09-01-2026, 07:09 PM
Joe, you replaced the 12AV6 Jury Rig with the 14B6? That small cap was likely used to filter RF out of the audio and probably would not cause motorboating.
Most of your voltages are well within what most people would accept as limits. the exception here is the 1st Audio plate and the IF and Converter screens.
Since the 50L6 Screen value, 12BA6 and 7A8 plate values are all good and that section of the power supply is the source of the 14B6 triode plate and converter and IF screens, check the resistors R202 for the 14B6 Plate and R300 for the converter and IF screens. The conundrum is that when you have several problems, look for the common. The common between the 3 tubes with out of spec voltages is the B+ from the second filter cap which is well within range. So you have either 2 separate problems, one of which is causing the motorboating or 3 separate problems, one of which (the motorboating) is not caused by the out of spec voltages discussed above.
Is the motorboating there at low volume? Does the frequency change with the volume? If so, look at the audio portion of the set. If not, look at the RF section of the set. There should be a rather large (0.05uF) cap between common and the AVC line this is C304 on the Canada Philco 83 schematic. There is a parallel cap C403, a 0.02 uF cap between the AVC line and common. I don't know why there are 2 caps across the same line unless the schematic is wrong (Schematic missing a resistor between the 2 caps in the AC line?) By common, I mean the junction of the low side of the heater string, the negative of the twin electrolytic caps, the return from the volume control, the cathode of the 1st AF tube, the cathode resistor of the 50L6, etc. There should be also a large value cap between this "Mecca" and the physical chassis. This is C303, a 0.1uF cap.
Re the power switch. Hear is a very useful link to the "Switch in the low side" conundrum from Philip Nelson's "Phil's Old Radios":
https://antiqueradio.org/safety.htm
If you did this mod (Using a polarized power cord, connecting the neutral directly to the "Common" (Mecca), placing the power switch between the "Hot" side of the power cord and the junction between the 35Z5 filament and plate, make sure that you removed any "jumper" wire from the switch to the "low" end of the volume control.
Hope this helps.
Most of your voltages are well within what most people would accept as limits. the exception here is the 1st Audio plate and the IF and Converter screens.
Since the 50L6 Screen value, 12BA6 and 7A8 plate values are all good and that section of the power supply is the source of the 14B6 triode plate and converter and IF screens, check the resistors R202 for the 14B6 Plate and R300 for the converter and IF screens. The conundrum is that when you have several problems, look for the common. The common between the 3 tubes with out of spec voltages is the B+ from the second filter cap which is well within range. So you have either 2 separate problems, one of which is causing the motorboating or 3 separate problems, one of which (the motorboating) is not caused by the out of spec voltages discussed above.
Is the motorboating there at low volume? Does the frequency change with the volume? If so, look at the audio portion of the set. If not, look at the RF section of the set. There should be a rather large (0.05uF) cap between common and the AVC line this is C304 on the Canada Philco 83 schematic. There is a parallel cap C403, a 0.02 uF cap between the AVC line and common. I don't know why there are 2 caps across the same line unless the schematic is wrong (Schematic missing a resistor between the 2 caps in the AC line?) By common, I mean the junction of the low side of the heater string, the negative of the twin electrolytic caps, the return from the volume control, the cathode of the 1st AF tube, the cathode resistor of the 50L6, etc. There should be also a large value cap between this "Mecca" and the physical chassis. This is C303, a 0.1uF cap.
Re the power switch. Hear is a very useful link to the "Switch in the low side" conundrum from Philip Nelson's "Phil's Old Radios":
https://antiqueradio.org/safety.htm
If you did this mod (Using a polarized power cord, connecting the neutral directly to the "Common" (Mecca), placing the power switch between the "Hot" side of the power cord and the junction between the 35Z5 filament and plate, make sure that you removed any "jumper" wire from the switch to the "low" end of the volume control.
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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