Rambler readers may recall my mobile HF radio install from last month’s issue where I installed my Yaesu FT-100D mobile transceiver and ATAS-120A antenna in my Ford F-150.  When testing the radio after the install, I encountered a problem with the radio microphone.  When I tried to transmit using the mic PTT switch, the radio “chattered”, switching rapidly between transmit and receive.  I had once previously encountered that with another radio when the battery voltage was low, but in this case it was a different problem.

After some elementary fault finding with a continuity tester I diagnosed a faulty microswitch inside the mic.  Before ordering a replacement mic or microswitch I took a shot and repairing it and found that it was just a build up of surface corrosion on the mic switch contacts that was causing the issue.

A quick squirt of “DeoxIT D5” and a scrub with a Q-tip cleaned up the contacts and returned the mic to normal operational use.  Here’s some pictures to show the repair.

With the microswitch enclosure opened, the spring contacts can be seen.  The interaction of the straight and semi-circular metal strips gives that satisfying “click” when making and breaking the contact, just like a computer mouse button.  The left most end of the semi-circular strip (see arrow in photo) is what makes contact with the flat strip, to put the transceiver into Tx.  It was this contact area that I cleaned well with DeoxIT.  It is a surprisingly small contact area.

Once cleaned and reassembled I was happy to see that the PTT switch was working correctly again.  I am expecting to give the mic PTT switch a good work out on a road trip to the Rocky Mountains in July.

Rob, VE3RXH
Appears in: Rambler Vol.66 Issue 9