Yaesu MH-31 Microphone Failure/Fix

Problem: No transmit audio on the FT-857D tranceiver.  FM/AM/CW produced a carrier.

mh31a.PNG       mh31cart.PNG

RFO: Dynamic microphone cartridge failure.

Detecting failure.  Besides no transmit audio, measurement of the resistance between the two cartridge leads enabled diagnosis.  The cartridge should have with a normal resistance of between 200 to 600 ohms.  Instead the faulty cartridge presented an open (infinite) resistance as measured by a DVM

.

mh31schema.PNG

This was a bad dynamic mic cartridge that checked out with infinite R instead of 200 – 600 R. Given the mic was dead, seemed like a no-brainer to do the electret mic conversion since I had the parts.  Used the electret mic cartidge that came with the uBITX and followed m0ukd’s schematic. The utube is excellent with a very similar solution. QSO on 40m confirmed decent sound on the FT-857D with the new refurbished microphone.

Performed this mod with parts on hand , an electret mic cartridge from the uBITX project and a couple of caps on hand (47nf and 1 nf).

It’s important that there be capacitors in the signal path to block the +5v dc voltage.  I took some pains to insure there were no shorts since 5v would be bad news for a mic pre-amp or other circuitry downstream that is not expecting it.

yaesu-mh-31-electret-condenser-mic-modification.jpg

The microphone now works and reportedly sounds good.

 

 

~ by marksun on February 17, 2019.

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