Philco model #20 - Printable Version

Philco model #20 - Printable Version

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RE: Philco model #20 - MrFixr55 - 04-04-2025

Hi Rick,

The volume control on the Model 20 has 2 sections and does 2 seperate things:

One part of the control, the 5000 Ohm section, shunts the antenna input from minimum shunt at full volume to maximum shunt (shorting antenna to ground) at minimum volume setting.

The other part of the control, the 250 Ohm section, varies cathode bias on the 1st and 2nd RF Amp, thereby adjusting the gain of these stages, from minimum bias at full volume to maximum bias at minimum volume.

Ohm the 2 sections end to end. You may find that antenna (5000 Ohm) sections to be open or miswiring at the 250 Ohm section. If the 250 Ohm section is open someone may have rewired the set to bypass that pot and provide fixed bias to the RF stages.

Troubleshoot by measuring between ground and either of the 2 "end" terminals (the resistance element connections). There should be several volts (maybe somewhere around 25V?). Measuring between chassis ground and the cathodes of either of the RF Amp tubes, the voltage should vary while rotating the poet from min to max.

If the pot has only 1 section, determine if it shunts the antenna or if it varies the RF amp bias.

Hope this helps.

RE: Philco model #20 - MrFixr55 - 04-05-2025

Hi Rick,

One other thing regarding volume on this set. Make sure that the tube shield is in place. this is that large metal "clamshell" with 3 "cavities" that surround the RF Amps and Detector tubes (the three 24As).

Having reread your thread, if you can turn the sound all the way down to silence on local stations, but the slightest increase causes the sound to be too loud, it is possible that the 5KW section is open. If so, there are 3 possible solutions:

Disassemble and try to repair the volume control.
Try to find another control from a chassis being parted out.
Put in an "auxiliary" volume control. Use a 5K linear taper pot across the antenna connections, either mounted to the back of the chassis or radio cabinet or external from it.

Hope this helps.

RE: Philco model #20 - RodB - 04-05-2025

I seem to recall in some TRF sets there being a Local/Distance toggle switch that removes the B+ from the first 24 rf tube screen grid, making it a triode, resulting in less gain. There's no automatic control in a TRF. I guess you could take up farming. :-) But you would need 9 kids like my great grandfather. That's a lot to take on just to get lower volume on the radio.

RE: Philco model #20 - onlyrick - 04-05-2025

AHH HA , I got it and can’t thank you fellows enough , when I changed the capacitors I took the advice and did not unsolder the wires, I heated the Bakelite container and pushed out the tar and condenser with a small allan wrench ,unbolted it from the chassis cleaned it out and put in the new condenser then bolted it back to the chassis , what I found well checking the volume control with my ohm meter was the centre wire on the back part of the volume control had come off , I guess from bending the #5 Bakelite box where the wire is attached so I could clean it out and put in the new condenser, the broken wire was hidden underneath the volume control out of sight , had I not been checking around the volume control with my ohm meter I would never had seen it , you know I had drawn a diagram of each cap block I changed and still missed it , oh well it works just fine now , thanks again for all the help , and looking back at this job it’s far easier not to unsolder the wires from the Bakelite condenser block , but making a drawing is so helpful when some thing like this happens , all the very best have a great weekend Rick
PS= As soon as I have photos of the radio I will post them