01-30-2022, 02:10 PM
Bill, if you haven't already, make sure you replace the coupling cap between the 75 tube plate and the 42 output tube grid. It is part of the bakelite encased cap marked #33 on the schematic and chassis layout drawing.
Any leakage through this cap will throw off the 42 grid bias, making it less negative and causing the plate current to increase. This can cause a runaway condition which can take out the tube, output transformer and power supply.
As to the sensitivity problem, probably an open winding in the antenna coil. It is the one in the can where the 6A7 grid cap wire comes from. This is a fairly common problem and usually results from a nearby lightning hit discharging through the antenna coil primary winding.
Good luck,
Mike, WA2YGA
Any leakage through this cap will throw off the 42 grid bias, making it less negative and causing the plate current to increase. This can cause a runaway condition which can take out the tube, output transformer and power supply.
As to the sensitivity problem, probably an open winding in the antenna coil. It is the one in the can where the 6A7 grid cap wire comes from. This is a fairly common problem and usually results from a nearby lightning hit discharging through the antenna coil primary winding.
Good luck,
Mike, WA2YGA
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