11-09-2025, 06:39 PM
Hi Bob,
Welcome to the Phorum and to this hobby. Please don't be afraid to ask questions; we are very happy to help!
Did it work before you disassembled the sub-chassis to replace the capacitors (aka condensers, aka "caps", not to be confused with grid caps)?
Make sure that there is continuity between the sub-chassis and the main chassis.
As said by others, the positioning of the wiring (called "lead dress) is important in all areas, but critical in the RF and IF areas.
Try this:
Remove all tubes except the detector / 1st AF, output and rectifier. Raise the volume, determine if there is excessive hum or motorboating. If so, the issue is in the audio circuit. If not, install the IF amp and repeat the test. If motorboating, troubleshoot the IF section. If not install the convertor tube, and so on and so on.
Check your work against the schematic. Unfortunately, because this is a multi-band set, it is not easy to follow the schematic. The screen grid circuits usually consist of a resistor from the common B+ circuit. There should be a cap (capacitor) between the screen grid connection on the tube to ground. Same thing for the B+ to the plate of the 1st AF tube. If these parts are bad, missing or mis-wired, motorboating will occur. On your set, it is cap # 20, a 0.1 uF cap
In addition, caps # 62 and 60 are drawn incorrectly. Ensure that the positive leads of these caps go to either side of the speaker field coil, which also acts as a filter choke for the power supply. the negative lead of C60 goes to chassis ground. Please note that the negative lead of C62 does NOT go to chassis ground but to the center tap of the secondary winding for the rectifier on the power transformer. This is common on pre-war sets, almost acting like a C- supply instead of cathode bias on post war sets. Note resistor 61, a "voltage divider. With power off, measure between chassis ground and the center tap of the HV secondary of the power transformer. You should read 245 Ohms give or take about 10 - 20%. If any part of the resistor is open than all sorts of bad stuff will occur.
Check Cap #18, which is the filter cap for the screen circuits and 1st AV B+, and cap # 20, the bypass cap for this circuit.
An important cap is the AVC filter cap. IN this set, there are two; #15 for the convertor and #5 for the RF. Also check cap # 35, the IF cathode bypass cap. Note that these caps connect to the AVC through the band switch. If the band switch is dirty, this can happen. Caig DeOxit D5 is the correct stuff to use.
Hope all of this helps. Keep us posted!
Another important "cap" is #54 which decouples the bias voltage for the 1st Audio Amp.
Welcome to the Phorum and to this hobby. Please don't be afraid to ask questions; we are very happy to help!
Did it work before you disassembled the sub-chassis to replace the capacitors (aka condensers, aka "caps", not to be confused with grid caps)?
Make sure that there is continuity between the sub-chassis and the main chassis.
As said by others, the positioning of the wiring (called "lead dress) is important in all areas, but critical in the RF and IF areas.
Try this:
Remove all tubes except the detector / 1st AF, output and rectifier. Raise the volume, determine if there is excessive hum or motorboating. If so, the issue is in the audio circuit. If not, install the IF amp and repeat the test. If motorboating, troubleshoot the IF section. If not install the convertor tube, and so on and so on.
Check your work against the schematic. Unfortunately, because this is a multi-band set, it is not easy to follow the schematic. The screen grid circuits usually consist of a resistor from the common B+ circuit. There should be a cap (capacitor) between the screen grid connection on the tube to ground. Same thing for the B+ to the plate of the 1st AF tube. If these parts are bad, missing or mis-wired, motorboating will occur. On your set, it is cap # 20, a 0.1 uF cap
In addition, caps # 62 and 60 are drawn incorrectly. Ensure that the positive leads of these caps go to either side of the speaker field coil, which also acts as a filter choke for the power supply. the negative lead of C60 goes to chassis ground. Please note that the negative lead of C62 does NOT go to chassis ground but to the center tap of the secondary winding for the rectifier on the power transformer. This is common on pre-war sets, almost acting like a C- supply instead of cathode bias on post war sets. Note resistor 61, a "voltage divider. With power off, measure between chassis ground and the center tap of the HV secondary of the power transformer. You should read 245 Ohms give or take about 10 - 20%. If any part of the resistor is open than all sorts of bad stuff will occur.
Check Cap #18, which is the filter cap for the screen circuits and 1st AV B+, and cap # 20, the bypass cap for this circuit.
An important cap is the AVC filter cap. IN this set, there are two; #15 for the convertor and #5 for the RF. Also check cap # 35, the IF cathode bypass cap. Note that these caps connect to the AVC through the band switch. If the band switch is dirty, this can happen. Caig DeOxit D5 is the correct stuff to use.
Hope all of this helps. Keep us posted!
Another important "cap" is #54 which decouples the bias voltage for the 1st Audio Amp.
"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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