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Philco 46-1226: Strange Volume Problem - Printable Version

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Philco 46-1226: Strange Volume Problem - Al Green - 04-11-2023

New member here. I'm in the process of restoring a Philco 46-1226, code 125. Unfortunately, the condition it came to me in prevented me from testing it prior to the work I did. I say that because I don't know if the issue I'm dealing with was preexisting or the result of something I did during my work. The issue? When listening to AM, increasing the volume (turning the knob clockwise) causes the output from the speaker to decrease. This doesn't happen when I inject a sine wave into the phono input, i.e. the volume works as you'd expect.

I've replaced all of the electrolytic and paper caps throughout the unit and also swapped out any grossly out-of-spec resistors. I tested all of the tubes with my B&K 550 and replaced any that tested weak. If anyone has any ideas or has encountered this oddity before, please weigh in.

Thanks.

RE: Philco 46-1226: Strange Volume Problem - morzh - 04-11-2023

How'bout a simple stupid thing like inverting the volume pot 1st dn 3rd pins?

RE: Philco 46-1226: Strange Volume Problem - Al Green - 04-11-2023

I'd love to find a simple, stupid fix. But since it works properly when using the phono input, I think the volume pot itself is fine.

RE: Philco 46-1226: Strange Volume Problem - morzh - 04-11-2023

Check the Phono/AM switch. It is possible it does not make a good connection when in "Radio" position, and the signal goes through C26, in which case the volume pot would increase volume whgen going CCW.
The volume itself should be pretty low, lower than normal.

RE: Philco 46-1226: Strange Volume Problem - Al Green - 04-11-2023

Yes, even when the volume is at max CCW, the output in AM is still lower than I would expect.

I'll check the switch (again). I've cleaned it a few times, but perhaps there's still oxidation on the contacts.

RE: Philco 46-1226: Strange Volume Problem - morzh - 04-11-2023

Do an experiment. Short out the switch by a piece of wire. Or simply desolder the wires and twist them together to fully exclude the switch and possible wrong connections.

RE: Philco 46-1226: Strange Volume Problem - MrFixr55 - 04-11-2023

+1 on Morzh's idea to jumper the switch.

Not much should come through C26 as it is 100 pF (0.00001 uF), unless it is shorted. Also, look at the tone control. Some weirdness could happen if the wiper no longer connects to the resistance track.

BTW, with the Push Pull 6K6s (Successor to the 41), Cathodyne Phase invertor and the negative feedback, this should be no slouch of a receiver.

RE: Philco 46-1226: Strange Volume Problem - Al Green - 04-12-2023

I measured the resistance between the point after C200 (which is new) to the volume pot, and it was around .03 ohms. So I think that rules out the switch.

   

RE: Philco 46-1226: Strange Volume Problem - morzh - 04-12-2023

Somehow Rider's sch has different ref. designators on parts, but that's OK.


How about the volume pot, does it show correct end-to-end resistance and the wiper action? (I am driving at the top pot lug being disconnected due to a crack in the top pafrt of the rheostat).

RE: Philco 46-1226: Strange Volume Problem - MrFixr55 - 04-13-2023

Check the volume and tone control wiring and parts carefully. an open or wrong 4.7 ohm resistor between the pot and chassis ground may wreak havoc, as that is the injection point for negative (?) feedback.

RE: Philco 46-1226: Strange Volume Problem - Al Green - 04-13-2023

Thanks guys. I'll check as suggested and circle back with what I find.

RE: Philco 46-1226: Strange Volume Problem - morzh - 04-13-2023

It is a neg. feedback.
Inversion of the output winding or the plate wires of the primary of the output xfmr would also create an interesting behaviour.
Though it probably would manifest itself a bit different....like howling.
Had it once in a Grundig. Boy did it howl.....

RE: Philco 46-1226: Strange Volume Problem - Al Green - 04-14-2023

The volume pot measures close to the spec'd 2 meg and tracks fine when rotating it while watching the needle of my Simpson 270. The 4.7 ohm and 68 ohm resistors connected to the one leg of the pot are ok.

The tone pot actually measures high compared to what it should be: 10 meg vs 6 meg. Granted, this is in circuit. The 33k resistor coming off of one leg is ok.

RE: Philco 46-1226: Strange Volume Problem - morzh - 04-14-2023

If the tone pot is higher in-circuit, it means it high or even higher (depending on the circuit) out of circuit.

This said, the tone pot almost does not matter in this situation.

What would happen if: (top/right is used as from the sch point of view, not physical locations)

1. Fully disconnect C200 top pin from J200 test terminal (fully disconnect, the pin should be unsoldered).
2. Fully disconnect C202 right pin from the volume pot's wiper terminal.
3. Connect that C202 pin to the J200 Test terminal.

Does the volume become loud, as if at maximum volume? (be careful with local stations).

RE: Philco 46-1226: Strange Volume Problem - Al Green - 04-16-2023

Figured it out. Turns out that when I replaced C206 I connected it to the lug for pin 6 of the 6J5GT, rather than pin 5. Lug 6 is unused for the tube, but acts as a tie point for where the radio signal goes to the bandswitch and C201 (part of the negative feedback?). I moved the C206 to pin 5 and correct volume operation was restored, and it was significantly louder than it had been before, which is no wonder.

Thanks for the help with this.