DRBILL Posted August 9, 2006 Share Posted August 9, 2006 I am restoring a 1939 Zenith radio for an old friend who is quite ill. I want to replace the field coil speaker with a PM speaker. The field coil provides the magnetic field for the speaker and it also serves as a power supply choke. I need to replace it when I install the PM speaker. How can I measure the value of the old one so I can replace it with something similar? I looked this subject up in the Radio Amateur's Handbook, and had a flash-back to my first week of Greek grammar back in graduate school! I don't need several decimal places. I just need a BPF (ball park figure). Anybody? DRBILL Quote Link to comment Share on other sites More sharing options...
BEC Posted August 9, 2006 Share Posted August 9, 2006 Bill, I am guessing you don't have an inductance meter or meter with LC functions. That is the easy way to directly measure it. Seems I had to do some inductance measurements while in school about a hundred years ago by using a resistor, a signal generator and an AC voltmeter. Then solve the inductive reactance formula for "L". Don't think I even want to remember how to do that. Bob Quote Link to comment Share on other sites More sharing options...
DRBILL Posted August 9, 2006 Author Share Posted August 9, 2006 Bob, You have probably fooled with old gear that used the "electrodynamic" field coil speakers. I wonder how critical a replacement could be? Most of the research that I have looked at says to measure the ohms of the coil and replace with a similar value resistor at 10-15W. I'm sure this would work, but I am also sure that it would compromise the power supply and probably result in excessive hum. I wonder how much damage I could do with trial and error. I have a box of old (but good) chokes from Dynaco amplifiers. I'm tempted to hang one in and see if it smokes. BILL Quote Link to comment Share on other sites More sharing options...
BEC Posted August 9, 2006 Share Posted August 9, 2006 Bill, Only once did I do what you are doing. It was easy in that case because I had a schematic and that schematic gave the value of the field coil as an inductor. I just put in an inductor of close to that value and everything worked fine. I would say that if you could get away with just a resistor in place of the field coil, the power supply is not using it as a choke. In some circuits that could be the case. In the case of large wooden cabinet radios, I have seen a dynamic speaker installed and the old field coil speaker just left connected and placed in the bottom of the cabinet. I don't know if that is a possibility in the case you are working on. Bob Quote Link to comment Share on other sites More sharing options...
DRBILL Posted August 10, 2006 Author Share Posted August 10, 2006 TO: BOB & GILL -- I managed to find a schematic that had the value of the coil noted. I don't know if I will ever have another occasion to use it but I bought an inductance/capacitance meter on e-bay. It was remarkably inexpensive. It will probably be remarkably unreliable! Or calibrated in Ferds instead of Henrys! Thanks for your help and suggestions. BILL Quote Link to comment Share on other sites More sharing options...
DRBILL Posted August 12, 2006 Author Share Posted August 12, 2006 I spent the better part of the day surfing the net on the subject before us. The consensus is to replace the electrodynamic speaker with a permanant magnet speaker. Replace the choke with a 10W wire wound resistor of the same impedance. Double the capacitance of the first stage electrolytic (In this case it would be 16µF!). It would seem that the field coil did little except introduce hum to the speaker cone. The choke effect to the power supply was minimal. I've got a lot of stuff on order. The NOS tubes were easily found and very inexpensive! We shall see. The under-chassis is a nightmare. This set is the same age as I. My wife wondered if my insides were as bad. Probably! DRBILL Quote Link to comment Share on other sites More sharing options...
djk Posted August 12, 2006 Share Posted August 12, 2006 With a 16µF cap it is likely to be a choke input filter. Look at the schematic and see. If so, removing the choke and replacing it with a resistor will cause the caps to go over-voltgage and may blow them up, not to mention a drastic reduction in filtering that doubling the cap value will not make up for. Check the schematic please. Quote Link to comment Share on other sites More sharing options...
BEC Posted August 12, 2006 Share Posted August 12, 2006 Bill, I have lots NOS radio and tv tubes. Let me know if you need some of those in the future and I will check to see what I have. Bob Quote Link to comment Share on other sites More sharing options...
DRBILL Posted August 12, 2006 Author Share Posted August 12, 2006 DJK, Thanks for the "heads up". I don't think there will be much of a problem. We are talking about 100vdc. The choke was not where we would have expected it to be in modern amplifiers. Remember, this is 1939 and we were still feeling ourselves around. I'll see if I can scan this schematic and post it. They were just trying to find some fairly clean DC to provide a magnetic field for the speaker. Any improvement in the B+ was largely negated by the hum induced in the speakerby the field coil (choke). This is why the PM speaker was on the drawing board. DRBILL Quote Link to comment Share on other sites More sharing options...
DRBILL Posted August 12, 2006 Author Share Posted August 12, 2006 Bob, Thanks for the offer. Frankly, I don't hope to repeat this excercise. I just wanted to bring some joy to an old buddy. On the other hand, being retired and somewhat lethargic mentally, I find these excercises to be very beneficial. Sometimes you have to spark the corrosion off the old synapses to restore them. It is interesting to get into the minds of the early engineers to see how they solved problems. My kids find me bent over at my bench and shriek, "God, it looks like an old horror movie!" "Momma, you are going to have to do something with him!" Yes. DRBILL Quote Link to comment Share on other sites More sharing options...
BEC Posted August 12, 2006 Share Posted August 12, 2006 Bill, If you need another challenge to the synapses, I could send this old rig I picked up yesterday at a yard sale. AM, FM and Short Wave. Probably from the 50's. Sort of works on AM. 8 tubes, I think if I counted right. Bob Quote Link to comment Share on other sites More sharing options...
Georges Posted August 16, 2006 Share Posted August 16, 2006 Hi here a simple way to do it. I test it and worfs fine. regards george http://engr.nmsu.edu/~etti/fall96/electronics/induct/induct.html Quote Link to comment Share on other sites More sharing options...
Klipschguy Posted August 16, 2006 Share Posted August 16, 2006 Doc, Try posting your question at the Antique Radio Forum. They have helped me out in the past. Andy http://antiqueradios.com/forums/ Quote Link to comment Share on other sites More sharing options...
DRBILL Posted August 16, 2006 Author Share Posted August 16, 2006 Bob, We'd better wait to see how the Zenith turns out! Bill Quote Link to comment Share on other sites More sharing options...
DRBILL Posted August 26, 2006 Author Share Posted August 26, 2006 The project is almost finished. The 2000O 10W wirewound resistor replaced the choke/field coil perfectly with no ill effects. The big surprise was how conservative religious groups have taken over short-wave. Now I've got to see if I remember how to align an AM radio! DRBILL Quote Link to comment Share on other sites More sharing options...
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