Lee Hooper KG5VU
Thank you for the comments and suggestions.
In my unit, I feed 12 VDC to the enclosure which then feeds a home-brew 5VDC supply with a basic LM317 circuit. I will start by adjusting the pot to raise the output voltage a bit. If that doesn't do it, I guess that I will test using a second 5VDC supply for the heater circuit.
I can easily imagine that I did advance R21 too aggressively. How long did it take you to adjust R21 properly. Hours I suppose??? (I saw another comment on the Group where someone shorted C1 and another that reduced R3 to 100K. Perhaps one of these can be my backup plan.)
Thanks again, Lee