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Replaced the capacitors but all I get is a hum
#46

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.
#47

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

A friend in need is a pest!  Bill Slee ca 1970.
#48

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
#49

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.
#50

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
#51

And that's the reason for that hum-bucking coil that runs in series with the voice coil. Icon_smile
#52

Maybe...if the winding of the transformer that is open is the primary one, yes.
#53

I've never seen an open low-impedance secondary.
#54

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
#55

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.




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