Hi All,
I picked up this nice radio with a home-made table-top cabinet last Spring (yes - makes a rather unusual table radio!!!), and was disappointed to measure the field coil at 4.7M ohms instead of about 1k ohm per Rider's, and it's the original "Giant Theatre-Sonic" speaker. I bypassed it with 4k ohms for testing, and got 157 VDC, but right away began getting more than minimal volume from the speaker. After a few minutes, I shut it down and the field coil now measured 6k ohms. After a few more minutes, 5k ohms. After awhile, I began getting full normal B+ through it even with my 4k bypass resistor removed. Did this field coil have some kind of "microgap" in it? Perhaps from a little corrosion? Is this coil going to open up again after a few hours operation? I sure wish I had a back-up field coil for it, and someone who can install these! (Looks like field coil can't be changed without major speaker surgery.)
Thanks,
Eddie

Did you disconnect the field coil when taking your resistance readings?

:Hi All,
:I picked up this nice radio with a home-made table-top cabinet last Spring (yes - makes a rather unusual table radio!!!), and was disappointed to measure the field coil at 4.7M ohms instead of about 1k ohm per Rider's, and it's the original "Giant Theatre-Sonic" speaker. I bypassed it with 4k ohms for testing, and got 157 VDC, but right away began getting more than minimal volume from the speaker. After a few minutes, I shut it down and the field coil now measured 6k ohms. After a few more minutes, 5k ohms. After awhile, I began getting full normal B+ through it even with my 4k bypass resistor removed. Did this field coil have some kind of "microgap" in it? Perhaps from a little corrosion? Is this coil going to open up again after a few hours operation? I sure wish I had a back-up field coil for it, and someone who can install these! (Looks like field coil can't be changed without major speaker surgery.)
:Thanks,
:Eddie

These old field coils often have breaks buried within the windings. And, maybe at the terminals, too?

My guess is that the coil temporarily healed itself due to the heat? Hope for the best, but expect the worst - the problem may come back. You can always replace the speaker with a PM type, and install a power resistor in place of the field coil.

By not disconnecting a component, the resistance might read low, but not high.
Doug

:Did you disconnect the field coil when taking your resistance readings?
:
::Hi All,
::I picked up this nice radio with a home-made table-top cabinet last Spring (yes - makes a rather unusual table radio!!!), and was disappointed to measure the field coil at 4.7M ohms instead of about 1k ohm per Rider's, and it's the original "Giant Theatre-Sonic" speaker. I bypassed it with 4k ohms for testing, and got 157 VDC, but right away began getting more than minimal volume from the speaker. After a few minutes, I shut it down and the field coil now measured 6k ohms. After a few more minutes, 5k ohms. After awhile, I began getting full normal B+ through it even with my 4k bypass resistor removed. Did this field coil have some kind of "microgap" in it? Perhaps from a little corrosion? Is this coil going to open up again after a few hours operation? I sure wish I had a back-up field coil for it, and someone who can install these! (Looks like field coil can't be changed without major speaker surgery.)
::Thanks,
::Eddie