-- Problem first showed up as a low level oscillation in the background. Annoying.
-- I suspected the simple AF amp from the TJ DC RX project was oscillating. Wasted a lot of time.
-- Isolated the problems by cutting off all of the RF input to the SBL-1 mixer.
-- While watching the carrier osc signal, noticed that the hum appeared as the waveform got fuzzy.
-- I had used the osc/product detector circuit from the BITX 20.
-- I had added an amplifier stage to get the signal to higher level. But it was poorly designed.
-- After some struggle, I decided to rebuild the osc/mixer board.
-- Redesigned amplifier stage using LTSpice.
-- Changed from an SBL-1 mixer to a singly balanced mixer like in the BITX 20.
-- This worked well. Hum disappeared.
-- But I had neglected to put the .001 uF cap to ground from the AF port! Thus the cartoon!
-- This is a very important cap. Circuit was a lot more balanced with this cap in place.
-- Noiticed that output on 10 was only about 25 watts PEP, while on 15 it was around 100.
-- Checked input to final amps. Input on 10 was a lot lower.
-- Tweaked the 10 meter bandpass filter and got the output on 10 up to about 80 watts.
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Bill, I can't tell you how valuable I find these sorts of debugging walkthroughs. I have oscillating amplifiers littering the workbench. 73 de VK3TPM
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