05-28-2019, 05:26 PM
A hot field coil indicates excessive current flow. That could occur within the FC itself, shorted or shorted to frame and frame connected to chassis. Beyond that a shorted output transformer, shorted IF, incorrect wiring, reversed filter condenser(s). Shorts in the C38, 39 or a defect in Sw37. Any other B+ connections would be via a resistor which would overheat. This radio uses the "back bias" design. If the new filters are connected to chassis or shorted because of sloppy stuffing, there would be no bias for the output tube, that tube would conduct heavily heating the output transformer and the field coil. Voltage divider R47, two resistors, could be open or wrong value, shorted to chassis.
Rather than risk smoking a component with prolonged live testing, with the AC power removed and unplugged from mains, use a VOM and measure the DC resistance across C46. negative probe on negative capacitor. After an initial sag and charging the resistance should settle at least 5K, better at 8K and 12k would be great. I am allowing 60ma for a total B+ load, could be more only a measurement from a working radio could confirm total normal load.
If the resistance is at say 3k that would account for the overheating and further resistance measurements made at the same point and disconnection of the various B+ sinking locations will isolate the fault.
This is the correct method to isolate a defect of this sort without , "try this or that"...
If you want to know just how much current is being consumed in this defective condition, first. Measure the exact resistance of R47, the 32 ohm resistor to the 10ths. Connect a DVM or VTVM to this resistor, negative to chassis and positive to the 32 ohm junction of the 235 ohm resistor. Turn on the set, allow a 30 second or so warm-up measure this voltage to the 10ths. Shut down the radio and do ohms law that will tell the total current. Exception would be a filter capacitor fault or other leakage in the bias to the 1st audio tube.
With the B+ resistance procedure and the total current measurement it should be very possible to zero in on the defect (a wayward solder blob on a tube socket) in short order...
YMMV
Chas
Rather than risk smoking a component with prolonged live testing, with the AC power removed and unplugged from mains, use a VOM and measure the DC resistance across C46. negative probe on negative capacitor. After an initial sag and charging the resistance should settle at least 5K, better at 8K and 12k would be great. I am allowing 60ma for a total B+ load, could be more only a measurement from a working radio could confirm total normal load.
If the resistance is at say 3k that would account for the overheating and further resistance measurements made at the same point and disconnection of the various B+ sinking locations will isolate the fault.
This is the correct method to isolate a defect of this sort without , "try this or that"...
If you want to know just how much current is being consumed in this defective condition, first. Measure the exact resistance of R47, the 32 ohm resistor to the 10ths. Connect a DVM or VTVM to this resistor, negative to chassis and positive to the 32 ohm junction of the 235 ohm resistor. Turn on the set, allow a 30 second or so warm-up measure this voltage to the 10ths. Shut down the radio and do ohms law that will tell the total current. Exception would be a filter capacitor fault or other leakage in the bias to the 1st audio tube.
With the B+ resistance procedure and the total current measurement it should be very possible to zero in on the defect (a wayward solder blob on a tube socket) in short order...
YMMV
Chas
Pliny the younger
“nihil novum nihil varium nihil quod non semel spectasse sufficiat”
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