U3 receiver element low volume

[Charles]

I have a U1 receiver element that was converted to a U3. It is dated 6-11-63 with the U3 painted over the U1. It has the green tic-tac shaped varistor on it.
I am getting low volume out of this U3. When I replace it with a newer receiver element (from my 1991 cortelco 500) the sound is loud and clear.

Can anyone tell me what resistance the green varistor is supposed to provide?
Would a bad varistor give me low volume?
Can I replace this varistor with a resistor?

Thanks

[Phonesrfun]

The varistors rarely give out, but it is not beyond the realm of possibility.  The more likely culprit would be some physical damage to the receiver from dropping.  You have nothing to loose and everything to gain by clipping the varistor out and seeing if the volume comes up to normal.  If not, you just have to get a new receiver, since they were never anything that could be serviced with any reasonable investment of time.  They are relatively easy to come by and cheap.

No, a resistor will not take the place of a varistor, but a couple of back-to-back diodes can reasonably replace the varistor, or just go without, if the receiver works with the varistor snipped out.  It's purpose is only to deaden the loud clicks that we're more prevalent when phones were connected to the older electromechanical switching equipment.

[Charles]

Thanks for the response, I will give that a try.

[Charles]

Bill, clipping out the varistor actually increased the volume. I don't think that it is at full volume, but it is good enough until I get another receiver element.

Thanks

Charles

[Phonesrfun]

Excellent.

If the Varistor was bad, it may be that way because the receiver took a massive "hit" of voltage at some point which could have bent the diaphragm to the point of making it less than full volume.  The varistor is actually a semiconductor which is the equivalent of two diodes in parallel back to back, and they are sensitive to high voltage spikes.  Your average audio voltage going to the receiver would never get to be so loud as to fry a varistor, but if wires got crossed with AC power or if the phone line took a zap from a nearby lightning storm, then that could fry one.