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Before I condemned the IF transformers (which, could very well be the trouble), I'd clean the tube sockets and controls with contact cleaner. Then, I'd check the solder connections on all tube sockets and surrounding areas. Tubes and PC boards really don't need to mix; but, companies thought differently and the result often ends up being damaged tube sockets, damaged PC board traces, or cracked solder connections. If the problem is still there, try tapping each tube with a pencil or similar object. If tapping a particular tubes causes the problem, you likely have an intermittent tube that will have to be replaced. The symptoms of bad IF transformers are static burst in the audio that sounds like a thunderstorm. When the IF transformer is bad in this way, you can usually check the grid voltage of the tube that's being fed by the suspected IF transformer and if the transformer is bad, you'll see a varying positive voltage on the grid.
Unfortunately, it's getting hard to find good IF transformers and even NOS ones can shortly develop the same problem. The only permanent fix is to take the IF transformer apart, determine what value capacitors are used, disable the internal capacitors, reinstall the IF transformer, and mount outboard capacitors to the underside of the PC board.
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