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Hi, my simple question is - is it possible not all 25K linear trim pots are alike? In my case could the sub I installed be the source of my bias problems? Here's the situation:
I'm having trouble with biasing a Fender amp. I purchased the amp as is and don't know if it biased ok before the trim pot was damaged (it's mounted on the back panel and was banged, breaking the pc board mount off).
It's listed as a 25K linear pot. I couldn't find the exact part so I bought a standard 25K linear pot (ie tone control pot) and installed it. The only apparent difference was the shaft and existence of a brace on the sub part.
I have since found the correct replacement part, but am trying to determine if I should buy and install that before I rip out the main board and try to diagnose/repair the issue.
Advice? AS usual, thanks in advance!
Follow Ups:
for those interested - I found the problem. It isn't the trim pot. There's a dpst switch that changes the output power setting between Full and 1/4 power. One side of that feeds the bias ckt and it had a defect. It was dropping 60Vdc across it. It's a brand new switch too.
So net-net I have the amp up and running now. I still have soem tuning to do, but at least it's getting close. This amp was a mess before I started, so it's come a long way.
Thanks again . . .
Are you sure that the pot you used has a linear taper? The picture that you included does not say so. In fact, if it was intended for use as a tone control, it most likely has a log, not a linear taper.
A log taper would really screw up bias adjustment ..
thanks, It says B25K on the pot, and on the Antique Audio catalogue page where I bought it it says "POT - FENDER ORIGINAL, HOT ROD DELUXE, 25K LINEAR". I thought that "B" indicates linear.
B is the standard code for linear pots.
I believe this is fine.
I've repaired some of these that actually damaged the boards, too. Seems the set nut comes loose and when the pot shaft is hit, the force transfers to the board. I try to use two nuts. One on the inside tighten up against one on the outside of the chassis. Helps keep the shaft from getting loose due to vibration from the amp.
that's a good thought, and since I found another damaged trace on the front edge of the main board I pulled the rear panel board to check it too. Sadly that wasn't the problem, so the cause and solution is still a mystery
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