The PHILCO Phorum
Philco 40-120C Restoration - Printable Version

+- The PHILCO Phorum (https://philcoradio.com/phorum)
+-- Forum: Philco Radio Discussions (https://philcoradio.com/phorum/forumdisplay.php?fid=5)
+--- Forum: Philco Electronic Restoration (https://philcoradio.com/phorum/forumdisplay.php?fid=8)
+--- Thread: Philco 40-120C Restoration (/showthread.php?tid=25622)

Pages: 1 2

Philco 40-120C Filament Wiring - bluecap - 02-13-2025

Hi folks, I'm in the process of restoring a Philco 40-120C and I'm finding the wiring to be a bit puzzling. According to the schematic, the filament resistor should be connected to Pin 1 (the heater of the rectifier tube 35Y4). However, it is attached to Pin 5 (a "no connection" pin in the tube) without a jumper from Pin 5 to anywhere else. Meanwhile, Pin 1 is attached to Pin 6 of the 7C6 tube (the tube's triode cathode). It all appears to be the original wiring. Any guidance would be greatly appreciated. Thanks.

RE: Philco 40-120C Filament Wiring - klondike98 - 02-14-2025

Let's try to sort it out. So we are working with the same info here's the schematic I have:
[Image: https://www.dropbox.com/scl/fi/mvw9qo0t7no3ouzvh5dbb/Philco40-120schematic.png?rlkey=xh7ukv0fojk6pnki84h5oxpxj&raw=1]

...and here's the pin out for the 35Y4 (only difference from the 35Z3 in the schematic is the center tap on the heater):
[Image: https://www.dropbox.com/scl/fi/xhbiio84h4psi5bcieh7d/35Y4.png?rlkey=bafklaa5cx97tx35qgb4s3wc3&raw=1]

I agree that if the 132 ohm end of resistor #34 is connected to pin #5 of 35Y4 and there is no other connection to pin#5 (and no other connection to the end of resistor #34) the unit should not work. With all the tubes in place, what is the resistance reading across the power plug with the switch closed?

RE: Philco 40-120C Filament Wiring - morzh - 02-14-2025

Is the tube in your radio actually 35Z3 or 35Y4, or something else?

RE: Philco 40-120C Filament Wiring - bluecap - 02-15-2025

Thank you for your replies.

That is the correct schematic and pin out diagram for the rectifier. The resistance across the power plug is 328.6 ohms.

At this point, I am getting some voltage to pass through the radio. (It is not correct, but that's another issue.) I've attached a picture of the 35Y4. It shows the key and the connection to the filament resistor. I replaced the new caps and resistors. The dark yellow wire from pin 5 to the 132-ohm end of the filament resistor is original.

The tube is a 35Y4. I believe it is the original one that Philco shipped with the radio, although the schematic shows 35Z3 because Pin 4 is connected and Pin 8 is not.

RE: Philco 40-120C Filament Wiring - morzh - 02-15-2025

I would disregard everything that is there now. I do not know who and when changed it.

1. Make sure that what you thing is the key is indeed the key.
2. Wire all according to the sch.

RE: Philco 40-120C Filament Wiring - klondike98 - 02-15-2025

I think Morzh has a good thought. Check the key position. The under chassis diagram in the Philco info shows an “H” for heater where you are thinking pin 5. It also identifies the “P”, plate pin 2, as what you would call pin 6.

RE: Philco 40-120C Filament Wiring - bluecap - 02-15-2025

You're correct. I was misinterpreting the key position. I now know where it is, and things are making sense. I appreciate your help!

RE: Philco 40-120C Filament Wiring - bluecap - 02-15-2025

BTW, someone had replaced the original 20uF/20uF filter cap with a 47uF/33uF cap. Do you think these values are safe on the Philco?

RE: Philco 40-120C Filament Wiring - morzh - 02-15-2025

With these values I think you are OK; the Cmax for this tube is 40uF.
Your Vbb could rise a bit, but probably not too much.
I would use 33uF as the 1st cap and 47uF as the second. Provided the voltages of both are adequate.

RE: Philco 40-120C Filament Wiring - klondike98 - 02-15-2025

Icon_thumbup

Philco 40-120C Weak, Distorted Ouput - bluecap - 02-17-2025

I'm getting weak, distorted output from a Philco 40-120C that I'm restoring. I traced the location of the distortion to the plate on the 35A5 tube.  I've checked the tube's grid and plate voltages, and they're a bit high (by about 10%-15%) but not too bad. I've done the obvious things (e.g., replaced caps, resistors, tested the tube, etc.). Any advice?

RE: Philco 40-120C Weak, Distorted Ouput - morzh - 02-17-2025

Have you tried a different speaker?
How did you trace the distortion?
How do you know it is weak? What is the reference?

Have you checked the tubes?
Have you tried replacing the 35A5?

RE: Philco 40-120C Weak, Distorted Ouput - bluecap - 02-17-2025

Thank you for your reply. I pulled a speaker from a Philco 41-221 and received the same result. I used a signal tracer to trace the distortion. At the grid of the 35A5, the sound was clear. At the plate, it was heavily distorted. The sound from the speaker is barely audible, so even for a 1-watt radio, I believe it's way lower than it should be.

RE: Philco 40-120C Weak, Distorted Ouput - David - 02-17-2025

The higher voltage may be due to higher mains voltage. My mains run 120-125AC when the set was new mains would be 110-115AC might want to put together a bucking transformer. Have you checked the output transformer? The tube test good maybe try another tube have had tubes that would not work in a set but tested good. David

RE: Philco 40-120C Weak, Distorted Ouput - David - 02-17-2025

Just checked were I buy tubes price is four dollars. So living in Florid we have a local source for tubes. David     https://vacuumtubesinc.com/