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Model 90 1st Detector Coil - Printable Version

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Model 90 1st Detector Coil - lasurveyor - 11-18-2017

Man this was disappointing. Started working on a Model 90 (1-47 tube) a while back and all has been going great. I started with the cabinet using lacquer toners and I'm still going to use pumice and rottenstone on it. Then the re-capping and resistors. That was a chore, re-stuffed the bakelite condensers and the two tobacco can ones. After the electrolytics and a third of the other caps were replaced I couldn't stand it and I powered it up. It played very well, although at low volume. Once more I powered it up, just before re-stuffing the two tobacco can caps, and again really nice, although low volume. Now it's finished other than alignment. I power it up, again it plays quite well, albeit low. After a minute or so, some static. Then some more, then a lot, then dead. Expletive!

First of course I inspected the last things I did and found no problems. Then I started checking coils and found the primary of the First Detector Transformer (part 7) open. I unraveled a little of it and boy is that wire fragile. It's thinner than the secondary wire and is evidently pretty corroded the way it falls apart. This is scary for me. Unwinding to a repair point doesn't seem possible with the corrosion. I'll have to rewind it. Never done that before so I've been reading up. There is a video on YouTube that rewinds the exact coil, that's cool, and elsewhere I read that the exact number of turns isn't real important but it's about 50 turns of about #38 wire.

Before I begin are there any pointers or tutorials that any of you could help me with. My plan is to start with a lot of photos, sketches and dimensions, then use #37 enameled wire for about 50 turns on the same physical location as the old winding. I'll use some cellephane between the windings and maybe have some clear nail polish handy. Should I aim for a specific impedence or ohms reading? My confidence needs a boost here.

Thanks,
Alan

RE: Model 90 1st Detector Coil - Radioroslyn - 11-18-2017

Hi Alan,
Well your lucky it isn't secondary it's fairly critical in terms of # of turns. The primary does two things it provides a path for the dc hv to get to the plate of the rf amp tube and it inductively couples the rf signal to the input of the mixer (some called it the 1st det).
The # of turns isn't particularly critical as it isn't part of the tuned circuit. What I've done is just measure the width of the winding and rewind the new winding to the old width. As long as your wire ga is within a few # the width is going to close enough.
Some bake the coil at low temperature after the rewind to dissipate moisture from the form. I've been lucky and haven't had to to make a rewound coil operate properly. The osc coil are more sensitive to the moisture issue, it lowers the Q (efficiency) of the coil.
Sound like you have a good plan dive it and do her!

GL

RE: Model 90 1st Detector Coil - morzh - 11-18-2017

Yes primary number of turns is less important. Secondary is.

RE: Model 90 1st Detector Coil - lasurveyor - 11-19-2017

Thanks for the encouragement guys, I'm feeling pretty good, I'll let you know how it comes out.

Alan

RE: Model 90 1st Detector Coil - morzh - 11-19-2017

To make things even easier: this transformer will work even if you switched winding direction to he opposite. It is recommended you always keep it, but in case you screwed up, in this case it is no big deal.