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smuprof
Intermediate Member
Username: smuprof

Post Number: 150
Registered: 12-2003
Posted on Saturday, March 02, 2013 - 4:43 pm:   Edit Post

I've got an F2-B I bought second-hand several years ago with a problem I haven't been able to resolve. Channel B will intermittently become very distorted/static-y.

It is isolated to Channel B, so it can't be the power supply. It shows up whether I use the Channel B output or the mono output. I've cleaned the jacks thoroughly - still have the problem. I've swapped the tubes - the problem doesn't follow, so it's not the tubes.

I'm down to the signal path / tone stack. Looking at the manual / schematic, it looks like I could literally jumper pin 6 of the tube to pin 2 of the tube and bypass the volume and tone stack (plate of the first triode amplifier to the grid of the second triode amplifier in the 12AX7 tube).

If I can do that and it solves my problem, I'm down to trouble shooting caps and pots.

Any thoughts? Can I do this without damaging my F2B?
gtrguy
Senior Member
Username: gtrguy

Post Number: 590
Registered: 9-2004
Posted on Saturday, March 02, 2013 - 9:33 pm:   Edit Post

Have you cleaned the contact points in all the jacks?
smuprof
Intermediate Member
Username: smuprof

Post Number: 151
Registered: 12-2003
Posted on Sunday, March 03, 2013 - 4:42 am:   Edit Post

Yes, I've cleaned the jacks/contact points twice.
gtrguy
Senior Member
Username: gtrguy

Post Number: 591
Registered: 9-2004
Posted on Sunday, March 03, 2013 - 10:21 am:   Edit Post

I don't remember if the pots on this unit are sealed or not but I would spray some pot cleaner/lube into the pots next (if they are not the sealed type). Use the good stuff and not just the stuff that cleans only. Then spin them around some.
smuprof
Intermediate Member
Username: smuprof

Post Number: 152
Registered: 12-2003
Posted on Sunday, March 03, 2013 - 1:48 pm:   Edit Post

Thanks, David, but they're sealed. Pulled them out yesterday just to confirm and no place to put any cleaner.

Doesn't mean one of them isn't the problem - hence my original question about bypassing the entire stack.
gtrguy
Senior Member
Username: gtrguy

Post Number: 592
Registered: 9-2004
Posted on Sunday, March 03, 2013 - 3:07 pm:   Edit Post

You should be able to, on old Fender amps you can often just lift a single resistor at one end and that will do it.
slawie
Senior Member
Username: slawie

Post Number: 478
Registered: 8-2002
Posted on Wednesday, March 06, 2013 - 12:02 pm:   Edit Post

If its an old unit that has been sitting around for a while I recommend replacing the capacitors. The large blue ones. The electrolyte in the caps deteriorates after time.

slawie
jimmyj
Senior Member
Username: jimmyj

Post Number: 435
Registered: 8-2008
Posted on Wednesday, March 06, 2013 - 4:33 pm:   Edit Post

Sealed pots also need to be exercised to stay fit (just like we do). Seriously, rotate the pots 50-75 times through their their full sweep and see if that helps. Beyond that, I think you know more about circuitry than I do... Electrolytic caps age and fail but I was under the impression that they are mostly used in the power supply section so, i dunno!

Jimmy J
terryc
Senior Member
Username: terryc

Post Number: 2066
Registered: 11-2004
Posted on Friday, March 08, 2013 - 1:11 am:   Edit Post

Personally I would send it to Alembic and let them fathom out the problem
gtrguy
Senior Member
Username: gtrguy

Post Number: 593
Registered: 9-2004
Posted on Friday, March 08, 2013 - 2:42 pm:   Edit Post

Probably a pot.
smuprof
Intermediate Member
Username: smuprof

Post Number: 153
Registered: 12-2003
Posted on Thursday, March 14, 2013 - 7:17 pm:   Edit Post

Well, interesting solution. I took Jimmy's advice and exercised the heck out of the pots - a modest improvement, but still the interimittent static/distortion, and even when no distortion, definitely a different tone between the two channels when set the same.

After a little homework, took it in the David Smith at Dabeck Custom Amps in Richardson, TX. Very nice guy, very knowledgeable. Tossed it on the bench while I was there, identified a broken solder joint on one side of one cap in the tone stack (so I wasn't far off, but it took a jeweler's loupe to find it).

He kept it for the afternoon, resoldered everything just to make sure, found a couple of weak filter caps in the power supply, repaced those, and went ahead and put in new tubes.

Bottom-line: as good as new, $150 for everything, picked it up the same day. I highly recommend David's work. He builds custom amps based on the original Fender circuits also. It's nice to find someone who takes pride in his work, knows what he's talking about, and appreciates sound tube design. He commented when I left what a nice piece my 1984 F2B was and I should enjoy it for a long time. (Hard to believe it's nearly 30 years old now).

Anyway, thought you'd enjoy hearing "the rest of the story" to borrow from Paul Harvey.
jimmyj
Senior Member
Username: jimmyj

Post Number: 437
Registered: 8-2008
Posted on Thursday, March 14, 2013 - 10:51 pm:   Edit Post

John,

Thanks for the update. You were definitely on the right track - good logical circuit fault tracing! And great that you found the exact guy to do the repair and tune-up for you.

The build quality on Alembic gear has always been outstanding. This stuff is built to last. 30 years ain't nothin'. Ha!

Jimmy J

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