Fender 'Twin Amp'

[Tom Roberts]

Having had trouble with fuses blowing on my Fender Twin Amp (c 2002), I finally took it to a tech. He replaced a resistor and chanrged me £80.00! On replacing the chassis back in the cab and connecting everything up there was a noise like the effect of distant thunder on a radio set. I removed the chassis and noticed small brown pimples on one of the group of 4 (piggyback) elecrlrolytic caps rated @ 22uF/500vdc and decided to replace all 9 - 4 of which are 47uF/350vdc. Also, the power transformer was so hot that it could not be touched by hand.
Problem! Do I have to remove the large PCB to get to the underside to de-solder the connections or can I simply trim the existing positive and negative connections from above and solder the new caps on to the stumps? The new caps are F&Ts - made in Germany and will replace the existing Illinois ones but are the same size and rating.

Regards Tom

[Jazz P Bass]

Fender Twin Amp

Exactly which amp do you have?
Fender "The Twin"?
Here is the schematic.
As far as I can surmise, those capacitors are on the B+high voltage rail.
You may just have a bad capacitor or something is amiss on the B+ circuit.
Before doing anything, remove the tubes. All of them. Check the B+ Vdc.
With the amp off, one at a time install the tubes, from V1 preamp to the power tubes. One at a time power up the amp while monitoring the B+ Vdc.
Chances are you will find one of the tubes is drawing down the B+ Vdc.
That will be your bad tube.
Yes, you will have to remove the board to properly change out the capacitors.
It would be nice to know which resistor the tech replaced.
Not to knock the tech, but he missed something, big time!

[Alex R]

Yep, check the valves first. You could try just replacing them with a single new one, one at a time, no need even to take the chassis out to do that.

I must admit that when a board is particularly hard to remove I have sometimes been reduced to clipping the old part out and crimping/soldering the new part in. It's not pretty but it works. If there's a risk of rewiring the board back in wrongly, or of doing some damage, then I would go for it. I see old repairs done that way in amps all the time, it's just not best practice, obviously. Beware of the heat generated by large resistors - that can melt the solder on your bodged joints - so don't go too close to the body of the part.