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1. 02:28 PM - Strange radio transmit problems - take 2 (Peter Pengilly)
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| Subject: | Strange radio transmit problems - take 2 |
In June last year I wrote about a problem I was having with the
transmitter in my airplane where the radio would not transmit on some
frequencies. I wonder if I could again ask for some advice?
I thought I solved the problem by replacing the radio to antenna coax -
but all I in fact did was shift the frequencies that were problematic.
Since then I have tried several different fixes without any success. I
have improved the grounding of the antenna, replaced the antenna and
tried several different headsets, all with no change to the problem. I
have found that on the ground without the engine running the radio will
transmit on all frequencies (or at least in 1 MHz steps through the
frequency band). In the air some frequencies cause a problem (such as
118.9, 125.25 and 127.05), but throttling back to idle sometimes
produces an intelligible transmission, although not great. When the
radio does not transmit it is obvious as no side tone is heard in the
headset, sometimes carrier wave only is transmitted.
The voltage regulator (PMR-1) for the SD-8 alternator is mounted very
close ahead of the radio, along with the large capacitor and OV thing,
I'm now wondering if there is some kind of electro magnetic interference
happening? Of course the best check would be to go fly with the
alternator off, and I should be able to do that over the next few days
(there's some wedding going on here tomorrow that is getting in the way
of everything ...). Assuming it is interference, how far away from the
radio should I mount the VR, is the back side of the firewall a suitable
location? Should the capacitor be close the the VR, or doesn't it
matter? Its a Becker AR-4201 radio, Becker say that if it transmits on
any one frequency there is nothing wrong with the radio. Transmit is
good on some frequencies (for example 122.6, 129.8) and receive is
always strong.
Thanks for your help, in anticipation.
Regards, Peter
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