[MrFixr55]

HI DM Blackburn,

This sounds like a a capacitor problem. First off, it is important to note that the 2 main filter caps do not ground to the chassis, they connect to the center tap of the HV secondary. Make sure that the The "can" of the capacitors must be insulated from the chassis. The second possibility is dress of the grid cap for the First Audio stage (labeled Detector - Audio on the schematic. Make sure the lead is dressed correctly. If there is supposed to be a shield on this tube, ensure that it is in place. touch the wire. Does the "beat" change?

Next, "divide and conquer"

Does the "beat" noise occur with the volume control all the way down? If yes, the problem is in the audio stage or power suppl. If no, the next step is to power down and pull the detector tube. Power back up and raise the volume control. You may hear a little hum, but you should not hear the beat.

If no hum, power down, reinstall the detector tube and pull the IF tube. If no hum, reinstall the IF but leave the grid cap off. Repeat the test. If no hum, repeat the same procedure with the First detector. If no hum, repeat with the 1st RF Amp tube. This should help you isolate the stage that is causing the issue. The issue may be a bad capacitor, bad resistor, gassy tube, etc.

If the Bakelite condenser blocks have not been serviced, do so. Remember, you can't just "bridge" the caps with new ones, the old ones must be take out of the circuit. Remember, some of these blocks have 2 caps or a cap and esistor inside. Several of these condensers are "B+ Bypass" condensers. Failure of these condensers can cause this problem. Service the "Filter Condenser Block" by replacing their components also. Check the continuity of the voltage divider resistors

Mike (Morzh) has written a great article on servicing theses Bakelite blocks without having to detach / reattach the lead wires. See the attached link:
https://philcoradio.com/library/index.ph...soldering/

Hope this helps