Long time problem with old amp yields interesting new clue

[peteb]

This is a mostly stock 65 champ.

A long time ago it would play fine and then when it warmed up it suddenly would play not so well, less output, a strong chord could bring it back, like it was a connection problem. I temporarily helped it by replacing the RCA jack with a 1/4 inch phone plug. This has been going on for many years. Another symptom, when it warmed up it would sound like radio waves, but it was actually a flatulence kind of sound at various frequencies.


Now the amp plays fine for a little while and then gets staticky. It quickly becomes unplayable. I had done the chopstick test some time back and found no problems that way.


What I just found out yesterday is that if the champ is directly on a concrete slab, the amp sounds good and delays the static problem for quite a while, close to an hour, where as if the amp is on vinyl flooring on top of the concrete slab, the static problem comes on very quickly, in just few minutes.


I will measure the chassis voltage as it warms up in both places to make sure something obviously dangerous is not happening. And I guess I will check the AC and DC voltages at the old filter caps, and see if anything has changed there compared to when I have checked in the past. The original filter caps are obvious suspects, but is this problem because of the them?

[telex76]

Sounds like it could be the caps. They need replaced whatever else is wrong.

[Paul G.]

I had what sounds like the same symptoms on a Princeton Reverb once. I finally tracked it to a bad connection on the bias circuit. Was fine cold and a loud chord would jerk it back to full output.

I wasn't losing signal, but bias was unstable. Took a while for me to find it after 2 techs couldn't.

P.

[printer2]

Originally Posted by Paul G.

I had what sounds like the same symptoms on a Princeton Reverb once. I finally tracked it to a bad connection on the bias circuit. Was fine cold and a loud chord would jerk it back to full output.

I wasn't losing signal, but bias was unstable. Took a while for me to find it after 2 techs couldn't.

P.

Intermittent problems are a bitch.

[peteb]

Because Paul had the same symptom with a bad connection in the bias circuit, I will hit the solder joints on the main bypass bias cap just to make sure that isn't the problem.


Thanks for the idea.

[jmiles]

" where as if the amp is on vinyl flooring on top of the concrete slab, the static problem comes on very quickly, in just few minutes."

Stop playing guitar in your bathroom! ;o)

[Ringo]

stock caps in a 65 Fender amp? you're running way way past borrowed time IMO. I'd bet that is the problem, even if it isn't you should change them as preventative maintainence.

[peteb]

Resoldering both connections on the bias cap seems to have solved this old problem for now. Thank you, that helped a lot.

I was not able to replicate that moving the amp to the slab once it was staticky could cure it as it had last night.


The biggest sign that the filter caps are more than ready for replacement, that I see, is there is around 7 volts AC on the chassis. My other old champ with old filter caps only has 1.5 volts AC on the chassis.

I most likely will replace the filter caps.

[Capn Rory]

Just a guess ... the eyelet fiberboard may be shorting to the chassis at times. They tend to absorb moisture and warp, also conducting parts of the circuit incorrectly.

[Middleman]

I have distortion in an old Vibrolux that only manifests itself on a G note. Otherwise the amp is clean as a whistle. Go figure. Not a tube rattle but distortion.

[jmiles]

"I have distortion in an old Vibrolux that only manifests itself on a G note. "

I have a buddy with a BF Showman. It does that same thing, but on F#. In his case, it's the speaker.

"the eyelet fiberboard may be shorting to the chassis at times. They tend to absorb moisture and warp, also conducting parts of the circuit incorrectly."

The eyelet board can also become conductive. I have watched my tech scrub them with a brush and, I think, pot cleaner spray.

[peteb]

Would the conductive circuit board occur after the amp warms up, like mine always does?


Resoldering the bias cap really did seem to help, but I was only able to play about 15 minutes.

When I was able to play longer, the static was back in only 5 minutes, back to how it was.

That's the way this problem has been, something may seem to help or fix it but it always returns. Would that be the ever worsening filter caps? I am going to replace them. This static is on the low notes. I don't think of static being a filter cap problem but that it's on the low notes does make me believe it could be a filter cap problem.

[BobbyZ]

Do the electrolic caps and see if that clears it up.
If not when it's acting up check voltages. Could be as simple a bad resister.

[peteb]

The cap can and big soldering iron are ordered and on the way.


I don't usually change something out until I can see that there is a problem, not only are there 7 volts AC on the chassis but the plate voltage is down about 15 VDC from low 400s to the 380s, ironically bringing the amp more in spec.


I really do have an interesting clue now. This old champ plays fine for extended periods, have verified for up to the time to consume five of my favorite beverages, if the amp is OUTSIDE of the chassis????

Has anyone heard of an amp that only plays fine if it is outside of the chassis? I almost think I have heard of that but I am not sure.


The reason I thought I had success by resoldering two connections is that when I tested it, it was on the bench, outside of the chassis.


This is some sort of heat issue because the problems only show up after the amp has warmed up, and now I believe the amp in the cab affects this by keeping heat in the chassis.


With it on the bench and testing and playing I tried an alternative to the chopstick test. Because the problem is heat related I was looking for heat in the chassis so I touched all of the components, avoiding touching all leads and wires. I found no heat but when I touched either of the brown turd caps in the tone stack there was a disruption to the sound signal. I will try resoldering those connections.

[jmiles]

Try this stuff!

[peteb]

What do you do with that stuff?

[jmiles]

"Operate equipment until problem appears. While equipment is still operating, spray suspected component until frost appears. If operation returns to normal, replace that component."

For heat-related problems. Follow all precautions when using it, cuz it's nasty to breath! But works quite well!

[Green Lantern]

You can try re-tentioning your tube sockets. You may not have a good connection with one of the tubes there. Cleaning them couldn't hurt, either.

[peteb]

Resoldering the joint where the brown turd tone caps meet really has fixed it and all of the old problems have gone away, tested for long time outside of the cab and now even in the cab works again.

Apparently the outside of the cab fix crutched it along somehow alleviating the problem.

What I learned on the chop stick test is that before I was using a chopstick to poke at the leads where this time I nudged the bodies of the components.


Thanks for the help and replies.



Soon I will replace the cap can and report back what improvements it makes.

[peteb]

That's interesting jmiles