05-24-2025, 07:32 AM
Hi CGL,
Yes, troubleshoot the 140V Power supply, something is definitely loading it down, and since this supply exclusively powers the IF section, some parts of the Video section and is involved in the audio output, your problem is there. It is likely that one stage has a shorted components your tuner power is somewhat but not greatly loaded down, not by the tuner but by the fact that the 140V supply is overloaded, pulling down the 280V Source of all power. You state that you receive 235V from the 250V tuner supply. Measure the 280V to determine if that is correct.
One can safely work on this set by ensuring that the power is connected in such a way that the chassis is connected to the neutral leg of the power source and not the "hot" leg. If the "interlock" is not polarized by one leg being larger than the other, then consider "tacking" a polarized power cord to the input in such a way that you confirm that the chassis is connected to the larger prong on the polarized power cord. Then confirm the correct wiring of your power source.
Other than the hot chassis, one issue in troubleshooting this set is the "Christmas Tree" arrangement of the tube heaters. My first troubleshooting technique for this power supply would be to pull all the tubes associated with it and while monitoring the 140V PS, insert each tube, one at a time, and troubleshoot the stage that pulls the PS down. Unfortunately, one can't use this technique because if you pull one tube, the heaters on all tubs go out.
First off, check the C3100 uF cap to verify the polarity and working voltage. Replace C51, the 470 pF cap or cut it out of the circuit. Next, verify or replace R75, the 18KOhm 2W resistor that connects this leg of the supply to the 280V supply. Next, disconnect B+ and cathode from the audio output tube. I have no idea why they sed the arrangement that they did on this stage, where the whole stage is 140V above ground, but a heater-cathode leakage could definitely mess up the supply.
Otherwise, the only way to troubleshoot this supply is to cut wires to the various loads to see which one pulls the supply down.
Hope this helps.
Yes, troubleshoot the 140V Power supply, something is definitely loading it down, and since this supply exclusively powers the IF section, some parts of the Video section and is involved in the audio output, your problem is there. It is likely that one stage has a shorted components your tuner power is somewhat but not greatly loaded down, not by the tuner but by the fact that the 140V supply is overloaded, pulling down the 280V Source of all power. You state that you receive 235V from the 250V tuner supply. Measure the 280V to determine if that is correct.
One can safely work on this set by ensuring that the power is connected in such a way that the chassis is connected to the neutral leg of the power source and not the "hot" leg. If the "interlock" is not polarized by one leg being larger than the other, then consider "tacking" a polarized power cord to the input in such a way that you confirm that the chassis is connected to the larger prong on the polarized power cord. Then confirm the correct wiring of your power source.
Other than the hot chassis, one issue in troubleshooting this set is the "Christmas Tree" arrangement of the tube heaters. My first troubleshooting technique for this power supply would be to pull all the tubes associated with it and while monitoring the 140V PS, insert each tube, one at a time, and troubleshoot the stage that pulls the PS down. Unfortunately, one can't use this technique because if you pull one tube, the heaters on all tubs go out.
First off, check the C3100 uF cap to verify the polarity and working voltage. Replace C51, the 470 pF cap or cut it out of the circuit. Next, verify or replace R75, the 18KOhm 2W resistor that connects this leg of the supply to the 280V supply. Next, disconnect B+ and cathode from the audio output tube. I have no idea why they sed the arrangement that they did on this stage, where the whole stage is 140V above ground, but a heater-cathode leakage could definitely mess up the supply.
Otherwise, the only way to troubleshoot this supply is to cut wires to the various loads to see which one pulls the supply down.
Hope this helps.
"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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