Motorboating Johnson VFO

[W1ATR]

Hi All.

I wanted to check with the collective to see if someone has any pointers on this problem with my Junkston 500. After about a half hour of running (talking or not), the vfo will start to throw a slight hum into the receiver. I believe this is called "motorboating". The sound tracks along with the vfo if I run off freq from the rx, and it show a tiny amount of driver grid current on the meter. This is happening with the tx in standby. If I kick it into zero or transmit, then all is normal. (While transmitting). If I switch the vfo off by moving the knob to one of the crystal positions, then the noise is gone from the rx.

I suspect it to be a bad cap somewhere in the buffer/multiplier/driver area, but a cold search showed all is well. This time around, I intend to get it out of the box, upside down on some insulators, jump out the cab switch, and fire it up. Because it only does it after warming for a half hour, I think I can hit parts with canned air and see if the hum goes away. But, before doing this, I wanted to ask this 'do all to end all', compendium of AM knowledge if you have any pointers. Other than don't let any cats jump on the open chassis while the maul is down looking for this bug.

Thanks

[WQ9E]

Jared,

This is almost surely in your keyer tube circuit (V107).

Try a new tube first since heater/cathode leakage will cause this problem.  You will need to readjust the keyer control if you replace the tube.

[KB2WIG]

Johnson made some nice outboards.


klc

[Jim, W5JO]

Johnson made some nice outboards.


klc

Better than most of their radios too!

[W1ATR]

The fix wound up being one of the straps on the adjustable bias resistor was loose enough to act up after a short period of warmup. Cleaned it up and tightened the screws a bit and all is well. I found it with a little burst of air on the connection wound stop the hum for a second. Nice easy fix.