[morzh]

Reversing "speaker leads" if it means the voice coil probably won't help much but could be worth a try.

reversing hum bucking coil will eliminate the possibility of it is adding to the hum instead of bucking it.

The idea of that coil is, it picks up the field created by the ripple in the field coil (ideally it should be DC, but when you use the coil as a filter it will have ripple) and then injects the current of opposite phase and of the same waveform into the voice coil.
If the ends are switched, the phase of that current will be the same with the ripple field and will add to it.

Switching the voice coil leads ideally should not matter.
However if we are to assume the output transformer picks up something (shouldn't - it is a closed magnetic core, but then it does have a gap as it is an SE transformer) from the field coil, switching the voice coil potentially might matter. Though probably way less than the bucking coil.

[Jamie]

Ok, I just double checked my work...

Resistor #65 had already been replaced with a new one.
Resistor #70 is a multi part wire wound resistor. The part number does not match the number listed on the schematic. The schematic says it should be part no 33-3037 (22ohms, 235ohms), and it is part no 33-3222 (22ohms, 47ohms, 275ohms).

[Jamie]

   

Ok, to reverse the hum bucking coil, I should swap these two wires? (marked in yellow)?

[morzh]

Yep. Those two.

[Jamie]

Ok, hum bucker is now reversed. Kind of a tough job for tired, old eyes. I somehow managed to poke a small hole in my speaker...arrrghh... As soon as the speaker repair glue dries, I'll put it back in the cabinet and give you a report.

[morzh]

Why put it in cabinet.
Also you could listen to hum without mending the hole.
Considering you are likely to have to undo the reversal and poke one more hole

Just listen to it.

[Jamie]

Ok, I listened to it. It still hums. No difference at all. Maybe even worse.

[morzh]

If it is worse then it needs to be put back the way it was.

BTW an interesting experiment for you and all of us would be, to connect the transformer directly to the voice coil without the bucking coil.

This can be done by unsoldering that end of the bucking coil from that post where the voice coil comes in and then running the wire directly to the transformer end where the bucking coil connects to it (you do not even have to unsolder this one).

The goal is to see if this has any effect whatsoever on the hum.
----------


I guess the next thing is to
1) try to swap the voice coil ends (as I said I doubt this matters) and
2) Ground the chassis.

BTW is the hum level really pronounced? Could it be a normal thing?

[morzh]

Last thing:

Are you sure it is the speaker hum?

If you disconnect the speaker do you still hear it? (only disconnect whith power unplugged).

Could it be the power transformer hum?

[Jamie]

I think I solved the problem. If you look at resistor #55 on the schematic, it connects to E-cap #67 and then also, to resistor #65. It was not connected to either. As soon as I connected it, the hum vanished. I could see no evidence that it was ever connected. I know Philco frequently made changes to schematics and parts, so I was a little concerned that maybe this was a late change, but voltages are good and hum is gone, so I think the problem is solved.

Edited to add - It was definetley speaker hum. No doubt about it. I also switched the hum bucker back. Hum was very pronounced. It wasn't so bad that you couldn't listen to the radio, but it would eventually drive you crazy. The hum was constant, no change with volume control or tuning. If you numbered the volume control on a normal radio, from 1-10... with 10 being loudest and 1 being the lowest volume you can hear from a 10 foot distance, the hum was a level 3.

[morzh]

What was the resistor connected to? If it was not connected to anything, your first audio triode (from the diode-triode tube) should have not worked at all as it would not have plate voltage.

[Jamie]

It was connected to a terminal point off the back of the chassis, and from there, three wires go to different locations - the 1st IF, 2nd IF, and RF, from what I can see.. I even did a continuity test to see if any hidden connection to the E Cap existed, and none did.

[Jamie]

I am sorry, one of the wires runs to the 42 tube. I think it is pin 3. My Mistake.

[morzh]

It seems that other than to the cap (power) it connected where it should.
But a good question is - what did provide the power to all the tubes if all these wires connected to the resistor (all of them are plate powers) did not go to the e-cap?

And the audio triode 75 then did not have the power either.

Can you describe where the power was going, and where was the #67 cap connected if anywhere?

[Jamie]

Positive end of Cap #67 was connected to resistor #65 (which ran to bakelite block #66) and also a wire from the speaker transformer. (There are two caps numbered 67 on the schematic, so I a not sure which one you are asking me about. I am referring to the one on the left.)