[quote author="rodabod"]If you disconnect the transformer from the ribbon assembly (only one side) then you can measure the resistance of the ribbon motor.
Just place an accurate digital ohm-meter on each clamp or terminal and compare the resistances. Should be in the range of 0.25 - 2.0 Ohms. It depends on the mic. Some people say this can be dangerous, but it should usually be ok. It is only really dangerous if you test through the transformer because the current gets stepped up.
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Why would you want to do that?
It would not tell you anything except there is an electrical contact, but if it gives at least some signal you already know that.
The DMMs in that range are exceptionally nonlinear and won't tell you if there is some clamps corrosion. But on the other hand, some excessive noise would tell you that immediately.
I remember, there were a few suggestions here from Jacob and PRR as for how to measure ribbon resistance accurately, make a search. I do it on GenRad 1650A bridge.
Andre,
The problem is most likely in the ribbon itself. The duralum these mics were ribboned, unlike aluminum tends to harden with time and lose its compliance. It can look OK, but the first symptom of this problem is LF and output loss.
I'd check the ribbon resonance, the first thing.
Alnico magnets lose their field with time, and should be also checked.
If you have some spare time, you could also try swap transformers between mics, to see if something has changed, although I highly doubt they would be a problem.
As far as I know, nobody anymore re-ribbons B&O with original corrugation. Actually, it would not be possible with use of aluminum foil as a ribbon.
If you cannot find anybody to re-ribbon them, send the mics my way, and I will do it for you. PM me if you are interested.