I've recently acquired an old Beyerdynamic BMS85. This comprises a BM85 ribbon head fitted to an S85 wireless transmitter body. The S85 I have is non-functional, and I'd like to modify the unit to be wired instead of wireless.

The BM85 appears to be a dual-element cardioid or hypercardioid ribbon. I'm told it is similar to (or may be the same as) an M500. I've looked at the output of the ribbon elements on a scope, and they both appear to work. One gives phase and the other an anti-phase output, surprisingly. In the head unit, the two element outputs are connected to opposite ends of a 100KOhm (!) pot, with the +ve head output taken from the pot wiper. The common end of the elements is the -ve head output. The pot adjustment hole has a symbol that might be a bass roll-off indicator, but it's not clear.

I have asked Beyer about the BM85, and they sent me the only information they said they had in their archive, which was a .pdf mainly about the S85 wireless part. However, it did have a frequency response curve for the ribbon head showing several bass roll-off curves. The text on the chart is too small to read, but the responses looks very similar to the M500, so they could well be proximity effect variations. Unlike the M500, there's no transformer fitted in the BM85.

My appeal to the ribbon community is for (a) any additional information about the BM85, and (b) whether the M500 had an adjustment potentiometer that varied the proportion of each element's contribution to the output, and, if so, what effect did it have on the sound.

Helpful comments gratefully received.