K3/DVD hum problem solved
Posted by
k6rb on
Mar 19, 2010; 2:07am
URL: http://elecraft.85.s1.nabble.com/K3-DVD-hum-problem-solved-tp4760735.html
I've been battling what appeared to be a ground loop problem when I joined the two K3s using a DX Doubler. I would hear low-level hum on both audio outputs, and it got louder when either was keyed.
Tonight, I hooked up the radios to separate dummy loads and removed all I/O except RS-232, ACC, key in. No change. Put both radios in "test" mode and still heard low level hum but the higher level hum upon keying was not so noticeable.
In TX Normal mode, again, I noticed an interesting thing - at 10 watts out into the dummy load, the hum on keying was lower but would jump to louder as soon as I started raising the power output passed the LPA to HPA threshold (around 15 watts). But, it did not get louder when I continued to raise the power to 100 watts. So RF feedback was ruled out. It appeared to be DC current related, since that will jump as the threshold is crossed.
Both radios had been DC sourced from a common 30 amp power supply figuring that only one would be transmitting at a time so it should handle it. My next step was to separate the two radios from a common DC power source. I used two 30 amp supplies, one to each. Bingo! No more hum, no more hearing radio one in radio two when one was keyed and vice-versa.
What threw me at first was I had a similar problem with the DX Doubler using two FT-1000s, and they didn't share a common DC source. That time, the problem was a ground loop in the audio chain. So, isolation transformers worked. This time, with isolation transformers, there was no change other than loss of frequency range and volume.
Rob K6RB
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