08-07-2021, 09:33 PM
Thanks MrFix,
Sorry I didn't see this earlier, sound advice. The issue is fixed, but first let me respond to your excellent notes. The chassis is wired to ensure connection of metal tubes. Being that the shields were missing from the chassis, I used metal tubes throughout while bringing it back to life. The oscillation certainly seemed to be in the IF, rather than in the audio stages. I chased down exactly the same things you had suggested.
With this in mind, I went back through the schematic, concentrating on the 6A8 and 6K7 first IF sections, tracing each component. Something looked odd, so I counted all the fixed capacitors (excluding trimmers and large cans) - 16 on the diagram, 15 on the chassis. Sure enough, the decoupling cap on the supply line to the mixer and first IF pentode was missing. Since I'd restuffed each cap one-by-one it can't have been on the chassis when it came to me. I added the indicated 0.1 uF at a convenient point close to the IF transformer and retested the set. Sure enough, all odd behaviour was gone, and the radio works even better on AM than it did before. In fact it is a real performer, excellent reception, sound quality and volume, a superb tuning dial, and handsome appearence.
After the electrical work, I pursued some more gentle rubbing of the cabinet with tung oil - the finish really is looking nice. The last steps will be adding grillcloth and replacing the dial escutcheon and glass. This one is practically finished. pictures tomorrow.
Ed
Sorry I didn't see this earlier, sound advice. The issue is fixed, but first let me respond to your excellent notes. The chassis is wired to ensure connection of metal tubes. Being that the shields were missing from the chassis, I used metal tubes throughout while bringing it back to life. The oscillation certainly seemed to be in the IF, rather than in the audio stages. I chased down exactly the same things you had suggested.
With this in mind, I went back through the schematic, concentrating on the 6A8 and 6K7 first IF sections, tracing each component. Something looked odd, so I counted all the fixed capacitors (excluding trimmers and large cans) - 16 on the diagram, 15 on the chassis. Sure enough, the decoupling cap on the supply line to the mixer and first IF pentode was missing. Since I'd restuffed each cap one-by-one it can't have been on the chassis when it came to me. I added the indicated 0.1 uF at a convenient point close to the IF transformer and retested the set. Sure enough, all odd behaviour was gone, and the radio works even better on AM than it did before. In fact it is a real performer, excellent reception, sound quality and volume, a superb tuning dial, and handsome appearence.
After the electrical work, I pursued some more gentle rubbing of the cabinet with tung oil - the finish really is looking nice. The last steps will be adding grillcloth and replacing the dial escutcheon and glass. This one is practically finished. pictures tomorrow.
Ed
I don't hold with furniture that talks.
