[EdHolland]

I spent a little time with this radio yesterday, trying to track down the initial problem which seemed to be a misalignment in the 455 kHz IF. It is quite reproduceable, and at first, seemed to be apparent only with the radio in AM mode, and using the wide audio filter. It sounds slightly more "tuned in" and indicates slightly higher signal strength with the dial set ~ 1-1.5 kHz below the true carrier frequency. With eyes closed, if one rocks the dial and listens for best tone, the result is the same.

The general scheme of the IF is as follows. Just like a tube radio, there are several filter/amp stages:
1st IF is mixed down to 455 kHz>Transformer>20 kHz crystal Filter>Noise blanker>Transformer>6kHz or 3 kHz filter>2 Stages of amplification>Detector. The output from the last IF amp is also buffered and detected seperately to generate the AGC line.

I went through the alignment stage another time yesterday, and may have made an improvement, but I still think something is not quite right. OCD is the enemy of good enough and I like the detective work. Investigation shows that the effect occurs with either the wide (6 kHz) or narrow (3kHz) filter, not just the 6 kHz as first suspected. I tried injecting a 455 kHz signal at various points along the IF chain, thinking this might expose the culprit. One suspect could be the 20 kHz crystal filter - I have seen one other case where a part of this vintage and manufacture had gone bad. However, injecting downstream of this filter, and tuning the 455 kHz signal +/- 2 kHz also seems to show the offset.

I am now back to a close study of the circuit, to see if there is anything else amiss! This is one situation where an IF sweep tool would be very useful.