Yaesu FT-857: No TX Audio Fix

A report of a microphone audio problem with the Yaesu FT-857, by Pete M0PSX

At possibly the most awkward moment possible (a recent training session in front of candidates), my Yaesu FT-857 packed up. The symptom was that it was transmitting carrier on FM, but no audio was being transmitted. Embarrasing, and potentially expensive.

For the event itself, we switched to another radio, and today’s mission was to resolve the problem…

Testing the problem

The first check was to make sure that I hadn’t done anything silly. Was the mic correctly plugged in? Was the Mic Gain (menu option 051 “FM Mic Gain”) correctly set? A ‘yes’ to both. The easiest test was to call on FM, whilst monitoring on a handheld, which quickly confirmed a good carrier, but no audio. I also tried switching the radio’s meter to MOD to see if I was modulating the carrier with audio. Again, nothing – I’d normally expect to see the on-screen graph to move as I spoke.

Testing the modulation on a Yaesu FT-857
Testing the audio modulation on a Yaesu FT-857

Looking for a fix

Fearing an expensive repair bill, I had a look around online, and the first match proved to be very helpful – a video from TRX Bench. This detailed video shows how the video poster (who’s name I couldn’t find) went about diagnosing the problem step-by-step, and eventually traced the problem to a failed capsule in the MH-31 microphone supplied by Yaesu for the FT-857:

After tests, and by putting a meter across the two terminals on the mic element, the video author was able to verify that the capsule was dead. I did likewise, unscrewing the mic’s three screws, and discovered the same failure.

Yaesu MH-31 Microphone and mic element
Yaesu MH-31 Microphone and mic element

The suggested fix on the video involved removing the faulty dynamic mic element and replacing it with an electret condenser mic element – the challenge with this being that the mic element in the MH-31 fist mic is ‘dynamic’, whilst the replacement condenser element requires power. Fortunatly, the fist mic is fed 5V from the radio, so with a minor modication, a powered element can be used.

Improving the mod

More internet research yielded another useful article, this one from John M0UKD (QTH Rainham, Essex) – The modification was not to resolve a faulty mic, but to improve the mic quality for his Yaesu FT-817, which uses the same mic. See the M0UKD  Yaesu MH-31 Electret Condenser Mic Modification

John has tapped off the fistmic 5V supply and used an electret condenser mic element to improve clarity, and also to make use of the switch on the rear of the mic to offer two different settings – one for HF, and one for VHF use.

As I was having to change the faulty element anyway, I thought I’d try John’s mod.

Making the modification

John’s kindly supplied the circuit on his site, and what’s needed is as follows:

  • An Electret condenser mic element (£5 from Maplin)
  • Capacitors: 1nF, 47nF and a 1µF electrolytic
  • Resistor: 8.2k ohm
MH-31 Yaesu Mic element prior to removal
MH-31 Yaesu Mic element prior to removal

The photos on John M0UKD’s site showed perfectly what was needed, and it was a  case of removing the 0.33uF capacitor that’s already there, adding the three capacitors and resistor, and glueing the replacement element in place.

My job’s not as neat as John’s – not helped by the shape of the electrolytic capacitor I had at my disposal, but it’s done the job.

Yaesu MH-31 Mic Repairs Pic 1
Yaesu MH-31 Mic Repairs Pic 1
Yaesu MH-31 Mic Repairs Pic 2
Yaesu MH-31 Mic Repairs Pic 2

Job done, and will be neatened up when I can find a better capacitor. I put out a test call this evening, and worked Steve G8UDD, who at first reported a punchy, but narrow signal, akin to a phone line. Disappointed, I flipped the switch on the back of the mic, and Steve reported a perfect signal. That’s allowed me to work out which position the HF / VHF switch lives in! As a result, a better sounding mic, hopefully with more oomph for calling on HF SSB.

All in all, a success. Rather than buy a new mic at £35, or send off the radio for repair, that was a £5.50 fix, and has given me what sounds like a better and more punchy mic. A win-win!

Thanks to the unnamed video author and to John M0UKD for the great online material that’s got me back on-air. No thanks to Yaesu for the duff dynamic!

Pete M0PSX

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