Vox Cambridge Reverb V1031 non-polar electro caps

[DrGonz78]

Perhaps this is oscillation. It starts out like it is creeping up in volume, but then it runs away into oscillation. So it builds with intensity into a full on howl. At that point must turn off amp as it is really really loud. Happens everytime it just runs away even with no signal applied and only turning up volume/reverb controls.

[R.G.]

It's oscillation. I'll have to think for a bit on this one. The Thomas Vox reverb circuit is prone to this unless everything's right.

[g1]

Does the position of the tank in relation to the speaker have any effect on the howl?

[DrGonz78]

No moving the tank around does not have any effect on the howl in relation to the speaker. Even without signal the feedback grows louder to the point that I have to turn it off.

I looked at the signal coming out of the secondary of the reverb transformer with a scopoe and it is a perfect sine wave. Perhaps it is too strong for the reverb? It is relative to how loud I turn up the master volume and it is nice solid sine wave. Then I tried to scope the return side from the reverb tank and I don't see a sine wave at all. Eventually you start to see an oscillating wave signal which must be that horrible noise I am hearing. All the scope tests were done with 1k sine wave injected at the input of the amp. I did voltage tests on Q5 without a signal present...
B= 624mv
C= 1.49v
E= 61mv

This is a brand new reverb tank so I can assume that it is working, still need to know if it is working. The only two ideas I have involve too strong of a signal overpowering the reverb on the input. Or that Q5 might have something to do with the issue. Note that when I tap on return plug it responds to my finger tapping the tip and the reverb pot makes it louder or quiet. So it appears to be working and passing signal through Q5 to the final output. Stange.

Edit: Also note that when I say feedback from the reverb circuit I mean oscillation. As Bill had pointed out a few posts back and I realized that I keep wanting to type feedback. Unplug the reverb tank and the amp sounds great. To touch on RG's questions... It is a high pitch single frequency tone and it does sound a little like speaker feedback. Moving tank around and viewing the behavior on a scope(no speaker attached) reveals to me that it is not caused by speaker feedback issues.

[52 Bill]

The reverb control sort of acts like a balance control CCW is straight signal and CW is reverb signal. So maybe the value of the pot is important enough to cause your problem. What does the resistance read?

And in general, ground/shielding problems are always a part of a reverb circuit. Does the tank have both RCA jacks grounded to the chassis? Is the return cable good?, etc.

[Jazz P Bass]

Try a small piece of foam under the reverb springs in the middle of the tank.
The springs should just touch the foam.

[R.G.]

Unplug the reverb tank and the amp sounds great. To touch on RG's questions... It is a high pitch single frequency tone and it does sound a little like speaker feedback. Moving tank around and viewing the behavior on a scope(no speaker attached) reveals to me that it is not caused by speaker feedback issues.

Does an ohmmeter show continuity between the RCA jack ground shells on the tank?
Does an ohmmeter show continuity between one or both RCA jack ground shells on the tank and the tank metal cover?
Does an ohmmeter show continuity between the RCA ground fingers on the tank ends of the cables and amplifier signal ground?

[DrGonz78]

Does an ohmmeter show continuity between the RCA jack ground shells on the tank?
Does an ohmmeter show continuity between one or both RCA jack ground shells on the tank and the tank metal cover?
Does an ohmmeter show continuity between the RCA ground fingers on the tank ends of the cables and amplifier signal ground?

Yes I forgot to mention that I had tested all the wiring with an ohmmeter. So the answer to all three question above is a yes. Which even then I will triple check this later tonight even still.

Try a small piece of foam under the reverb springs in the middle of the tank.
The springs should just touch the foam.

I will try the foam idea out later too.

The reverb control sort of acts like a balance control CCW is straight signal and CW is reverb signal. So maybe the value of the pot is important enough to cause your problem. What does the resistance read?

And in general, ground/shielding problems are always a part of a reverb circuit. Does the tank have both RCA jacks grounded to the chassis? Is the return cable good?, etc.

I will test the resistance on the pot and report back.

Thanks for all the replies and I will follow up with more info later on tonight.

[DrGonz78]

Okay the foam did the trick! Thanks for that one Jazz! Only problem now is some distortion on the reverb recovery side. I think Q5 might be pretty much gone just like the send side of the reverb transistors were bad. Amp sounds great with reverb turned off then turn it up I get reverb but signal is distorting a bit. It is somewhat similar distorted signal as I got from Q3 or Q4 before I replaced those but not quite as bad. Anything to check before I pull the board? I will wait just in case till tomorrow to do the work(also don't feel like pulling out the board tonight) as other's here might have some other ideas first.

Q5 is just like Q3 so I have some of the 2N3392's on hand already.

Edit: I just tried it out again and it sounds pretty good. However, does adding the verb cause the amp to overdrive a bit? If not then Q5 might be the reason why. I am also playing around with different foam pieces to get the reverb to sound as good as it can.

[DrGonz78]

So I finally got it straightened out! Of course I owe so much thanks to RG especially for guiding me on this project and to everyone else who helped too. A big thanks to all, you are the best.

I was doing most of my testing with amp out of the cab and with the tank springs pointed upwards. I finally figured out that when I turned the springs downward they hang and that was all the problem. I was putting foam pieces between the aluminum float and the springs. Sounded great till I turned it facing down the way it would be mounted in the cab. I ended up just placing a foam piece between the middle of the springs and the cab bottom. Then I kept fine tuning it till I got a nice balance. The distortion I was getting is not really a problem now and definitely not worth pulling the board out to replace Q5. It just sounds like a nice vintage solid state amp!

[bohrmanator]

Dr. Gonz, Just finished reading all 3 pages of this Cambridge repair from June '014. I recently obtained a V-1031 Cambridge with "that volume problem"-The amp is cosmetically a 9.5 and everything seems to function correctly except that the reverb seems to go over the top at the 2 setting. And Volume at 10 is what it should be at, say 4. I definitely do not have the skills to do what you've done and my usual repair guys won't work on Solid state amps. Any advice?

[DrGonz78]

These amps are a real chore to work on at any level of amp tech repair. Do you have any experience working on amps? This is not necessarily the amp to learn how to repair. Post your location and perhaps there is someone near you that can be of service.

[bohrmanator]

Thanks-I'm in Monterey, California.