Fender Princeton '65 Bias (?) Problem

[Jazz P Bass]

Seeing that C22 is being used to block Vdc, if it has failed, a parallel cap will not 'fix' it.
(By paralleling the bias capacitor, as it is being used as a ripple filter, any added capacitance can only help.)
But it may change the symtoms.(leaking less voltage?)
Any change will most probably indicate that C22 is bad.

Unfortunately the only cut & dry answer is to simply replace C22.
(Even then, you may get a 'bad' one. Isn't this fun?)

[nickb]

You could clip through one of the leads of C22 and see if the problem is solved.

[John R Smith]

You could clip through one of the leads of C22 and see if the problem is solved.

Yes, I might as well do just that. I've got nothing to lose, really. The fault has now returned, so it should be a meaningful test. It's just that there is no way of putting C22 back to stock afterwards without removing the PC board, which looks like a total nightmare to me. There are 10 screws and standoffs, 4 ribbon connectors, the bias pot has to come off the chassis, the chassis ground has to be removed, and then there are 8 power transformer connectors as well. Once you've done all that the board is still attached all along the rear edge by the soldered wires to the valve bases, so all you can do is fold it back very carefully to access the underneath. Then I don't see how you can test it without putting it all back together again. The whole thing is just a nightmare for maintenance, I have to say I am really disappointed with this amp, especially for what it cost here in the UK. I thought it would be a proper reissue but it is nothing like the old ones at all when you get inside.

Back in the early 1970s I had a s/h Bandmaster Reverb and that was just a gorgeous amp, I gigged it for years and it went in and out of clubs several times a week. It was never a moment's bother, I never changed a valve and it sounded wonderful till I sold it in the '80s. Over the last ten years I had been playing with a little rehearsal band up the road and borrowing the studio Silverface Deluxe. Again this was a magic little amp and even though it is pretty old it has never been a problem, you just switch it on and go.

I bought this Princeton for my retirement just to have a nice little "proper" valve amp around for the occasional rehearsal or pub gig, and then health issues meant that I could not use it. Now when I finally might be able to play a bit again, the damn amp doesn't work. There you go

Anyhow, none of this is your fault folks, of course. You have been extremely helpful to a novice and I do appreciate that. I will keep you informed of progress (if any).

[Jazz P Bass]

It's just that there is no way of putting C22 back to stock afterwards without removing the PC board, which looks like a total nightmare to me. There are 10 screws and standoffs, 4 ribbon connectors, the bias pot has to come off the chassis, the chassis ground has to be removed, and then there are 8 power transformer connectors as well. Once you've done all that the board is still attached all along the rear edge by the soldered wires to the valve bases, so all you can do is fold it back very carefully to access the underneath. Then I don't see how you can test it without putting it all back together again. The whole thing is just a nightmare for maintenance.

Welcome to the world of amplifier repair.

Actually, it's not that bad getting the board out.
Relax, take your time, grab a cup of joe.
Document all wires that are removed (as you remove them) & what is left will simply flip up.

My advise once you do have it flipped, is to resolder the board.
It takes about 10 minutes but in the long run it will be well invested time.
Being a combo amp & having a single sided printed circuit board, sets up every component to vibrational stess.



As a side note, if you think this amp is a PITA, hah, wait until you pop open a Mesa Boogie Nomad.
Octopuses Garden.