[Flexradio] RF-feedback to microphone preamplifier via Heil AD-1 Y adapter

[Tom Brown]

Setup: FLEX-5000A with ATU, Heil Pro-Set Plus and PTT foot-pedal connected
with AD-1 Y adapter.RF-grounded & bonded station, new tested RF-choked coax
to DX-CC multiband dipole 40' up.

Problem: Gross audio distortion or "ALC-motorboating" when transmitting SSB
audio on certain bands (20 meters in my case).

Analysis and solutions: The AD-1 Y (recommended by Heil Sound for Pro
headsets with FLEX-5000s) has some RFI-engineering flaws that couple even
low levels of RF (which this analysis also supports) into the microphone
preamplifier inputs, hence detection and feedback. If you have or suspect
similar problems, even minor audio distortion, try a jumper wire from the
PTT (TS) connector "Sleeve" to FLEX-5000 chassis ground (front-panel
socket). And of course, if you have the option then why not use the separate
(RCA) PTT connector on the rear panel?

For analysis details, more permanent repair options and some cool references
see http://tinyurl.com/4ld8kf (Note: IE 7 does not fully render the
schematic diagrams but you may guess what's omitted. Google Chrome and
Firefox 3.0.1 have no problem.)

I am indebted to Dudley Hurry (FlexRadio Technical Support) for the
suggestion to chassis-ground the microphone-cable shield and to leave PIN 7
(MIC- input to the balanced preamplifier) unconnected, a key part of the
more permanent AD-1 modification described.

-Tom Brown, KG6RRU