01-12-2023, 12:06 PM
(This post was last modified: 01-12-2023, 12:08 PM by Mike Watterson.)
And it can't be said too often, the biggest issue isn't electrolytics, resistor values can be way out, but what will destroy valves, audio transformer and mains transformer is capacitors coupling anodes to grids. They go leaky and it's only evident with a high voltage tester, not a regular capacitor meter or DMM.
The leaky caps from g2 pins will lower gain, but do no damage.
The paper dielectric caps had a 10 to 15 year life. They can be in waxed card, tar or even plastic.
One way to see is if your DMM is 10M or 20M on volts, is to power up the radio with no valves in it. Connect -ve of DMM to any cathode or 0V line.
Without valves and powered on (the dead HT resistor can be temporarily be replaced by a short):
Check g1 on all sockets. Should be zero (won't be with valves in, be negative or zero compared to cathode.
Check g2 on all sockets. Should be HT volts (won't be with valves in, be less then).
If any g1 is not zero the series capacitor needs replaced. Exact value not critical, can be x2.
If any g2 is not at HT, the capacitor to 0V needs replaced. Exact value not critical, can be even x4.
Rating should be HT +50V (minimum) and polyester types are fine.
The leaky caps from g2 pins will lower gain, but do no damage.
The paper dielectric caps had a 10 to 15 year life. They can be in waxed card, tar or even plastic.
One way to see is if your DMM is 10M or 20M on volts, is to power up the radio with no valves in it. Connect -ve of DMM to any cathode or 0V line.
Without valves and powered on (the dead HT resistor can be temporarily be replaced by a short):
Check g1 on all sockets. Should be zero (won't be with valves in, be negative or zero compared to cathode.
Check g2 on all sockets. Should be HT volts (won't be with valves in, be less then).
If any g1 is not zero the series capacitor needs replaced. Exact value not critical, can be x2.
If any g2 is not at HT, the capacitor to 0V needs replaced. Exact value not critical, can be even x4.
Rating should be HT +50V (minimum) and polyester types are fine.
