Glad to hear that you found your problem, which wasn't anything most of us suspected. I'm curious about one thing, though. On Continental/Collins FM transmitters a quick easy way to drop power is to trip off the screen breaker, which lowers the output to around 15% of normal. In those rigs that doesn't seem to affect the input impedance to the PA tube, like it apparently does on Harris rigs. Is that because of the grounded screen topology of the Collins rigs?
FM does it with frequency!
The screen breaker trick works with the Harris HT-10 also. I have one of my clients running that way now due to icing. This site has always been ice-sensitive, and even if the screen voltage is driven down by the VSWR foldback control, it sometimes isn't enough. Even with screen off, we've had a couple occasions where backing down the exciter was necessary to stay on the air.