[BrendaAnnD]

Ah... that would do it.

Yes, the speaker must be connected when taking voltages. The field coil is the large coil on the back of the speaker. It has a dual purpose: It serves as a magnet for the speaker and as a filter choke for the power supply. Without that in line, no voltage.

[jerryhawthorne]

Yes Andrew, as Brenda indicated with the field coil open (disconnected) you will get no voltage anywhere but the rectifier. Need to have the speaker plugged in to give it a chance. The field coil is part of the speaker and the power supply.
Jerry

[cwirth]

I recently purchased a 42-380 console at an auction. I replaced all the caps and had the same problem, just a low hum. I took the radio to a local repair person and he found that the output transformer under the chassis was open. We bought a new one and it corrected the problem. The radio works great now.

Charlie

[morzh]

Charlie

I find this story a little strange: if your output transformer is open you should not hear any hum whatsoever. Maybe you could hear the transformer buzz, but there is nothing to make the speaker hum.

[Mondial]

Morzh, you still can get a hum because of the pulsating DC flowing through the field coil. The resultant varying magnetic field induces a voltage in the voice coil which causes it to move.

Same concept as an AC induction motor

[BrendaAnnD]

And that's the reason for that hum-bucking coil that runs in series with the voice coil.

[morzh]

Maybe...if the winding of the transformer that is open is the primary one, yes.

[BrendaAnnD]

I've never seen an open low-impedance secondary.

[TV MAN]

Andrew, I am quite curious how this turned out. Did you find the problem? I would really like to know where you got a "capacitor kit" for your radio. I have a model 70 I am working on, and am sure I will need to replace the caps also.

Tim

[codefox1]

Welcome. Many times the low wattage carbon resistors have gone way out of spec, and a fistfull of replacements is only a couple of dollars. Do this and report back.