Masco MA25N repair

[RancidAltoid]

I just got to working on a Masco MA25N for someone and I've run in to a couple rather weird problems that I've never seen before.

First off, I have a weird oscillation problem in one of the channels of the amp. Normally I'd go through and find the offending part and/or wire and fix it, but I'm stumped with this one. The problem that I'm having is that one of the 6J7 tubes seems to have an oscillation problem, so I started off by disconnecting the grid cap which makes the oscillation stop. Given this, I rebuilt the circuit up until the grid cap(which consists of a .005uF cap from the tip of the jack to the grid cap and a 15meg resistor from the grid cap to ground). This didn't help the problem at all. Right now, I'm thinking that I may have grounding issues, so when I get home, I'll take a look at that.

The other problem that I have(which may or may not be a problem) is that there are grounded shields that go over the grid cap and make contact with the body of the metal 6J7 tube, with those shields taken off of the tube, no signal seems to be getting to the grid which make less than no sense to me. Coincidently, removing the grounded shield also solves the first problem that I have, but creates the new problem that no signal seems to be getting to the tube.

Here is a schematic of the amp:
http://www.angelfire.com/vt/audio/masco.html

Any one have any suggestions for me?

-Adam

[HollowState]

Adam,

I would check this:
Swap input tubes around to see if problem shifts channels.
High meg resistors tend to increase with age. (15M is very high to begin with)
Open deteriorated input cap or open connecting wire.
Are the input jacks grounded when not in use?
The lack of input signal getting to the grid is the key I would think.
Possible grounding issues as you said.

Victor

[RancidAltoid]

Already tried swapping the tubes for the first gain stage around, it didn't make a bit of difference. As I had mentioned, I replaced the caps, resistors and wire of the offending channel, so I don't think there would be a problem with the components. The jacks are not shorting jacks(not grounded when not in use), even so, the offending channel still squeals like a stuck pig weather or not I have an instrument or mic plugged in to it.

To clarify, signal does get to the channel with the grounded sheild connected over the grid cap, but no signal when I remove the grounded sheild. Also, by grid cap I do not mean a capacitor, I mean the electrode sticking out of the top of the tube.

At this point, I'm definitely thinking I may have grounding problems with that channel, so when I get home I'll check all the grounds for that channel.

-Adam