'93 Blues Deville 4x10, problem with noise when volume is low

[jskeen]

Hey guys,
I've been working on this gen1 blues deville for a few days now, off and on. I've replaced the filter caps, retouched the solder on all the tube sockets and generally cleaned up most of the issues with it. Replaced the 1 watt 470 ohm resistors on the suppressor grids of the power tubes, because one of them was just gone, as in missing. strange but true. replaced the input jacks with Isolated switchcraft grounding jacks, cleaned all the sockets and pots with Deoxit and retensioned the pins.

The only problem left is that the volume channel has a very odd noise issue. when the volume control is anywhere above 4, it's dead quiet and sounds good. but below 4 there is a seemingly random noise issue, both when moving the pot and when it's at zero. Relatively loud hissing, popping and generally being obnoxious. It is worse at zero, and comes and goes as you move toward 4, then after that it's quiet. I recleaned the volume pot, and when that didn't help, swapped it with the drive control pot, no change.

It goes out with the preamp send, and is still present if v1 is out, goes away if v2 is removed. Not a tube, so rather than just randomly replacing components around v2, I figured I would ask you guys.

Here's a link to the schematic; https://www.thetubestore.com/lib/the...-Schematic.pdf
Thanks
James

[glebert]

Do you have a scope that you can check for oscillation in that area of V1 and V2? Otherwise maybe just a DMM in AC mode. Seems odd to me that the volume knob affects it and the noise is still there when you pull V1, but I am kind of a newb.

When you say "volume channel" you mean clean correct? Noise goes away when Drive is engaged?

[mozz]

DC voltage on the pot?

[nickb]

It still happens with V1 removed???? That makes no sense as the volume pot is not in circuit then. Just so there is no confusion, I wonder if by "Volume Channel" you mean "Drive" or "Clean"? What is the VR number of the volume pot that has the problem?

Have you made any mods in that area?

[jskeen]

Yes, sorry, I meant the clean channel, that is controlled with the pot labeled volume. And I forgot to mention that the channel select switch does not work, because the owner specifically said he never uses it and doesn't care if it works or not. And while the noise is dependent on the level on the volume pot, it is not exactly controlled by the volume, IE, is is present at pretty much the same volume anytime it is there. Thus my suspicion that it is somehow related to the boost circuitry which feeds in just before v2a. But yes, with v1 removed, the noise is still present if the volume is below 3 or 4 and gone above that, Although there is no actual signal.

[glebert]

For no reason that I can articulate I think the channel switching problem is part of the noise issue.

Does twiddling the EQ knobs affect the noise?

What is U4B doing? Is that a momentary mute during channel switching? Just wondering if oscillation there can get into the signal path via Q1.

[nickb]

Yes, sorry, I meant the clean channel, that is controlled with the pot labeled volume. And I forgot to mention that the channel select switch does not work, because the owner specifically said he never uses it and doesn't care if it works or not. And while the noise is dependent on the level on the volume pot, it is not exactly controlled by the volume, IE, is is present at pretty much the same volume anytime it is there. Thus my suspicion that it is somehow related to the boost circuitry which feeds in just before v2a. But yes, with v1 removed, the noise is still present if the volume is below 3 or 4 and gone above that, Although there is no actual signal.

Did this happen before you started work on it?

With V1 removed, the only path I can see thru the volume pot involves the local HV supply node "X" (unless there is a mod deliberate or otherwise, is there?).

If it's hooked up per schematic, then for that to happen, C2 or (R6 or C6) or R4 AND the filter cap for node X i.e. C46 would have to be bad. So let's find out if that is the path. Temporarily tack a 450v electrolytic say 10uf or so, between pin 1 and 3 of V1. Remove V1 and test to see what the effect is.

BTW what test gear do you have?

PS: Also plan on fixing the channel switch issue. It just seems like a really bad idea to me to be working on something that is not as it should be. Why? It's possible relays might not be set as you might expect. Anyway, it just rubs me the wrong way.

[glebert]

Are the new switchjacks PCB or panel mounted? If panel mounted maybe also check the lead placement to make sure they aren't creating an odd antenna that can get past V1. Just spitballing on this.

[jskeen]

Thanks for the reaponses guys. Some more background info. All the problems were there when I started, there are no mods to the amp afaik, this is a hurricane Harvey victim, it didn’t get wet, but sat in a storage room on a shelf above a foot of water with no power for several weeks, and had some corrosion issues, but grounds are all clean and tight and ohm out at zero or darn close. This issue is the last problem, I’ve got everything else sorted out by now.

I don’t have a working ‘scope, due to Harvey as well... but I do have a tone generator and tone probe, cap tester, and digital and analog meters.

When I hit the channel button, nothing happens, no red led, no momentary mute, no relay, and no jump in gain. I can rig a foot switch to verify if the same thing happens via the jack in the am.

I do think the noise is somehow coming through from the switching circuits, I just can’t figure how. I don’t think it is a normal noise signal propagating through the signal chain though. It’s almost like a zener breaking down prematurely at a low voltage or a cap arcing through when the volume is below a certain level. Can’t say why that would be though.

More test results in the am guys, thanks.
James

[nickb]

In light of what you said about moisture, I'd add a path thru RLY1 to the list. I have seen relays leak from coil to contacts before. Also bad contacts might have something to do with the nature of the noise. You might have to pull components one by one in the electrical area around the volume control to isolate the path.

For the channel switch issue measure the supply and signal voltages around U3A. The expected values are given on the schematic.