Hot Rod Deville III - clean channel problems

[Tone Meister]

Actually I meant the C1 and C56 coming off the cathodes of V1. Check for correct value, orientation, bad joints etc as you mentioned that they had been changed earlier on.

Also does the amp fault change with thumping / chopstick / vibration etc?

Voltages at C1 and C56 are ~ 2.02V DC. Values are correct (47/35) and the caps are oriented correctly. The fault does not change with smacking or chopstick.

[mikeydee77]

I wonder if you can check one other thing on the coax effect.

Sometimes pre-amp stages are sensitive to the proximity of hands especially when they are away from the shielding of the cabinet. I am pretty sure hand proximity can either increase or decrease the noise - not sure I understand the science of why but presumably because we are all aerials or ground-planes.

In order to clarify whether the coax is an issue you could try bringing your hand close but not actually touching the cable (so are your hands reducing the noise) and conversely you could try moving the the coax with a chopstick (to remove the hand factor).

PS The amp should have 12AX7 according to the schematic. I am sure it would function with the other ones but not optimum.
PPS The cabinet screening is important on all the those Fender amps so make sure it is intact and secure.

Mike

[Pedro Vecino]

If you have examined the new components and their values and everything is fine, there remains only the own line of sound (volume potentiometer, basically) and switching. When removing the board out is common that potentiometers are forced and it´s possible they may have some battered welding or even an internal rupture (mechanical contact) of a pin. Also make sure you have the original potentiometer values. Would not be the first time ...
I see less likely that the problem is associated to the switching.

[Tone Meister]

With a 1K signal into the input, I get a strong signal at R52 and R4 but nothing at R9 or R44 unless the CHANNEL SELECT switch is depressed. With the CHANNEL SELECT switch depressed, I get a weak signal at R9 and R44 but get a strong signal at R11, R16, and R22.

Pedro, all the values I have checked have been correct so far.

[mikeydee77]

With a 1K signal into the input, I get a strong signal at R52 and R4 but nothing at R9 or R44 unless the CHANNEL SELECT switch is depressed. With the CHANNEL SELECT switch depressed, I get a weak signal at R9 and R44 but get a strong signal at R11, R16, and R22.

Pedro, all the values I have checked have been correct so far.

If I am following this correctly, then you have drive channel is fine. Clean channel is good up to C18. Doesn't that just leave R9, R6 (pot) and relay as suspect. And of course it could be a solder wisp/blob from earlier repairs bringing the signal down.

With the 1k signal, what do you get at each point in that chain? either side of R9, pot input and wiper?

[Tone Meister]

If I am following this correctly, then you have drive channel is fine. Clean channel is good up to C18. Doesn't that just leave R9, R6 (pot) and relay as suspect. And of course it could be a solder wisp/blob from earlier repairs bringing the signal down.

With the 1k signal, what do you get at each point in that chain? either side of R9, pot input and wiper?

Yes, you are absolutely following it correctly, Mikey. Replaced C18 and pulled the R6 VOLUME pot and re-soldered it back in. Now the amp has full volume on the clean channel, BUT the hideous (120 Hz?) hum is still there when I remove that jumper that Pedro suggested in post #13.

EDIT TO ADD: Supplemented the shield at WJ20A with a wire to ground and she's quiet as a titmouse. Gonna burn it in for an hour or so with a Stratocaster and see if it holds up under pressure. Thanks Dude, Mikey, and Pedro for helping point me to the right places.

[mikeydee77]

Yes, you are absolutely following it correctly, Mikey. Replaced C18 and pulled the R6 VOLUME pot and re-soldered it back in. Now the amp has full volume on the clean channel, BUT the hideous (120 Hz?) hum is still there when I remove that jumper that Pedro suggested in post #13.

EDIT TO ADD: Supplemented the shield at WJ20A with a wire to ground and she's quiet as a titmouse. Gonna burn it in for an hour or so with a Stratocaster and see if it holds up under pressure. Thanks Dude, Mikey, and Pedro for helping point me to the right places.

Good news!

I think I saw a mention of cutting out problem earlier in the thread so watch out for that. If you don't know already this is often to do with switch jacks on the fx loop or pre amp power amp sockets. Check out the sticky thread on this board.

[Tone Meister]

Good news!

I think I saw a mention of cutting out problem earlier in the thread so watch out for that. If you don't know already this is often to do with switch jacks on the fx loop or pre amp power amp sockets. Check out the sticky thread on this board.

Yes, I am aware of those gotchas with these and have already serviced & reflowed all the tube sockets and serviced those jacks.