Blues Jr issue

[daz]

When you turn the volume up at some point it goes into that theremin routine....you know, like a whistle that changes tone as you turn it up. So you can actually use the volume knob to play a tune with nothing plugged in. It changes a bit if you turn the bass or mids, but the treble will eliminate it altogether if turned fully down unlike the bass or mids. I swapped all tube first thing even tho i figured it's a signal cap. It's not a PSU cap i don't think, and i replaced 2 of them that were looking suspect due to leakage. Tried freezing all suspect caps with varied results. Replaced the treble cap because thta one was affected big time when i froze it. Seems to be before the master. Any thoughts on how to troubleshoot this and whether anything besides a cap might be worth looking at? By the way, if you have a schematic, there are several versions that are considerably different and this one is rev A with the volume control labeled R5 instead of R7 like the rest.

EDIT; lifted the resistor that takes the output of the treble pot into the next stage and took the treble pot output to a PA and got normal sound into the PA. So it's at V2 or beyond. Hopefully someone will have a suggestion, but if not i suppose eventually i'll find it. But too much time spent already so if anyone has any ideas at all.....

[Enzo]

Why do you think it is a cap? You have feedback, which is oscillation.

Look at the primary wires of the OT where they plug onto the power tube socket board. Seems to me there is a ribbon cable and one of the wires (blue?) crosses the ribbon. MAke that crossing as close to 90- degrees as possible.

There are other ribbons to the preamp tubes, try moving the ribbon cables and see if it changes anything.

When you lifted the resistor, you broke a loop. It may be after V2, but don't rule out something earlier.

[daz]

Why do you think it is a cap? You have feedback, which is oscillation.

Look at the primary wires of the OT where they plug onto the power tube socket board. Seems to me there is a ribbon cable and one of the wires (blue?) crosses the ribbon. MAke that crossing as close to 90- degrees as possible.

There are other ribbons to the preamp tubes, try moving the ribbon cables and see if it changes anything.

When you lifted the resistor, you broke a loop. It may be after V2, but don't rule out something earlier.

Well, i just always relate the sound of a changing tone that goes up in pitch after the amp starts warming up with a cap, so i just initially thought that i guess. I don't think it's a cable because it just started doing this out of the blue, plus i had the board hanging out of the chassis with all the cables having been bent way out of place and nothing changed. Also tried chopsticking them while the tone was happening and no change.

To give you a better idea of what it sounds like, imagine holding a key down on a digi keyboard and letting it sustain infinately with a flute like sound and then start pushing the pitch wheel up and down. Thats what it's like turning the volume knob up and down or the tone knobs.

[52 Bill]

Check all of the filter caps, I've seen bad first stage filter caps cause oscillations like this. Try paralleling a good one across the existing one and see if the oscillation stops.

[daz]

Check all of the filter caps, I've seen bad first stage filter caps cause oscillations like this. Try paralleling a good one across the existing one and see if the oscillation stops.

I did. I replaced 2 then lifted the others with no change.

[lowell]

Have you tried new preamp tubes? What if you ground the input with a grounded dummy plug? Just a thought. Does it go away with no PI installed? I had amps oscillate from tubes that were biased too hot, but not redplating, and had become microphonic too. So just to be thorough check the power tube current draw too.

[Chuck H]

I did. I replaced 2 then lifted the others with no change.

Do you mean to say that you disconnected the filters from the circuit? That's not the same as paralleling a good cap across them. Which is a good thing to try.

[soundguruman]

It's very often a broken connection on a control. A bad solder connection, or cracked connection.
Or a broken connection on the input jack.
The BJ has flimsy weak controls, that are not attached to the chassis with hardware, they break frequently.
The internal parts on the controls break all the time. The solder connections on the jack break all the time.
Almost everyone I work on has at least 2 defect controls, and a broken input jack...
It's that, almost every time.

[daz]

Have you tried new preamp tubes? What if you ground the input with a grounded dummy plug? Just a thought. Does it go away with no PI installed? I had amps oscillate from tubes that were biased too hot, but not redplating, and had become microphonic too. So just to be thorough check the power tube current draw too.

Current draw is fine, even a bit low, all tubes replaced, and it stops with ANY tube pulled. Also does it with or w/o a plug/guitar in the input

Do you mean to say that you disconnected the filters from the circuit? That's not the same as paralleling a good cap across them. Which is a good thing to try.

Yes, because i had no others to try anywhere near the voltage. I may try that later but i'll have to go all over town to find values like that.

It's very often a broken connection on a control. A bad solder connection, or cracked connection.
Or a broken connection on the input jack.
The BJ has flimsy weak controls, that are not attached to the chassis with hardware, they break frequently.
The internal parts on the controls break all the time. The solder connections on the jack break all the time.
Almost everyone I work on has at least 2 defect controls, and a broken input jack...
It's that, almost every time.

I'm real familiar with blues jr's and i know that well. And yes, i checked them all.

[daz]

I'll be damned Chuck...you nailed it ! I DID check the filters as one of my first things and as i said even replaced 2, but had no more to replace so i just clipped the remaining 2 to no avail. I figured if they are out of circuit it would stop if they were responsible. I didn't stop to think the reason one may be causing it was that it might be open which i guess was the case because why else would clipping the leads do nothing. In any case, i stole a couple from another amp, not the same values but the voltages were high enough and sure enough it stopped ! Thanks Chuck, and thanks to all who replied.

[Chuck H]

I have to concede this one to Bill. It was his diagnosis first. I was just policing the trouble shooting method.

[stokes]

I've seen way too many of these and the Devilles,and Hot Rods with exactly the same oscillation.Each case turned out to be a filter cap in the preamp stage.I think Fender had a big batch of bad filters,way too much of a coincidence in my experience.Clip a good cap across each filter one by one,it'll stop when you get to the bad one.Then change all of them.

[stokes]

Sorry,Bill,I shoulda read all the responses before chiming in.

[52 Bill]

Like I said, I've seen this happen before, especially those tiny Illinois brand 20uF caps. I don't know if it was a bad run of caps or not, but I've done maybe 15-20 warranty replacements on these.

Paralleling a known good cap is a quick, easy test to find a cap that has lost value and is causing hum or has increased in ESR and is no longer decoupling the power supply.

[Chuck H]

The thing that strikes me right off is that the cap failure always seems to result in oscillation. Obviously this is because there are equal phase signals that are not being decoupled. A failing cap is a failing cap. And should be replaced. But I always try to decouple anti phase stages at filter nodes. After all, there is ESR/impedance that is measureable in electros of high value. And modern guitar amps do amplify some tens of thousands of times for clipped tones. It just seems like Fender missed on this one. With both the filter arrangement and the cap QC.