Fender "the twin" hum

[timgman]

All the solder joints seem ok. I'm no amp tech, but it looks a bit nasty in here lol

might it be possible for a bad jack in the effects loop to cause this?
Anything else I might want to look at?

[timgman]

I cleaned the loop jacks and still the same issue. I guess I'll look at the circuit that attaches to v3 ?

[52 Bill]

I cleaned the loop jacks and still the same issue. I guess I'll look at the circuit that attaches to v3 ?

Did you clean the loop level switch? Does moving any of the wires that connect the loop board make any difference?

G1's question about the loop break jack not cutting off the hum is a poser. There are two power amp input jacks listed on the schematic, which one did you try?

Have you tried taking a signal out from the preamp and sending it to another hum free amp?

[timgman]

Seems that a yellow wire on v3 is directly related to the microphonic sound. If I tap or touch it with an insulated tool while powered, it makes slot of noise.

I tried both the inputs to the power section with the same result.

I cleaned the jacks but not the loop level switch.

[g1]

I think there are maybe a few symptoms here that are just confusing things? The microphonic sound of the yellow wire on V3, is this the same sound as the hum we are discussing? Or the same microphonic sound as the V9 tube mentioned in post #11 ?
It will be easiest to just deal with the hum for now, and leave other sounds for later.

[timgman]

Ok, the yellow wire is not related to the hum.
The yellow wire movement makes the microphonic sound.
Ill test the preamp section.
-

Singling out the power sectio...
If I remove all the preamp tubes and work with the power section ..( the power tubes and pi tube) the power section should still work.correct?
Maybe I'll focus on that?

[timgman]

I'll pull up the rear loop board and look for anything strsnge.

[oc disorder]

Had a few with that 30 k disintegrating .. I replaced it with 2 x 15k on a tagstrip mounted on one of the transformer bolts .

As it has no "hum adjust pot" suggest checking those 2 x 47 ohm R302 and R303.

While you're there might as well check the diodes and the other resistors.

Oh credit for the photo to Tom Phillips! Might refresh a few memories !

[timgman]

Had a few with that 30 k disintegrating .. I replaced it with 2 x 15k on a tagstrip mounted on one of the transformer bolts .

As it has no "hum adjust pot" suggest checking those 2 x 47 ohm R302 and R303.

While you're there might as well check the diodes and the other resistors.

Oh credit for the photo to Tom Phillips! Might refresh a few memories !

[ATTACH=CONFIG]35835[/ATTACH]

r302 and r303 each read 23 ohms but that with them still on the board.

[timgman]

r302 and r303 each read 23 ohms but that with them still on the board.

Looks like it got fried at some point!

[oc disorder]

Well you can eliminate those then (i.e. not faulty) - the heater winding and the resistance of the heaters makes them appear in parallel.

(P.S. nothing there at the photobucket link )

[timgman]

OK I installed the tubes from my mesa nomad 100.. just for a sanity test.. same results. it IS v9 that's microphonic. when a cord is in the effects return, the power section works (LOUD) with the hum. all tubes inserted.... if I take out v3 and 9, the effects return does not power the amp.
The input labeled "power amp thru" does not give signal to the power section with or without v3 and v9.
Here's a shot of the 2 resistors I tested 2 posts up.. one was obviously fried at some point.
http://i122.photobucket.com/albums/o...924_170230.jpg
**I talked to the guy I got this amp from and he stated that the amp "developed the hum" after he was in the middle of playing and there was a power loss**

[Tom Phillips]

1) Please post your related photos directly to this MEF thread. That way they will stay with the discussion and, though not guaranteed, there is a better chance that they will be intact when people look at this thread in the future.

2) Please measure and post the AC voltage of each side of the nominal 6.3V heater supply with respect to chassis ground. Those readings will tell us if the heater balance circuit is functioning properly.

[timgman]

1) Please post your related photos directly to this MEF thread. That way they will stay with the discussion and, though not guaranteed, there is a better chance that they will be intact when people look at this thread in the future.

2) Please measure and post the AC voltage of each side of the nominal 6.3V heater supply with respect to chassis ground. Those readings will tell us if the heater balance circuit is functioning properly.

1) I'll direct upload images for now on.
2) I do not know how to measure this. is it just the green wires in the power supply section to ground?
Thank very much you for your time.

[timgman]

On the main power board using numbers on board on page 6 of the technical papers (on post 37 of this thread)

From terminal 12 to chasis ground = 2.9 v terminal 13 to chassis ground= 2.9