Originally Posted by Cifrocco

Just received Photofact by email from Sams. Quality isn't the greatest, but I guess that's expected since it's an ancient document. Still, you gotta be impressed that such things even existed.

So, everyone ready for the answer to the quiz?

The device is none other than a "Ratio Detector Network" (never heard of such a thing). It has 2 capacitors and 3 resistors (no wonder DMM measurements were weird). Contrary to what I thought it has little or nothing to do with the output stage and is instead connected across one side of the 10.7MHz IF transformer at the output of the 12BA6 limiter circuit.

It's designated as part K4 on the schematic, enclosed within the dotted lines and known as Transonic part no. Q542D. If anyone has an idea what its function is I'd be delighted to hear it. Thanks.

Google "ratio detector".
It demodulates the FM carrier, with baseband audio as the output.
The part is probably a thick film network. They were usualy made on a ceramic substrate, and conformally coated.