— 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.
Re-Built board for carrier osc/BFO and mixer for TX and RX
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