[Radioroslyn]

You may want to approach the problem this way. Use a good sounding amp and speaker and poke around the output of the detector and audio stages of the '90 and hear if it is coming from the detector or other parts of the amp. If you have a signal tracer take a listen before the detector and see if you can hear it. To be honest with I've got a National NC-100X with the same problem. Low audio frequency sound garbled. I did a lot of work on the set to eletriclly restore it but so far I haven't been able to pin this one down. It too uses a plate type of detector. It's too bad it's a very kewl looking 1936 deco set.
Terry