Posts: 1,106
Threads: 88
Joined: Jun 2011
City: Tacoma
State, Province, Country: Washington
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.
Posts: 1,562
Threads: 56
Joined: Nov 2008
City: Sedona, AZ/Placentia/CA
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.
Posts: 21
Threads: 17
Joined: Jul 2013
City: MD
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
(This post was last modified: 11-12-2013, 05:00 PM by
cwirth.)
Posts: 15,307
Threads: 532
Joined: Oct 2011
City: Jackson, NJ
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.
Posts: 1,113
Threads: 14
Joined: Feb 2013
City: Irvington, NY
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
Posts: 1,106
Threads: 88
Joined: Jun 2011
City: Tacoma
State, Province, Country: Washington
And that's the reason for that hum-bucking coil that runs in series with the voice coil.
Posts: 15,307
Threads: 532
Joined: Oct 2011
City: Jackson, NJ
Maybe...if the winding of the transformer that is open is the primary one, yes.
(This post was last modified: 11-12-2013, 09:13 PM by
morzh.)
Posts: 1,106
Threads: 88
Joined: Jun 2011
City: Tacoma
State, Province, Country: Washington
I've never seen an open low-impedance secondary.
Posts: 1,083
Threads: 73
Joined: Jan 2014
City: Annapolis, IL
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
Posts: 2,128
Threads: 18
Joined: Oct 2008
City: Merrick, Long Island, NY
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.
(This post was last modified: 01-09-2014, 09:54 PM by
codefox1.)
