A friend of mine has a B-52 Matrix 200 PA system he uses for small events and the volume pot busted. Adjusting the master volume would result in intermittent sound on, often way more on than wanted. Pity, as it’s a decent sounding rig for a small to mid-size room.
In this case the master volume pot seems to have failed internally – probably from getting smacked at some point, but according to the internet these units seem to have various other quality issues as well. Rather to be expected with this bad a solder job – pretty crappy wetting on most joints:
Anyway, nobody on the internet seems to have fixed this particular problem but it’s pretty simple: the Panasonic EVJ-Y15F03A54 is a drop-in replacement for the obscurely branded OEM part. The EVJ-Y00 and EVJ-Y10 series should also work – the Y10 is actually the exact match – but DigiKey didn’t stock it and you don’t need the threaded bushing in this application anyway.
Forgot to take good photos of the tear-down, but if you’ve been into gear before it should be pretty straight-forward:
- Remove the amp chassis from the case – screws on top (in the handle recess) and on the bottom.
- Unplug the ribbon cable to the DSP (effects) board
- Remove the six screws holding the edges of the front panel to the main chassis.
- Figure out how to lay the front panel on the workbench without yanking wires too much (or unplug the wires from inaccessible locations on the power board in the chassis – make sure you label which goes where, they’re both the same size plug)
- Take out all the screws on the front panel and remove the nuts from all the jacks – the front panel will lift off the circuit board so you can access the volume pot.
- Desolder the old pot (don’t overheat like I did and lift traces)
- Solder in the new pot
- Reassemble in reverse order
If you can’t get the Panasonic for some reason it should be possible to use a standard dual 50K audio taper pot, as long as it will fit between the front panel and the circuit board. (Measure carefully!) Make careful note of the pinout in the Panasonic datasheet – it’s not what I would have expected. I did a quick trace back to the closest op-amp to confirm pinouts were identical, here’s the schematic:
The op-amps are standard JRC/NJM4558s but in SIP packages – took me a few minutes to find the SIP pinout.
So that’s pretty much it. It took me longer to find the replacement pot than it did to take the unit apart and replace it, and that was nearly more time with a screwdriver than the soldering iron. I also need to work on my desoldering skills, they ain’t what I thought they were.