Best Regards,
Bill Grimm
:I RECENTLY BOUGHT A 1940 RCA VICTOR AM RADIO AT A SWOP MEET. IVE SINCE RMEOVED THE CHASSIS FROM THE CABINET AND HAVE GOT ALL THE INFO I NEED TO REPLACE ALL THE CAPS AND RESISTORS WITH UPDATED VERSIONS. MY QUESTION IS THE SPEAKER. IT IS A 5" SPEAKER THAT HAS A TRANSFORMER MOUNTED TO IT. IT HAS SEEN BETTER DAYS AND IM WONDERING IF I NEED TO REPLACE IT. I MEASURED BETWEEN BOTH THE LEADS THAT GO TO THE TUBES AND IT MEASURES 660 OHMS. I MEASURED BETWEEN THE LEADS THAT GO TO THE VOICE COIL AND IT MEASURES .5 OHMS. THE SERVICE MANUAL THAT I FOUND ON THIS SIGN SAYS THE SPEAKER IS A 3 OHM. DOES ANYONE KNOW WHERE TO POSSIBLY FIND A REPLACEMENT TRANSFORMER OR IF I CAN UPDATE IT TO A NEWER VERSION? I ALSO NEED TO RECONE IS BECAUSE IT IS SEPERATED AND TORN ALL THE WAY AROUND THE FRAME. OR IS ANOTHER OPTION TO REPLACE IT WITH A NEW SPEAKER ALL TOGETHER? ANY ADVISE WILL HELP. THANK YOU.
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I would see if I could get the radio playing before I committed any work to the speaker. Even with its cone in tatters, the present speaker will work enough to tell if the radio is worth further investment of time and money. If you get the radio playing, it's no trick to replace your present speaker (which is probably an electrodynamic type) with a permanent magnet version. Contributors on this board can help with the technical details, if needed.
The direction that you go in depends if you value "originality" more or less than "functionality". It may be difficult (but by no means impossible) to find an exact replacement 5" ED speaker for your set; but it is absolutely no problem to find a 5" permag replacement with dropping resistor to replace the field coil.
I think you might be getting 3 Ohms impedance confused with .5 Ohms DC resistance. Also, you can do wonderful things to a speaker cone with a little white glue and some tissue paper.
Lewis.
This set uses a permanent magnet speaker, so the fix is easy- no need for a voltage-dropping resistor, since the original speaker doesn't have a field coil.
The audio output transformer is used to match the output tube circuit's high impedance to that of the low-impedance speaker. You have quite a bit of latitude in the speaker impedance but 4 or 8 ohms should work fine and are the impedances that are most common.
You can remove the output transformer from the old speaker and mount it anywhere it fits- the new speaker won't have a stamped "feature" to accept it like the old one did, but that doesn't matter. Just make sure that the output (secondary) wires are extended/insulated so that they can reach the new speaker. Where ever you mount it, the output transformer will have to be bolted in place since it is fairly heavy and leaving it "loose" in the set would be inviting problems.
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You can check the output transformer by unsoldering one of the two (secondary) leads that go to the speaker voice coil, and ohming across the secondary. If the reading is a few ohms or less, the secondary is probably OK. Next, if you ohm across the primary winding, you should see several hundred ohms. A very low reading or "open" reading means that you likely have a bad transformer. These transformers can go bad and when they do, the primary usually burns out due to a shorted plate bypass cap. I suppose primary shorts are possible, too, but I've never seen one in 48 years of repairing old radios.
It should be possible to find a new replacement transformer but you might have to order one from a guitar amp specialty house or antique radio supply vendor. EBay is also a good source for parts like this. If you do replace the transformer, buy one that matches the impedance (or tube type) of your output tube.
Finally, the transformer can be in horrible cosmetic shape and still work fine. I'd only be concerned about it if some of the windings were exposed and/or apparently damaged.