[EdHolland]

I managed a bit more detective work on the radio today. Sometimes, it's helpful to have too many radios, and this was one of those times. I used a neat little communications receiver I own as an IF sniffer, just using a piece of insulated wire as a capacitive probe, with no risk of zapping it's delicate (solid state) front end.

The signal generator, set to 455kHz was loose coupled up to the Zenith, and the radio also tuned to 455 kHz. With this setup, I could listen in to each IF stage by setting the probe near the output of each IF transformer. The Zenith was allowed to warm up, at which point the fault started to make itself heard while receiving AM. I turned down the Zenith, turned on the generator, and listened on the probe radio. First and second IF stages seemed to be producing a clean signal, but the third stage was noisy. Switching the Zenith to a broadcast station I could confirm that the same noise heard at the third IF was also present on the Zenith's audio.

I'll think about whether there's any other possible source for the noise (check resistors etc.,) before tearing into the IF can, but - if you'll pardon the pun - I'm getting warmer

Ed