12-01-2009, 12:32 PM
Don't give up yet!
Your resistance readings seem to indicate the old power transformer might still have some hope. The imbalance in the high voltage winding may indicate a potential issue, however...but those two resistance readings on either side of the center tap will never be equal, anyway, due to the way these transformers were wound. One half of a center-tapped winding will always have a somewhat lower resistance than the other half.
Here's a quick and dirty (and very easy) test you can perform.
Remove the rectifier tube from the radio. Unplug the speaker connector from the speaker.
Plug the radio in and turn it on.
If it goes POOF, forget about it...you need a new power transformer. But it probably won't go POOF.
Assuming it does not make any strange noises nor begin to emanate smoke, leave it on for a few minutes. Do nothing else. Do not assume all is well and put the rectifier back in.
Now turn it off, unplug it (important), and once you have unplugged it, see if the power transformer is hot by (carefully) placing your hand upon it. Do this very carefully...be prepared in case it is hot!
If it is hot:
The power transformer is probably toast.
If it is cool:
It may have some life in it yet.
Now go and doublecheck that field coil resistance. Measure it while the speaker connector is still disconnected. Remember, the field coil is the large coil in the back of the speaker, and should read in the neighborhood of 1100 ohms. (There are four pins on that speaker connector. You should read around 1100 ohms between the two inner pins, and only a few ohms across the two outer pins. That is the voice coil winding.) The small transformer under the chassis that is somewhat close to the power transformer is the audio output transformer, so make sure you are measuring the right thing.
Your resistance readings seem to indicate the old power transformer might still have some hope. The imbalance in the high voltage winding may indicate a potential issue, however...but those two resistance readings on either side of the center tap will never be equal, anyway, due to the way these transformers were wound. One half of a center-tapped winding will always have a somewhat lower resistance than the other half.
Here's a quick and dirty (and very easy) test you can perform.
Remove the rectifier tube from the radio. Unplug the speaker connector from the speaker.
Plug the radio in and turn it on.
If it goes POOF, forget about it...you need a new power transformer. But it probably won't go POOF.
Assuming it does not make any strange noises nor begin to emanate smoke, leave it on for a few minutes. Do nothing else. Do not assume all is well and put the rectifier back in.
Now turn it off, unplug it (important), and once you have unplugged it, see if the power transformer is hot by (carefully) placing your hand upon it. Do this very carefully...be prepared in case it is hot!
If it is hot:
The power transformer is probably toast.
If it is cool:
It may have some life in it yet.
Now go and doublecheck that field coil resistance. Measure it while the speaker connector is still disconnected. Remember, the field coil is the large coil in the back of the speaker, and should read in the neighborhood of 1100 ohms. (There are four pins on that speaker connector. You should read around 1100 ohms between the two inner pins, and only a few ohms across the two outer pins. That is the voice coil winding.) The small transformer under the chassis that is somewhat close to the power transformer is the audio output transformer, so make sure you are measuring the right thing.
--
Ron Ramirez
Ferdinand IN
