11-23-2008, 10:57 PM
The most common reason for the failure of a field coil is the corrosion of the copper wire due to the acids in the paper insulation inside the field coil combined with humidity. It causes a green type of corrosion which will eventually cause the coil to become open.
Other reasons for the failure of a field coil in the Philco 70 would be a leaky or bad capacitor at location 5 or 28. Also, if one of the 24 tubes developed (unlikely, but possible) a plate to screen or screen to grid short, the field coil could have failed.
And yes, if your 3100 ohm field is rated at 40ma, then it should be able to handle about 124 volts. I have a Philco 70 and and a couple Philco 90 chassis here and I never bothered to check the voltage across the field or current drawn. I just made sure to replace all the tar block caps and all caps in the metal boxes and the electrolytics. I also replaced a few out of tolerance resistors. The only other problem I had was that two of the 90's had open RF coils and had to be rewound.
Ed
Other reasons for the failure of a field coil in the Philco 70 would be a leaky or bad capacitor at location 5 or 28. Also, if one of the 24 tubes developed (unlikely, but possible) a plate to screen or screen to grid short, the field coil could have failed.
And yes, if your 3100 ohm field is rated at 40ma, then it should be able to handle about 124 volts. I have a Philco 70 and and a couple Philco 90 chassis here and I never bothered to check the voltage across the field or current drawn. I just made sure to replace all the tar block caps and all caps in the metal boxes and the electrolytics. I also replaced a few out of tolerance resistors. The only other problem I had was that two of the 90's had open RF coils and had to be rewound.
Ed
