Internal capacitor values for MR65 mixer output transformer?

chazix

AK Subscriber
Subscriber
Hoping for a life line, and a long shot at that!

I've got a very pretty MR65 (s/n 107R0) on the bench from a fellow AKer. The mixer output transformer (T2, Automatic Radio part T-107-134A) appeared to be suffering from the silver migration problem. Have done the surgery to lose the internal capacitors, and that went pretty well. I'm planning to install external silver mica parts.

I don't suppose anyone has already sussed out what values might work well for external caps?

I did try some bench testing in isolation, but I'm not sure whether to trust that for any better than an order-of-magnitude estimate. Seems like the order of 10-100pF is the right ballpark for both primary and secondary. I'm not even sure whether to expect primary and secondary to be the same.

Thanks!

chazix
 
My notes show the caps are 24pF ea. You can probably use anything betw 22 and 27 pF. Realignment nec after surgery.
 
Thanks, will give that a try! Realignment, for sure - that was always in the cards anyway.

Cheers,

chazix
 
I installed the post-surgery transformer back into the tuner and did some resonance testing with tacked-on capacitors. The secondary was indeed happy with 24pF. Had to go hunting on the primary side, though - it turned out to be happy with 68pF.

I'm not sure whether to be worried that I arrived at such different values for primary and secondary, but I guess I'll press on with a permanent installation and see how things go from there. For sure the tuner is closer to healthy than it was pre-surgery.
 
A little surprising pri cap would be so different from sec. Doesn’t match my experience. Maybe the pri slug was way out of position so proper resonance would be achieved with greater cap. FWIW, the removed mica cap discs can be measured after migrated silver is cleaned up, Overall, I wouldn’t worry about the difference; if it works it works.
 
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