Blues Jr oscillation-

[drewl]

Wow, didn't know this was such a problem with these amps.
I don't think I've ever had one in that had this problem....- until now -

oscillation comes right off the first preamp tube stage, pull the 2nd tube and it's gone.
Volume and treble pots have the largest effect on the osc.
Plugging a cable in looks like it kills the osc.
Aside from that amp is working properly, I already reflowed the pc board and retensioned the pot housings, everything looks good and normal except for the oscillation.

Any fixes for the osc. at this stage?
if not, I'll start by removing/rewiring the ribbon cable

Thanks.

[Mark Black]

Before reworking the ribbon cable (which can lead to unintended consequences in my experience) try moving the output transformer wiring relative to the ribbon cable.

It's been a while, but I recall being able to find a "sweet spot" with the wire routing that would pretty much eliminate the oscillation, and it didn't take much movement to make a big difference for better or worse.

[Mark Black]

After doing a search on "blues jr oscillation" (without quotes) here on the forum I was reminded that it is particularly the blue OT primary wire that is the usual offender. In a couple of those posts Enzo recommended routing that wire as close to 90 degrees as possible relative to the ribbon cable.

Just to add to the fun the oscillation can change depending on whether the back panel is on or off. I remember monitoring those on a scope and seeing all kinds of very high frequency oscillation coming & going based on where my hand was relative to the amp - kinda like a Theremin...

[drewl]

I don't even have the power tubes installed so I don't think it's coming from the output tranny wires.
Even with the phase inverter tube removed the oscillation is still there.
I'm getting negative voltage on V2 input grid when the osc. is present.
I know I've seen this before and can't remember the cause.

Edit: after reading through some other posts on this problem, it may be the filter cap for this node.
It's got some "gunk" on the positive lead I thought was melted glue that may be cap goo, I know I've seen this happen on the Hot Rod series.
Let you know in a bit.

[Mark Black]

Ah - OK - that certainly changes things.

Sounds like you might be on the right track with that cap. In any case good luck!

[drewl]

Pulled it out, 22uF cap is measuring 22pf.....not much filtering going on there.
Have to wait until I get home to swap it out.

[woodyc]

You probably know this, but the tube sockets in those amps tend to loosen up and cause several problems including intermittent hum, oscillation, and extreme microphonics. I used to retension them, but they'd come back a year or two later, so now I just replace them with a ceramic type.

[52 Bill]

You probably know this, but the tube sockets in those amps tend to loosen up and cause several problems including intermittent hum, oscillation, and extreme microphonics. I used to retension them, but they'd come back a year or two later, so now I just replace them with a ceramic type.

There was a run of bad sockets that were made from the wrong type of metal, so the contacts would loosen very easily. They were black plastic sockets. Fender has since replaced them and are using tan or light brown sockets that do not have the loosening contact problem.

Most of the lead dress problems with these amps have been worked out through the years, so the first thing that I test is the preamp filter cap. Just clip a new one across the existing one and if the cap is bad the oscillation will go away.

[g1]

You said plugging in a cable killed the oscillation, could you clarify? Open circuit, grounded or what? Just curious, thanks.

[drewl]

That was a false conclusion, the filter cap caused the problem.
From all the other work I did, and bending the ribbon cables up it's really quiet now.

[woodyc]

There was a run of bad sockets that were made from the wrong type of metal, so the contacts would loosen very easily. They were black plastic sockets. Fender has since replaced them and are using tan or light brown sockets that do not have the loosening contact problem...

Thanks for the info. It must have been a long run. Sorry to continue with this tangent, but I was working on another blues jr this morning and took a pic -- the bum sockets are on the left...

[Enzo]

I have replaced quite a few Blues Junior sockets over the years under warranty. Never really noticed what color they were.

[okabass]

I’d like to thank you for the tip of bad PSU filter lyte. I was in real trouble with all kind of strange oscillations. Then I put a fresh lyte to C10, and everything works beautifully now.
Thank you very much.

[xtian]

While we're here, I'll add one thing. Years ago I had a BJ with oscillation that was driving me nuts. In the end I found that it oscillated only with the back panel removed. With panel installed, no oscillation.

[Helmholtz]

While we're here, I'll add one thing. Years ago I had a BJ with oscillation that was driving me nuts. In the end I found that it oscillated only with the back panel removed. With panel installed, no oscillation.

I guess the back panel has some kind of shielding foil on the inside which covers the chassis?