07-02-2014, 09:24 AM
Murf
I forgot if I asked that already - does your field coil produce resistance of 860 Ohm (or whatever that is shown on the sch)?
Also - how good is the contact between the coil and the socket? When you measure resistance between the 80's filament pins (the fat ones) and the #3 pins of either 42 tube when the speaker is plugged (the radio is powered down and unplugged) - is it the same 860 Ohm? You have to measure at actual points I asked - electrically it is not the same as just measuring across the field coil.
Shot in the dark....that Candohm, or whatever it is that connect the Power Transformer Centertap to the Chassis (three resistors shown under the field coil in the sch) - when you buzz between these two points do you get roughly 200 Ohms (208 to be exact) ?
PS. Now let's summarize: we are dealing with a low B+ voltage.
It can be explained by:
1. Bad field coil (excessive resistance).
2. A large drain on the B+. Either short or a bad leakage.
3. Bad connection between the Field coil and the socket down the B+ line.
The B+ has very few direct loads:
1. Output 25uF el. cap. Check for leakage / polarity.
2. Output transformer. Will load if the 42 tubes are bad OR if the caps at their plates are bad.
3. Both 7B7 tubes. Check tubes and sockets for short/low resistance between the plate and cathode (chassis).
4. #32 resistor of the #33 cap (8uF) is bad. But - probably won't bring the voltage down to 50V and the resistor would smoke.
5. Simply check the B+ wire going across for insulation damage and touching things it should not be touching.
How I would start:
1. Ohm out the B+ wire to Chassis. There is no DC path so the only thing you should see is the capacitive leakage which could result in MegaOhms readings or open. If there is any finite resistance, it should be investigated.
2. If it seems OK, I would then remove EVERY SINGLE TUBE, plug in the speaker and briefly measure the B+ voltage. It should be practically the same as the rectifier output. Meaning both ends of the field coil should read the same voltage. If the output is much lower then input like you are having it now, there is a leakage.
BE VERY CAREFUL - UNLOADED B+ COULD BE CLOSE TO 500V. AFTER YOU DONE MAKE SURE THE CAPS ARE DISCHARGED. You can do it by plugging the output tubes in and turning the radio on for a minute, and then turning it off. Check the voltage to make sure it is down to safe low levels.
I forgot if I asked that already - does your field coil produce resistance of 860 Ohm (or whatever that is shown on the sch)?
Also - how good is the contact between the coil and the socket? When you measure resistance between the 80's filament pins (the fat ones) and the #3 pins of either 42 tube when the speaker is plugged (the radio is powered down and unplugged) - is it the same 860 Ohm? You have to measure at actual points I asked - electrically it is not the same as just measuring across the field coil.
Shot in the dark....that Candohm, or whatever it is that connect the Power Transformer Centertap to the Chassis (three resistors shown under the field coil in the sch) - when you buzz between these two points do you get roughly 200 Ohms (208 to be exact) ?
PS. Now let's summarize: we are dealing with a low B+ voltage.
It can be explained by:
1. Bad field coil (excessive resistance).
2. A large drain on the B+. Either short or a bad leakage.
3. Bad connection between the Field coil and the socket down the B+ line.
The B+ has very few direct loads:
1. Output 25uF el. cap. Check for leakage / polarity.
2. Output transformer. Will load if the 42 tubes are bad OR if the caps at their plates are bad.
3. Both 7B7 tubes. Check tubes and sockets for short/low resistance between the plate and cathode (chassis).
4. #32 resistor of the #33 cap (8uF) is bad. But - probably won't bring the voltage down to 50V and the resistor would smoke.
5. Simply check the B+ wire going across for insulation damage and touching things it should not be touching.
How I would start:
1. Ohm out the B+ wire to Chassis. There is no DC path so the only thing you should see is the capacitive leakage which could result in MegaOhms readings or open. If there is any finite resistance, it should be investigated.
2. If it seems OK, I would then remove EVERY SINGLE TUBE, plug in the speaker and briefly measure the B+ voltage. It should be practically the same as the rectifier output. Meaning both ends of the field coil should read the same voltage. If the output is much lower then input like you are having it now, there is a leakage.
BE VERY CAREFUL - UNLOADED B+ COULD BE CLOSE TO 500V. AFTER YOU DONE MAKE SURE THE CAPS ARE DISCHARGED. You can do it by plugging the output tubes in and turning the radio on for a minute, and then turning it off. Check the voltage to make sure it is down to safe low levels.
