Squealing Microphones: Hints & Tips

The following advice is from our South African dealer, Peter How, regarding
Squealing Mikes.

I have experienced some problems in different installations, mostly due to
my own fault, but here are a few suggestions and tests:

1. Don't plug in any mic at all. Just plug in a headphone. If it squeals on
the other receiver during transmit, it can't be the mic.

2. If you are using Microair's amplifier in the mic line, for a low level
dynamic mic as in a glider, try disconnecting that. It might not be stable
during transmit.

3. When the mic is plugged back in, if the squeal changes amplitude and
frequency slightly as you move the mic around, it is usually a sign that the
RF is getting into the mic or mic amp.

4. I initially used a diode from the co-pilot's mic to the PTT line in order
to kill the co-pilot's mic during transmit. It squealed sometimes, until I
bypassed the diode with a 1000pf ceramic cap.

5. Some makes of headsets can be very bad with RF getting into the mic. One
type of Telex headset is bad. We had one type of David Clarke which was bad.
Early LightSpeeds were not good, although new ones are OK. My experience is
that when they squeal there is not much that one can do. Its usually the
amplifier for the electric mic which is the culprit. Try another make of
mic. Pilot Communication headsets generally seem to be OK.

6. If you eventually decide that it is the electret mic capsule or the
amplifier, try a 330pf disc ceramic cap over the electret capsule. In
desperation I have fitted a 22000pf disc ceramic cap over the mic jack, but
it might alter the audio frequency response a bit.

7. One way to test if it is RF getting into the mic, is to transmit into a
50 ohm dummy load. There will be almost no radiated RF energy, so if the
sidetone still squeals, then it can't be RF problems into the mics. In any
case its best to place the antenna at least 4 or 5 feet from the mic.

8. If the mic gain and sidetone is turned up too far, you may get squealing
from audio feedback at transmit, from the earphone output to the mic. This
can happen if one headset is being worn, and the other headset is plugged
in, but just lying on the bench. In practice, the mic gain and side tone
pots should only need to be at about 20%.

9. Have you followed the wiring suggestions exactly? Have you used a single
core screened cable for each mic input, with the screen terminated at pin 2?
All other grounds, eg headphone and PTT are to be brought back separately to
the power supply ground pin on the radio.

10. Have you checked the SWR of the antenna? Is it lower than say 2:1

11. Check the voltage drop at the power supply connections during transmit.
You must ensure that the meter is not sensitive to RF energy, or you can get
false readings. I had a client who used a 1 amp fuse which did not blow at
transmit, but which dropped the voltage down to 8 volts during transmit. The
radio made a loud noise when transmitting. Replacing with a 3 amp fuse fixed
it. A voltage drop of more than about 0,5v at the connector during transmit
is not really acceptable.

12. Check that you have not accidentally reversed the pair of wires
connecting the phone jack. Sometimes it may work OK, but when you mount both
headphone and mic jacks on the same metal plate, you short the audio systems
out, and perhaps this might cause a strange fault.

13. Have you played around with the modulation pot? Its best not to, unless
you can measure the modulation depth properly.

14. If you have a Rotax motor, it must be fitted with a proper regulator
which limits the voltage to 14v and this must be connected to a large
battery, to help filter the ripple. Then you also need at least 10000 mfd
connected across the supply, within a foot or two of the radio, to filter
out the residual ripple or it will be heard during transmit as a low level
whine. That's all I can think of. Let me know what you find. Its all good
experience. They are good radios. Its usually an application problem that
catches me out.