Repair of a microphone

Those that have read the blog entry of my New Years Eve trip to The  Horn on Mount Buffalo may recall that the SOTA component was interrupted by static electricity, a relocation to below the peak and then a failed microphone.  At the time, I was not concerned about the microphone fault, rather I was more interested in packing up and descending the hill due to the increasing loudness and frequency of thunder claps! Clearly storm cells were approaching.

Whilst I was away, I used other radios for Park activations. The weather was hot and very dry, with  most days having a Severe or higher rating. In addition, much of northeast Victoria was shrouded in smoke and this made activating very unattractive. The smoke was coming from several sources: there were uncontrolled bushfires across the eastern ranges of NSW and Victoria, including most of the region around Walwa and Corryong and the ranges to the south, in the areas south of Bright, threatening Mount Hotham and Omeo east towards Whitfield including the Abbeyard and Mount Buffalo fires, a fire near The Bluff, and extensive fires in East Gippsland.

I returned to home on 6 January, simply taking the run down the Hume Highway to Melbourne and then working my way around the road system to head east on the Princes Highway back to Morwell. Smoke haze was a feature of the drive and was dominant for the next couple of weeks.

A couple of weeks later, I decided to investigate the microphone fault……

The microphone is an Elecraft MH3 unit for the KX2 and KX3 transceivers. The KX2 was last used on The Horn as mentioned above, when I started to feel static discharges through the transceiver, which was sitting on my thigh, with me sitting on the ground….. I had quickly switched off the radio after disconnecting the coaxial cable from the antenna connector.

I assembled the radio and connected it to a dummy load. The transmit function was fine on CW. The NH3 microphone keyed the radio into transmit, but speaking into the microphone produced no RF output. Disconnecting the MH3 activates the inbuilt microphone and the radio worked fine on SSB. This confirmed my thoughts – a fault in the microphone itself.

I took out the screws on the microphone and separated the two halves. I examined the circuit board and hunted on the internet for a circuit diagram. Gerard VK2IO sent me a copy of MH3 circuit that was no longer on the Elecraft page – he had downloaded it a while ago when he had an issue with his MH3, described on his blog.

Out with the multimeter. I checked voltages with the MH3 plugged into the radio. The voltage across the electret element was only 0.4 V, when it should have been at least 3 V. I disconnected the connector from the element and the voltage at the pcb side of the connector was fine. My suspicions were confirmed – probably a blown FET in the electret element.

I submitted a query to Elecraft support. A response a couple of days later indicated that they did not repair or service the MH3 microphone and that I could purchase a new microphone for US$69 plus postage.

In the interim, I had started hunting through the various boxes that make up my “Junk Box”. I could not find an elecret microphone element, although I was sure that I had some. I needed some other components and so ordered them plus two elements from RS Australia. Once ordered, the invoice indicated nil stock in Australia and they would take a week to arrive….. I was prepared to wait. I could have tried the electronics parts outlet in Traralgon if I was in a hurry, but they charged a significant premium over items carried by a well known supplier for which they are a “stockist”.

Whilst waiting the delivery, I received an email which meant that I would need to travel to Melbourne. I checked the Rockby website to find a suitable electret element on special for only 20 cents! Even with the minimum purchase quantity of 20 units, I placed an order, indicating that I would pick up the order in a couple of days.

After the trip to Melbourne, I revisited the microphone. I took the element out of its mount. Next was to carefully desolder the SMD capacitor which sat across the element terminals and remove the connecting wires. I took a new element and soldered the capacitor in place and then the connecting wires. I then placed the new element in position and checked voltages when connected to the radio. All looked good, so I tried the microphone as it was – just the pcb in place and no back on the microphone – a little fiddly! RF was produced, so I reassembled the microphone.

I again checked the microphone was working correctly – all OK!

The RS ordered elements arrived several days later.

I now have a working MH3 plus many spare elecret elements, now stored in a labelled drawer of a component storage drawer cabinet. I still need to use the microphone on air, but do not anticipate any significant issues.

Apologies – I neglected to take any photos of the disassembled microphone.

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3 Responses to Repair of a microphone

  1. Shane Loucks says:

    I have the same problem with my MH3. What part number did you use for the replacement mic element?

    • vk3pf says:

      Hello Shane,
      I used RS Stock Number 724-3153. RS PRO Noise Cancelling 9.7mm Microphone Condenser, 100 → 10000 Hz -40dB. Cost of AUD1.80 each.
      Good luck,
      Peter

  2. Shane Loucks says:

    I just dug through my parts tray and found a mic element with specs close to that one. Soldered it up and I’m back in business!! I’m building an order at mouser.com and I’m going to get a couple more.

    Thanks again.
    Shane

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