Hi. I hope WB2WIK is readin' this.... Hi Steve.
I have a nice example of a Swan 250C (I know, NO such thing, but I love these old room heaters).....
Here goes. I purchased this radio as a 'parts' radio. No rcv, no xmit. It had a couple of bad tubes, and a dead xtal in the marker.
After a cosmetic refurb, rcv is fine, no xmit. The 'manual' is pretty slim on transmitter alignment. I next acquired a second 250C and PS. This one is also a very clean unit. After some work, it's fully functional.
BOTH radio receive EXACTLY the same. Both were aligned using an HP 606A and counter. Perfect receive and very sensitive.
Now I have both of these radios on their back. I did the voltage checks as listed in the manual. One was way off (on the 'bad' radio). It was caused by a 47K resistor going into the Meg ohm region. Still NO output. No carrier balance. No carrier.
I have swapped every possible tube from radio to radio, changed PSs, no change. BTW, all the tubes were checked on a Gm tube tester, as well as substitution. NO change....
Today, I was in them using a 'scope. I noticed that the xformer that feeds the balanced modulator had very low RF voltage compared to the 'good' radio, when ptt was actuated.
Next, I measured the in-circuit resistance of the xtal filter. The 'good' radio measured 300K ohms, the 'bad' radio measured 36 MEG ohms. oh no.....
Since I can't get a spare filter this side of Mars, am I on the right track? It still puzzles me why both receivers are identical. The block diagram shows both xmit and rcv paths through the filter.
I was going to measure every resistor point-to-point when I found the anomaly with the filter.....