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Philco 96 work around? - retiredinprescott - 07-12-2013

I've started work on a cosmetically nice Philco 96 (one of Philco's last TRF sets, I believe). This set seems clean yet it has a lot of electronic problems with open power resistors, coils and an open input transformer.

The primary of the input transformer (no. 38 in schematic) from the plate of the 27 first audio tube to B+ is open. Secondary goes to push pull 45s and is OK. Question: I know it will be difficult to find another input transformer to drop in so can I use RC coupling as a work around for this set? I have done similar things with battery sets with open interstage audio transformers. I've used about 22K resistor across the open primary and a 0.05-0.1uF capacitor between the primary and secondary of the transformer. Do you think that would work here?
I've attached part of the schematic showing the input transformer.

Thanks,
Mark
   

RE: Philco 96 work around? - codefox1 - 07-12-2013

I'd like to know what caused the primary to become open. A failed capacitor in what appears to be the tone control could have caused it, but you will change them out anyway, right? Also could be a corroded connection between the winding and the lead or terminal of the transformer primary.

Dunno about an alternative phase inverter plan without hacking up the set, but if there is a simple enough solution that surfaces, why not try?

RE: Philco 96 work around? - BrendaAnnD - 07-12-2013

Hammond has transformers that work very well in this application. Better to use those than to try to make a phase inverter that would work on this set. http://www.tubesandmore.com/products/P-T124B

RE: Philco 96 work around? - jerryhawthorne - 07-12-2013

Mark, at least you have a schematic! Nice to see you back on the old ones. I would go with Brenda's advise, after all, it's only money. We just got back from 4 days in Tahoe, wife would like to go back if we didn't have to drive and could be teletransported some how. Back to working on the Monarch of undefined province, chassis seems to work fine, on to the cabinet. Post a picture or two of your project and did you get my e-mail on some tubes you might like?
Jerry

RE: Philco 96 work around? - Ron Ramirez - 07-13-2013

Mark

Of course it will work, and it will work surprisingly well. I've written about using a cap and resistor to sub for an open interstage primary here:

http://www.philcoradio.com/tech/audio.htm

RE: Philco 96 work around? - retiredinprescott - 07-13-2013

Ron,
Thank you for jogging my memory. I have seen your article before but it was a few years ago. I think your modification B is just what I need and it can be done under the chassis , discreetly, in a way that is easily reversed should I find a good original transformer some day. Best of all, the cost is very low!
Thanks,
MarkIcon_biggrin