08 December 2022

Behringer BX1200 Ultrabass

A friend's son plays bass. He was given a Behringer BX1200 Ultrabass combo amplifier with some problems, and he asked me for help.

The amplified bass was distorted, but using a back panel input the output was ok. Then the fuse blew up and that was the end of their troubleshooting. So they brought it over together with a schematic diagram found online (revision H in our case), which is very helpful.

At the first glance I noticed something weird at the front input sockets, that I had to rebuild following the schematic. Actually I wired it for a passive instrument because someone had removed the switching sockets in favour of simple 3-terminal ones, and the boy has a passive bass. It's a compromise I will document.

I went all the way to remove the large heatsink to check capacitors and PSU voltages, that were within specs. Hint: don't touch this area if voltages are correct. You can pick voltages across the diodes. I added the screws that secure 7815 and 7915 to the heat-sink since who came before me had forgotten them.

This gave me a working BX1200 that still needed a new master volume potentiometer. The diagram just mentions 50kB. The specific part is marked ALPS. Digging through the Alpine technical documentation I came up with these specifications:

Alpine ALPS 50k 30mm (total height) 3 pin, vertical, with collar, flat shaft, without detent, shaft length 25 mm. The closest part I could get is RK09D1130A1L, giving up the logarithmic response. We bought an Alpine part but there are alternatives too. In 2022 money one of these Alpine potentiometers costs 2€/US$.

If you have to replace the master volume pot chop off its pins and then desolder and remove what is left on the board: you will not be reusing that pot anyway! Remove all solder leftovers or the new one will not enter the holes.

That was an easy fix, at last!