Hmmm... Good news and bad news.
1. I took voltage readings from Q507 (AOD403) with my 12v QMX powered on as follows:
Source: 11.98v
Gate: 11.88v
Drain: 6.6v
With the device powered off, I measured a 49.8ohm resistance between the source and the gate.?
2. Then I proceeded to remove the Gate (left-most) pin of Q507 from its pad, so that it was isolated in mid-air. Powered on, I measured:
Source: 11.97v
Gate: 11.97v
Drain: 6.6v
Powered off, I still measured 50ohms between Source and Gate and, incidentally, 5.58k across R507 (which had been reading 50ohms).
The voltage into the transmitter was still high at 5.8v in receive and in transmit. I had kind of expected that to fall to 0 with Q507 half-way removed.?
3. Therefore I determined to lift the other leg of Q507. Which I did, resulting in a transmitter voltage of .6-.9v. Beautiful! It seems to me there's a pretty significant chance that Q507 really is the culprit, having been shorted to the transformer (T501) when I initially powered on.
Bad news: In try to lift that second leg of Q507, I plugged in my (smart!) soldering iron but neglected to ensure I had turned it on. I lifted the pad on a perfectly cold connection. If only the operator was as smart as the soldering iron (a nifty Pinecil 2). Argh!
So, my new question: is there any point in ordering a replacement AOD403? Or is my QMX now a receive-only device and expensive code oscillator?
Presently two of the three pads are still good. I wonder if I could make it work by tacking a 5.6k resistor and a 0.1u capacitor on top of the device. I would need to find myself a convenient ground and, more challenging yet, a +12V source.
Todd W2TEF