11-08-2021, 09:54 PM
Hi Cap'n Clock,
Divide and Conquer! Inject "Line Level" audio at the volume control, or use the Phono input. As an alternative, touch the grid cap of the "75" 1st AF with and without the grid lead connected. If the Audio output is loud and proud, then work backwards with injection. Check the plate and screen voltages in the section at issue Plate voltage on the 1st AF is usually not very high (50 - 100V), but the IF, Convertor and 2nd AF Tubes should have between 180 and 250V. Screen voltages are probably about half the plate voltage on the 2nd IF, somewhat less on the Screens of the Convertor adn almst equal tothe plate voltage on the 42 Output tube. The 42 AF Output should have no more than about 20V between Cathode and the junction of R16, the negative terminals of the 10 uF electrolytic caps and the center tap of the B section of the power transformer. There are only 2 coupling caps, C19 and C22 which are 0.1 uF. The Rider's schematic shows what might be a ground at one end of C19 but that is not the case; that is shielding. Note that there are several mica caps, at the primary and secondary of the IF xformers and C21, a 220 pF cap between the 1st AF Plate and Ground on the "75" Detector / 1st AF Amp tube. Much has been said by Experts about "SMD"- Silver Mica Disease shorting these caps.
Other than the caps within the IF Cans and in the tuned circuits, bypass and coupling caps are usually not critical. For example, if a screen bypass cap or AVC Cap was 0.5 uF and your replacement is 0.47uF, that is only a 6% difference and is just fine.
If I had a nickel fore each time i made a wiring mistake (Schematic in hand, one part at a time, etc.) I'd be rich. also look for a nasty little solder bridge hiding on you.
One other thing, check the resistance of the IF transformer windings especially the grid windings. An open winding in the grid circuit of the first IF transforer can do this. You can also force bias the radio's AVC at -3V or just short the AVC line to ground. if it blasts when doing so, lower the volume and if you pick up as many stations as before, check the AVC. Make sure that R5(?) is a 2.2 meg resistor.
Good luck! You will find the issue, learn more about these radios and have another fine radio for the rest of your life.
Best Regards,
John "MrFixr"
Divide and Conquer! Inject "Line Level" audio at the volume control, or use the Phono input. As an alternative, touch the grid cap of the "75" 1st AF with and without the grid lead connected. If the Audio output is loud and proud, then work backwards with injection. Check the plate and screen voltages in the section at issue Plate voltage on the 1st AF is usually not very high (50 - 100V), but the IF, Convertor and 2nd AF Tubes should have between 180 and 250V. Screen voltages are probably about half the plate voltage on the 2nd IF, somewhat less on the Screens of the Convertor adn almst equal tothe plate voltage on the 42 Output tube. The 42 AF Output should have no more than about 20V between Cathode and the junction of R16, the negative terminals of the 10 uF electrolytic caps and the center tap of the B section of the power transformer. There are only 2 coupling caps, C19 and C22 which are 0.1 uF. The Rider's schematic shows what might be a ground at one end of C19 but that is not the case; that is shielding. Note that there are several mica caps, at the primary and secondary of the IF xformers and C21, a 220 pF cap between the 1st AF Plate and Ground on the "75" Detector / 1st AF Amp tube. Much has been said by Experts about "SMD"- Silver Mica Disease shorting these caps.
Other than the caps within the IF Cans and in the tuned circuits, bypass and coupling caps are usually not critical. For example, if a screen bypass cap or AVC Cap was 0.5 uF and your replacement is 0.47uF, that is only a 6% difference and is just fine.
If I had a nickel fore each time i made a wiring mistake (Schematic in hand, one part at a time, etc.) I'd be rich. also look for a nasty little solder bridge hiding on you.
One other thing, check the resistance of the IF transformer windings especially the grid windings. An open winding in the grid circuit of the first IF transforer can do this. You can also force bias the radio's AVC at -3V or just short the AVC line to ground. if it blasts when doing so, lower the volume and if you pick up as many stations as before, check the AVC. Make sure that R5(?) is a 2.2 meg resistor.
Good luck! You will find the issue, learn more about these radios and have another fine radio for the rest of your life.
Best Regards,
John "MrFixr"
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55
