New transmitter has audio distortion

[KB5MD]

I have a home brew transmitter running 813’s modulated by 811’s.  When I transmitt into a dummy load the audio is good and clear.  When transmitting into an antenna the audio is very distorted.  Any adjustment to audio does nothing to correct the distortion.  The speech amp is a tube type p.a. amp. Any ideas, anyone as to cause and remedy?

[w8khk]

My first suspicion would be RF getting back into the audio amplifier.  This could either overload stages in the audio chain, or upset the bias on one or more stages, causing severe distortion. 

I would suggest monitoring the signal at various points i the PA amplifier with an oscilloscope, and with a DVM or VTVM, compare bias levels of the earlier stages, with and without the antenna. 

Instead of voice modulation, you might try injecting an audio oscillator into a later stage of the audio amplifier, after disabling the output of the previous stage.  Then compare the signal and bias at the earlier stages, with dummy load then the antenna.  If RF is present, it should give obvious clues.  These tests may well reveal the cause, and indicate a cure rather quickly.  You did not mention what amplifier you are using, and whether it is shielded or enclosed.  Perhaps some RF abatement right at the microphone input might be all that is needed.  Has this transmitter and/or speech amplifier worked either separately in the past?  If so, what has changed in the configuration before the problem was discovered?  Prior successful operation gives clues as well.

[WD8BIL]

I think Rick is right. Most PA Audio amps are not very well RF hardened. You may need to do a bit of bypassing in that PA amp.

[w7fox]

I had a similar problem with RF getting into a DDS VFO.  It worked fine on the dummy load, all kinds of distortion on antenna.  Better shielding solved it.

[K8DI]

Sometimes RF feedback can be the really simple stuff.  I have a Shure mic feeding a fancy audio DSP, that feeds my RCA BTA-1R1, that worked fine...Then I made some low level changes to the RF drive circuit, and a couple months went by before I fired it up.  I had RF feedback with the antenna (vs. dummy load) that I chased for a week...bypass caps here, ferrites there, to no avail...till I replaced the bad mic cable!

Ed

[KL7OF]

I have found that two door knobs to ground with a Z-50 choke between them, all in series in the B+ and modulated B+ lines cured that problem in 2 of my HB rackmounted transmitters.  I suspect long lead length allows the leads to pick up RF...  Good Luck    Steve

[KB5MD]

Problem solved!  Just a note to say thanks for all the replies and suggestions.  The distortion was being caused by rf getting into the audio chain.  I removed the preamp/equalizer and relocated the antenna coax, viola, the distortion went away.    Thanks again, you guys are great!