dean.psiropoulos(at)veriz
Guest
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 Posted: Mon Sep 21, 2009 9:17 pm Post subject: Transmit problem on both comm radios. |
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Hi all:
Maybe you can help with my transmit problems. I have a UPS Aviation
Technologies (now Garmin AT) radio stack with SL-30 Nav Comm, GX-65
GPS/Comm, SL-70 transponder and SL-10MS audio panel (actually a PS-6000 with
UPS AT nameplate). I checked out everything according to the installation
manuals while the airplane was sitting on the ramp and the engine NOT
running. Now that I've been flying a lot, I'm getting a crackling in the
sidetone and when I look at the "TX" light on the radio it's flickering as
if someone were pulsing the press-to-talk switch 4-5 times per second, even
though I'm holding the PTT down tight. This happens on both radios and
people in the pattern say I'm breaking up and garbled. I've tried several
things looking for a solution, they are:
1) Turn down the MIC gains on both radios (in steps, a little at a time,
over several flights). In the end I had them turned down so much I couldn't
get a side tone in the earphones and suspect no-one could hear me at all.
2) I tried a couple different headsets (inexpensive non ANR types,
Sigtronics and Pilot Avionics units, both around 15 years old but well taken
care of and worked just fine in the Cessnas I used to rent). No joy.
3) I tried both headsets with and without foam muffs over the microphones.
No difference that I could tell with or without the muffs.
4) Placed ferrite beads over both coax lines running to the transmit
antennas (about 3/4 of the way down to the antennas, don't have access to
the base of the antennas right now, but will when I remove the floor panels
for annual inspection). No help!
5) Shut off the field in my B&C LR-60 alternator (with external regulator)
to operate on battery only. No change (I'm using forest of ground tabs ala
AEC philosophy and I'm NOT getting alternator noise in the reception of
either radio so I didn't expect this to help but it was easy to test).
6)Since I used fine (24 gauge 5 condutor MAC servo cable, no shielding) wire
to take the PTT signal from the PTT on the control stick to the pilots mic
jack, I thought maybe that was causing a problem so I tried using the
passengers PTT (wired with 22 ga shielded tefzel) instead of the pilots. No
change.
7) The audio panel has a built in fail-safe feature that routes the PTT and
mic/earphone signals direct to the number 1 NAV/COM in the event of audio
panel failure. I shut off the audio panel and tried that. The NAV/COMM TX
light comes on but still flickers and is crackly so it's not that. I of
course could not test the #2 radio this way but would expect it to be the
same.
Transmit antenna location is under the belly of the airplane (all metal
RV-6A). Two Comant bent whips about 40 inches apart just aft of the wing
spar.
The GPS portion of the GX-60 and VOR/ILS portion of the SL-30 work just
fine. Comm reception on both radios is loud and clear (at least I can hear
people in the pattern even if I can't talk to them).
When I originally ran through the installation checkout I left the settings
on the radios at the factory recommended (transmit worked fine so I left the
settings alone) and have been making changes to the Mic gains only. These
are computer controlled radios so I'm wondering if I need to adjust any
other parameters such as squelch (no pots to control squelch like the old
days) after making the Mic gain adjustments. Could the two be somehow
related through logic in the computer program that runs the radios?
I'm really stumped, any suggestions? Any UPS AT radio stack users have this
problem? How did you fix it? Should I run through the installation checkout
with the engine running so as to have more noise impinging on the microphone
while making adjustments? Don't like the idea of head down in the cockpit
with engine running but I can tie down the airplane while doing the checks
(I'm talking ground running here not trying to make adjustments in flight)!
Thanks for the help.
Dean Psiropoulos
RV-6A N197DM
Flying and tweeking.
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