Intermittent HF oscillation on a 6505+, kind of at my wit's end here.

[DFuller]

Like it says in the title, got one of these in to repair. I've worked on this one before.

After some time, the amp will start oscillating near the ultrasonic region - I can hear the output transformer ringing. But the problem is, it did it once in my possession and I haven't been able to do it. Chopsticked the amp to no avail. No broken solder joints that I can see without lifting the board. All HT filter caps are new (within the last couple of years) and are a mix of Nichicon, F&T, and TDK/Epcos. As far as mods: I added a filter choke (swapped the screen resistors for 1K 5W parts and moved the standby switch to before the HT bridge rectifier), and a "normal/plus" switch for the preamp (which switches the interstage filter between V1A and V1B from a 470pf cap to 2n2+470K in parallel). Everything else is stock.

So what am I missing here?

[Enzo]

How close to the machine was the guitar when this happened? With all this gain available, the guitar can cause such feedback. When you replaced the big resistor with choke, how much did that raise the B+ throughout the preamp? And have we scoped the output for this noise? You might hear the transformer singing, and I sure hope there was a dummy load on it, but what was it singing TO?

You put wiring to C17 to a panel switch? (with parts)? If so, try disconnecting those wires and leave a cap or something in place of C17. Those wires are like antennas in this gain monster.. (antennae?)That should tell you if that is involved.

[DFuller]

Going to answer these in order:

How close to the machine was the guitar when this happened? With all this gain available, the guitar can cause such feedback.

That's the thing - I played on it for near on 20 minutes at high volume in the same location (amp on my bench, me sitting there, cab next to me) without this happening. I had messaged the customer that I didn't notice anything wrong, and... off it goes.

When you replaced the big resistor with choke, how much did that raise the B+ throughout the preamp?

I can't remember at this point, been a couple of years since I had the amp in for service when this happened. I'll get the number for you tomorrow, however. Still shouldn't exceed the voltage rating of the preamp filter caps which are 500V F&Ts, power amp plate node B+ maxes out at 480V-ish unloaded (so more like 465 in practice).

And have we scoped the output for this noise?

Didn't have the chance. It wouldn't do it again after the first time, but this was the customer's complaint so it's happened before. I'm thinking it must be heat related, but for the life of me, I can't figure out what.

You might hear the transformer singing, and I sure hope there was a dummy load on it, but what was it singing TO?

Hooked up to my 2x12 cab. The transformer was ringing right at the top of my hearing (so above 15k) but the output of the cab was just louder white noise. Sounds right otherwise.

You put wiring to C17 to a panel switch? (with parts)? If so, try disconnecting those wires and leave a cap or something in place of C17. Those wires are like antennas in this gain monster.. (antennae?)That should tell you if that is involved.

That's right, but the part that's confusing me is that it won't do it again. I even let the amp sit and heat soak for an hour and it wouldn't do it. It just sat there and worked right.

[Enzo]

Well that is the hardest thing to fix, long term intermittents

The guitar feedback I am concerned with is not cranked high volume stuff. With super high gain amps, some pickups can feed back at low levels, even you can talk over.. And could be his axe is more sensitive to such than yours.

I was not worried about over volting the filter caps, I was more thinking that increasing teh B+ through those stages migh alte the amp performance in some way causing stability issues. Not probably likely, but worth knowing.

You night lay a scope on th amp output just to see what there is to see. There could be some HF stuff going on.

yeah the old won't do it again. Whatever... play with the lead dress with those wires just to see.

Yes, this will be hard, and how can we be confident if it behaves for two weeks that it won't throw a tantrum the next day? We can't. The only hope is to find a definite thing.

[glebert]

Just a thought, what about vibration? Is the amp sitting on a cab when you are testing it? I bet it is when the owner is using it.

[DFuller]

Well that is the hardest thing to fix, long term intermittents

The guitar feedback I am concerned with is not cranked high volume stuff. With super high gain amps, some pickups can feed back at low levels, even you can talk over.. And could be his axe is more sensitive to such than yours.

Right, I get that - it did it with the guitar's volume down.

I was not worried about over volting the filter caps, I was more thinking that increasing teh B+ through those stages migh alte the amp performance in some way causing stability issues. Not probably likely, but worth knowing.

Definitely a thought, but I've done these same mods probably 10 times without it happening.

You night lay a scope on th amp output just to see what there is to see. There could be some HF stuff going on.

Yep, that's the next move.

yeah the old won't do it again. Whatever... play with the lead dress with those wires just to see.

I actually did this just to see if I could get it to start up again, and nothing. I thought it would have been this, too, since it seems the most likely reasoning.

Just a thought, what about vibration? Is the amp sitting on a cab when you are testing it? I bet it is when the owner is using it.

Nope, it isn't for me - it's on my bench. I'm sure the owner is, but it did it for me at no-signal conditions.

[Greg Robinson]

I've mentioned this before, but most times I've encountered instability issues with the Peavey 5150/6505 series, it has been related to the effects loop, and a good cleaning or replacement of the jacks if they were particularly worn-out has always managed to solve it for me. Certainly can't hurt, and if you're lucky might solve the problem.

[Enzo]

That reminds me of one of my shop rules. The cutout contacts or shunts in jacks serve in some cases to ground the signal path or to carry the signal past as in FX loop returns. In FS jacks they serve to keep something on when pedal is removed. I always measure the resistance of the contacts. If they are more than an ohm, I consider it bad. Usually cleans up. It is tempting to think something low like 3 ohms is OK, but it is not. The existence of that three ohms means the contacts are not pristine, and a little vibration or heat later and the resistance can grow to where it interferes. SO do that, measure actual cutout contact resistance on the jacks. I expect too get less than a half ohm.

[DFuller]

Okay, finally got the chance to sit down with it again today. First power on it squealed - volume controls didn't affect it. Went away when I plugged a cable into the effects return, so I'm thinking Greg Robinson is probably right on the money here. I cleaned the jacks with contact cleaner and we'll see if it does it again.

Update: Exercising the return jack caused it again. Reflowed the solder joints on the jacks... seems to have gone away. We'll see.