1]

I think if it is a 6 pin cliff style jack, and used in place of jk1 on schematic, it should be able to be wired up to work. Ring would need to be grounded, ring switch would go to the isolated tab on jk2.
Maybe that is what has been done, if it is indeed switching when using the different inputs. Pictures would help.

Note that jk1 may not be called "1" on the front panel, they may even be reversed. jk1 will be the one that has the 1M connected to the tip.

[Randall]

Not sure if this helps or not.

[g1]

Not sure if this helps or not.

That picture is not showing, please re-post.

[Randall]

Try again. Switchcraft doesn't even know. Don't know what to do about this.

[g1]

That would have been a custom jack ordered from switchcraft by Fender. It matches their jack schematic XVII. They do not currently make any such part except in the long open-frame telephone style, which is of no use here.
Clearly it is no longer wired up 'stock' but could be wired up to work as described in post #11. It would be in the schematic 'JK2' position, and you would need a TRS double switched jack in JK1 position, like a 6-pin cliff jack or switchcraft 14B.