Hot Rod Deville III - clean channel problems

[Tone Meister]

Drive/More Drive "channel" works and sounds fine. Clean channel has a nasty (120Hz?) hum that is not affected by the VOLUME pot. Power amp works fine with a signal into the POWER AMP IN. I can grab hold of the shielded coax cables (WJ19/WJ20 & WJ21/WJ22) and mitigate the noise somewhat. Started out the clean channel had a weak output, but then it progressed to the uncontrollable hum.

Someone had just gone through and installed all new plate resistors, cathode bypass caps, and filter caps. I've looked at all the solder joints and touched up a few, checked the ribbon cables for breaks, and checked for obvious mistakes, but nothing looks hacked and nothing is jumping out at me. Complaint was the amp was "cutting out" but there is more going on than that.

Tried known good preamp tubes in all slots, and tried putting the back panel on just to see. NOTE: I can grab either coax wire and mitigate the noise.

Whatever the problem, it's definitely in the vicinity of the V1 tube socket.

[The Dude]

Check your plate voltages on the preamp tubes. It sure sounds to me like your missing one; probably from a bad connection since the resistors were replaced.

[Tone Meister]

With V2 and V3 tube slots populated with 12AX7s, the V1 plate readings are:

430V DC on pins 1 & 6 with no tube installed in V1
264V DC on pins 1 & 6 with a 12AX7 fitted in V1

[The Dude]

All preamp tubes?

[Tone Meister]

Yes, I have swapped all preamp tubes for testing purposes. But, when I took the above readings, V2 and V3 had tubes fitted and I took the V1 plate measurements with and without a tube

[The Dude]

Looking at the schematic as it relates to V1, the signal take off point switches from R6 (volume) to R7 (drive) and is switched by K1-A relay. My next thought would be bad relay contacts or a bad R6 volume control. If you have a scope, you could inject a signal and follow the path through the volume control and relay to the 2nd grid of V1. My thought is that the hum is being caused by something "hanging in the wind" via an open circuit somewhere.

[Tone Meister]

OK: Cranking the TREBLE pot exacerbates the problem in a very big way. According to the owner, the amp was working fine before the work was done, EXCEPT it was "cutting out"

OK, now this is really strange ... at present I have the TREBLE dialed back and clean channel now has very low volume, but no hum. When I crank the TREBLE pot, full volume is restored and hum returns with a vengeance. Dial back the treble pot and channel gets weak again and no hum.

Bad treble pot?

[The Dude]

Perhaps C7 is leaky? Do you have DC voltage on the Treble pot?

[Tone Meister]

Perhaps C7 is leaky? Do you have DC voltage on the Treble pot?

C7 was changed to a silver mica. Not a drop of DC on the treble pot.

I get 1.6V AC ripple at the (+) of C32 and 3.2V AC ripple at the (+) of C31. No appreciable AC on the remainder of the filter caps, the readings are in the low mV range for C33, C34, C35, and C36.

[mikeydee77]

How about v1 cathode voltages. Any AC there?
Is the decoupling cap properly grounded correct value?

[Tone Meister]

How about v1 cathode voltages. Any AC there?
Is the decoupling cap properly grounded correct value?

EDIT: Cathode voltage is ~2.02V DC at pins 3 & 8 of V1 and at (+) of C1. Very low AC (~4 mV) at C1 and pins 3 & 8 of V1


Do you mean the .047 at C2?

[Tone Meister]

In the meantime let me clarify what it's doing.

With TREBLE pot at full CW both channels exhibit the hum, and that hum completely takes over the amp. Back off the treble pot even a smidgen and the DRIVE channel works fine, but the clean channel is very weak but it's quiet with none of the nasty hum. The owner says this wasn't what was happening before the work - it was dropping out occasionally but working normally otherwise.

If I max the TREBLE pot and hold the shielded coax cables (WJ19/WJ20 & WJ21/WJ22) with my hand, the clean channel volume comes back. Let go of the coax cables and the hum completely takes over. The hum is not subject to the VOLUME control.

NOTE: The amp was loaded with the following preamp tubes:

V1 - 12AT7
V2 - 12AT7
V3 - 12AU7

I've changed them all back to 12AX7s for testing purposes.

[Pedro Vecino]

If moving the two coaxial cables to the socket by hand you perceive changes, you can do this test:
Make a temporary bridge with a wire between the mesh of the coaxial cable into the socket board and the ground point on the chassis (green wire) that is next to the input jack.
Sometimes I found strange things in that line.

[mikeydee77]

I will report that information once I get back to the amp this morning. Do you mean the .047 at C2?

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?

[Tone Meister]

If moving the two coaxial cables to the socket by hand you perceive changes, you can do this test:
Make a temporary bridge with a wire between the mesh of the coaxial cable into the socket board and the ground point on the chassis (green wire) that is next to the input jack.
Sometimes I found strange things in that line.

You nailed it Pedro as far as the hum goes. I clipped in a wire from that ground point to the shield of one of the coax cables and the hum is gone. BUT, the clean channel still has a very weak output.