11-26-2021, 12:31 AM
Hi Rob,
First off, in addition to looking at the Rectifier plates, watch the '42 plate and screen grid for glow. If the plate glows red or if the Screen grid glows, shut down immediately. If the output transformer is open or the connection is broken, the screen grid will draw enough current to glow bright yellow, burn up the screen grid and short the tube. Check all 3 wires between the radio and the speaker (42 plate, and both field connections), especially if you separated the radio from the speaker. The wire from the '42 plate goes to the transformer, the connection that is common to the transformer and the field goes to the 4uF cap connection of 29 and the other field connection goes to the junction of the 8uF cap connection of 29 and the '80 filament
The draw of 0.5A is likely good and not excessive. 343V between chassis ground and Filament of the '80 is an indication of low current draw. Check between the negative terminal of Cap 29 (the power supply filters and each "+ Terminal+ of the cap. The 8uF cap will likely read the 343V that you saw or higher, because there is a 325 Ohm resistor between the negative return of the B Supply and the Chassis, mainly for cathode bias of the '42 Audio Output tube. If there is current draw, the voltage between the negative terminal of 29 and the 4 UF terminal will be lower, usually in the order of 250V. If the 2 voltage readings are the same, the output transformer may be open.
If the voltage is zero, either the field is open or there is a short totally loading down the B+ and a 450 some odd volt drop across the field coil. This would cause a strong magnetic field to be developed at the field pole of the speaker.
If Cap 24 is shorted, that would kill the audio output and cause all B+ to drop through the field coil and output transformer. A hum would likely be heard, however.
In Summary, connect the negative of your DVM to the junction of the B- of the power transformer, the common negative of C29 and R30. Note that this not the same as chassis ground. With the positive lead of the meter, check the following points for the following voltages (all voltages are +/- 20%, especially with the Line voltage being 90V):
8uF terminal of C29: 350V
4uF terminal of C29: 250V
Plate of '42: 235V
Chassis ground 15V
Let us know the results.
Best Regards, and a Blessed Thanksgiving,
John, MrFixr
First off, in addition to looking at the Rectifier plates, watch the '42 plate and screen grid for glow. If the plate glows red or if the Screen grid glows, shut down immediately. If the output transformer is open or the connection is broken, the screen grid will draw enough current to glow bright yellow, burn up the screen grid and short the tube. Check all 3 wires between the radio and the speaker (42 plate, and both field connections), especially if you separated the radio from the speaker. The wire from the '42 plate goes to the transformer, the connection that is common to the transformer and the field goes to the 4uF cap connection of 29 and the other field connection goes to the junction of the 8uF cap connection of 29 and the '80 filament
The draw of 0.5A is likely good and not excessive. 343V between chassis ground and Filament of the '80 is an indication of low current draw. Check between the negative terminal of Cap 29 (the power supply filters and each "+ Terminal+ of the cap. The 8uF cap will likely read the 343V that you saw or higher, because there is a 325 Ohm resistor between the negative return of the B Supply and the Chassis, mainly for cathode bias of the '42 Audio Output tube. If there is current draw, the voltage between the negative terminal of 29 and the 4 UF terminal will be lower, usually in the order of 250V. If the 2 voltage readings are the same, the output transformer may be open.
If the voltage is zero, either the field is open or there is a short totally loading down the B+ and a 450 some odd volt drop across the field coil. This would cause a strong magnetic field to be developed at the field pole of the speaker.
If Cap 24 is shorted, that would kill the audio output and cause all B+ to drop through the field coil and output transformer. A hum would likely be heard, however.
In Summary, connect the negative of your DVM to the junction of the B- of the power transformer, the common negative of C29 and R30. Note that this not the same as chassis ground. With the positive lead of the meter, check the following points for the following voltages (all voltages are +/- 20%, especially with the Line voltage being 90V):
8uF terminal of C29: 350V
4uF terminal of C29: 250V
Plate of '42: 235V
Chassis ground 15V
Let us know the results.
Best Regards, and a Blessed Thanksgiving,
John, MrFixr
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
Best Regards,
MrFixr55
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