After spending $$$ to get the speaker cone replaced, I've determined there is a break internal to the field coil (and not at the lead connections--I checked that first). So I'm faced with the task of removing the field coil bracket and freeing up the coil so I can rewind it. I understand the model 90 coil retainer is press fit but I'm not sure how to go about removing it. Any suggestions on disassembly and rewinding would be appreciated. I understand the field coil resistance to be around 3100 ohms. The voice coil is fine, it shows .7 ohms.
Thanks Bob, that gives me a great start! I will need to soak the edges of the cone with acetone to soften the glue that holds it on. But I think I can manage the winding. I wind guitar pickups and it should be similar...Just hope I can force the pin out!
OK, progress! A liberal application of acetone to a small section of cone edge, followed by very careful lifting of the edge by an exacto blade. I got off the cone with most of the edge intact. Enough to use to put it back...
Then a piece of round key stock just the size of the pin along with a couple of 2x4's under the speaker assembly and a good pounding with a hammer eventually persuaded the pin to drop out. Field coil is free to remove and I'm a happy camper!
Thanks for the encouragement and help. Now on to the winding--let's see where did I put that magnet wire?...
The original wire was .005" in diameter, or 36 guage. I've got a large spool of 38 guage so I'm going to try that.
(12-01-2016, 04:39 PM)rrova Wrote: [ -> ]The original wire was .005" in diameter, or 36 guage. I've got a large spool of 38 guage so I'm going to try that.
The 38ga is good for 130ma vs 36ga is 210ma. But then the 80 tube is only good 125ma output.
(12-01-2016, 05:23 PM)Radioroslyn Wrote: [ -> ] (12-01-2016, 04:39 PM)rrova Wrote: [ -> ]The original wire was .005" in diameter, or 36 guage. I've got a large spool of 38 guage so I'm going to try that.
The 38ga is good for 130ma vs 36ga is 210ma. But then the 80 tube is only good 125ma output.
Ah, good point! Thanks for letting me know. I've got a stash of unmarked wire, I'll see if I can come closer.
I found some .005" diameter wire and wound around 10,000 turns on the coil. But it only reached 1K ohms, not the 3.1K specified for this coil. I pretty well filled it, so here's hoping it will work. I can add a 2K resistor to match the spec I guess. I've read it's not the resistance it's the turns that count...
Resistance should be ok @ 1k. The fc isn't used to develop the bias voltages. May have to dink around with the values of the filter caps to get rid of any residual hum and get the HV close to spec.
OK, re-installed the field coil (used an old socket from a wrench set to hammer on to push in the force fit pin--needed to clear the spider screw holder) and re-installed the cone (broke the spider--aaarrrggghhh! But repaired it, mostly) Added new leads to replace the old rotten wire and resoldered them all. Hooked the field coil to 40V DC, and fed the Voice coil from my stereo with some good Fathead blues and it sings again! Whew! Now to attach the 4 pin cord that connects to the Model 90 radio and back to troubleshooting the 1931 Cathedral radio. Thanks to all for the help!
