I have an EM speaker from the 1940's in a zenith console that someone cut all of the wires, including the wires on the speaker. I took some ohms readings and was wondering if someone can tell me if my speaker system looks close enough with resistance measurments to function. Also, is there any way to test the system with it out of the chassis circuit?

Tramsformer:
Primary:528ohms
Secondary: .5ohms
Field: 1kohms
Cone: 4.5ohms

:I have an EM speaker from the 1940's in a zenith console that someone cut all of the wires, including the wires on the speaker. I took some ohms readings and was wondering if someone can tell me if my speaker system looks close enough with resistance measurments to function. Also, is there any way to test the system with it out of the chassis circuit?
:
:Tramsformer:
: Primary:528ohms
: Secondary: .5ohms
:Field: 1kohms
:Cone: 4.5ohms
Hi Dave
Well the resistance measurements sound close enough for the speaker to work, as far as checking out of circuit, if your console is like the Zenith I have, its going to be pretty difficult to check out of circuit, you won't have an amp or a load, unless you have an external amp and load/speaker to test the chassis with.
Mark

You could test it out by supplying a B+ voltage of appropriate value across coil and then provide AF signal to speaker voice coil. This would not check output transformer but the resistance readings would indicate it is OK. Actually, the readings seem to indicate it will work barring some mechanical problem with the cone. PL

Don't put B+ across the field coil - something would burn out, either the coil, the recitifier tube, or the transformer. In normal service, the field coil is in series with the plate circuit. For example, see: http://www.nostalgiaair.org/Resources/689/M0024689.htm

: You could test it out by supplying a B+ voltage of appropriate value across coil and then provide AF signal to speaker voice coil. This would not check output transformer but the resistance readings would indicate it is OK. Actually, the readings seem to indicate it will work barring some mechanical problem with the cone. PL

I thought I've seen some early circuits where the field coil was fed DC at one end and goes directly to ground on the other. It wasn't even used as a choke in the power supply.
If fed pulsating DC such as that right out of the rectifier then the inductive caracteristics should present a resaonably high impedance so as not to look like a simple 1k ohm path.
Now with clean DC..I'm not sure what power it can dissapate but if you feed a 1k ohm coil with 100 vdc it would need to disipate 10 watts.. is that correct?

Peter, I think your math is correct. But, if this is a conventional pre-war Zenith, the field coil is used as a power supply choke, and should not be hooked up across B+, 100VDC, or anything like that. Those things can't safely disipate 10W (or much higher if its hooked up across B+ on an AC set).

When you say hook it up right out of the rectifier to ground, the filter caps, if there are any, are still there in parallel with the coil, smoothing the DC that the coil sees.

:I thought I've seen some early circuits where the field coil was fed DC at one end and goes directly to ground on the other. It wasn't even used as a choke in the power supply.

: If fed pulsating DC such as that right out of the rectifier then the inductive caracteristics should present a resaonably high impedance so as not to look like a simple 1k ohm path.
:Now with clean DC..I'm not sure what power it can dissapate but if you feed a 1k ohm coil with 100 vdc it would need to disipate 10 watts.. is that correct?

Doug, you are correct about the Speaker field. The system I have is just like the link you provied (field in series) except my primary on the transformer is not a center tap. Cap you tell me in a simple mans way exactly how this system works.

The speaker needs a magnet to operate, which is what the field coil is, an electromagnet. You need to run DC current thru it. By putting it in the power supply, it does double-duty as a choke, helping to smooth the ripple in the B+ supply along with the filter capacitors.

If the field coil is open, there will be no B+ to the rest of the radio, and it won't play.

Hope this helps.

:Doug, you are correct about the Speaker field. The system I have is just like the link you provied (field in series) except my primary on the transformer is not a center tap. Cap you tell me in a simple mans way exactly how this system works.

I noticed something else. The seconday of the transformer doesn't seem correct. 0.5ohm seems a little low for an output transformer secondary. Are you sure you read a half ohm?

:The speaker needs a magnet to operate, which is what the field coil is, an electromagnet. You need to run DC current thru it. By putting it in the power supply, it does double-duty as a choke, helping to smooth the ripple in the B+ supply along with the filter capacitors.
:
:If the field coil is open, there will be no B+ to the rest of the radio, and it won't play.
:
:Hope this helps.
:
:
::Doug, you are correct about the Speaker field. The system I have is just like the link you provied (field in series) except my primary on the transformer is not a center tap. Cap you tell me in a simple mans way exactly how this system works.

I am sorry, I thought you had a push-pull output tranny. I was thinking of something differnt. Yes, a half ohm seems correct for the secondary.

:I noticed something else. The seconday of the transformer doesn't seem correct. 0.5ohm seems a little low for an output transformer secondary. Are you sure you read a half ohm?
:
::The speaker needs a magnet to operate, which is what the field coil is, an electromagnet. You need to run DC current thru it. By putting it in the power supply, it does double-duty as a choke, helping to smooth the ripple in the B+ supply along with the filter capacitors.
::
::If the field coil is open, there will be no B+ to the rest of the radio, and it won't play.
::
::Hope this helps.
::
::
:::Doug, you are correct about the Speaker field. The system I have is just like the link you provied (field in series) except my primary on the transformer is not a center tap. Cap you tell me in a simple mans way exactly how this system works.

Doug:
Here's an example where the field coil goes diectly to ground right off the rectifier.
http://www.nostalgiaair.org/PagesByModel/737/M0005737.pdf

I guess I should have said a DC voltage of appropriate value across coil, not B+ of appropriate value. Ether way the voltage would have to be computed by using the drop across the field coil which you should be able to get from the schematic. That value should produce a sufficient magnetic field to test the speaker.