Replaced the capacitors but all I get is a hum - Printable Version
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+--- Thread: Replaced the capacitors but all I get is a hum (/showthread.php?tid=7826)
RE: Replaced the capacitors but all I get is a hum - BrendaAnnD - 11-12-2013
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.
RE: Replaced the capacitors but all I get is a hum - jerryhawthorne - 11-12-2013
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
RE: Replaced the capacitors but all I get is a hum - cwirth - 11-12-2013
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
RE: Replaced the capacitors but all I get is a hum - morzh - 11-12-2013
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.
RE: Replaced the capacitors but all I get is a hum - Mondial - 11-12-2013
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
RE: Replaced the capacitors but all I get is a hum - BrendaAnnD - 11-12-2013
And that's the reason for that hum-bucking coil that runs in series with the voice coil.
RE: Replaced the capacitors but all I get is a hum - morzh - 11-12-2013
Maybe...if the winding of the transformer that is open is the primary one, yes.
RE: Replaced the capacitors but all I get is a hum - BrendaAnnD - 11-12-2013
I've never seen an open low-impedance secondary.
RE: Replaced the capacitors but all I get is a hum - TV MAN - 01-09-2014
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
RE: Replaced the capacitors but all I get is a hum - codefox1 - 01-09-2014
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.