October 16, 2012 at 2:32 pm #3528Jim MacleanForum Participant
Hi there, I’m new to the vintage radio repair hobby and have encountered a small problem. I’m repairing a radio where the speaker field coil is the inductor for the B+ filter. Unfortunately, I can’t remove the speaker from the case without damaging the grill; needless to say I’m reluctant to do this.
My question is, can I substitute a resistor, of appropriate wattage and resistance, for the field coil for repair purposes. Is there a better solution?
RegardsOctober 20, 2012 at 4:38 pm #3534Bob MasseForum Participant
The info from these 2 links should help.
BobOctober 23, 2012 at 1:22 pm #3547philForum Participant
I would agree that there are better solutions , because as mentioned in the links, the field coil functions as a choke, wheras a resistor won’t. that said I have successfully gotten radios to work using a resistor, I have even seen where a previous owner had replaced the speaker with a more modern permanent magnet speaker,but retained the field coil so it works as a dropping resistor as well as a choke again a repair but perhaps a Kluge.
Now you didn’t mention the size of the speaker. Keep in mind that there are a lot of radio/phono combo radios that were somewhat boring and may have been gutted so larger speakers ( say 10 inch) are a little more abundant. The speakers for cathedrals and tombstones are often a bit smaller and since those radios are a bit more desirable to many collectors it seems that smaller ( say 5 inch) speakers are a bit harder to find. If you find a donor you will like to find one that is close to the same as far as the resistance of the field coil goes. It mayor may not be on the schematic.
Depending how many parts sets you have at your disposal, the best fix may be just to look for a donor radio, or if you want it perfect you may seek out the services of a speaker repair business that will work on older field coil speakers.
Older radios such as ones from the 20’s may have a high resistance fild winding in the neighborhood of 2000 ohms and you may find it a bit more difficult to find replacements, but many of these type of radios had separate speakers.I am kind of making the assumption that your radio is one from the 30’s or 40’s just because they are a bit more common. Dont take any of this as gospel. I have limited electronic bench work experience but thought I might add a tiny bot of insight and perhaps some ideas.
PhilOctober 30, 2012 at 10:31 am #3549Jim MacleanForum Participant
Phil and Bob, sorry for the slow response but thank you for the spot on advice. I think the speaker (10″) is Ok but after some testing I now believe the power transformer has some shorted winding – the high voltage secondary is asyemtrial meatured from the centre tap. I think once I get this issue sorted the audio issue will be resolved.
Again, thanks. JimNovember 9, 2012 at 12:43 pm #3555philForum Participant
I have noticed that they are sometimes a bit asymetrical when measured from the center tap and have heard that with some, since the first set of windings are closer to the core, they use less turns than the outer winding, which is further from the coil and having more windings. The resistance should be close, but it might not be exactly the same for both sides. I have seen power transgormers that still worked sort of , but ran hot and that was because a portion of the windings do work but they were shorted part way through the windings. If it runs excessively hot or if you see the tar stuff dripping out that might be a bad sign. You should replace the filter caps first, as it is common for the power supplies’ electrolytic filter caps to short and then burn out the power transfomer because of that.
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