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model 86 fm detector problems - Printable Version

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model 86 fm detector problems - daddydonn - 06-13-2012

Hi guys: Just new to this site. I have a model 86 which is a mantle am/fm radio from ~1949. it's not the earlier unit from the 20's. I have repaired it and it works, sorta! Fm is extremely distorted and an attempt to align it fails at the ratio detector secondary trimmer. There is no effect in adjusting the coil slug. No null no nothing. Just very distorted. I would suspect an open cap across the secondary but the schematic shows none. Another weird thing. I get no audio unless I remove the 2mf cap in the detector! (new cap=no improvement) The 19c8 tests ok and swapping with another one makes no change. Anyone with any ideas? It would be greatly appreciated. This is not the most sophisticated radio and rather lo-fi but it belongs to a friend and I would not like to let her down. Thanks

RE: model 86 fm detector problems - codefox1 - 06-13-2012

Please post schematic or link. Can't help you without it. If set has a 12AT7, they are notorious, so swap in another one.

RE: model 86 fm detector problems - daddydonn - 06-13-2012

Hi: thanks for quick response. The fm lineup is 12ba6 rf amp, 12ba7converter, (2) 12ba6 if amps & 12c8 ratio detector diodes (and first af amp)
I'll scan & upload schematic tonight. BTW am works fine and all tubes have been tested in Jackson tube tester (all ok)

RE: model 86 fm detector problems - daddydonn - 06-25-2012

Hi guys. Finally got the schematic scanned. I'm showing only the last I.F. and the ratio detector. I'm wondering if there should be a cap in the secondary of the ratio detector trans. Some other schematics show one but not here. If it has one and it's open that might explain my not being able to get a null. Any ideas? Thanks[/quote]

RE: model 86 fm detector problems - Radioroslyn - 06-26-2012

Check to see if the coil is open, other than that check the voltages around the 19T8 to see if they are good
GL
Terry

RE: model 86 fm detector problems - daddydonn - 06-27-2012

Hi Terry:
Thanks for the reply. I did check the coil both primary and secondary and they are not open and the tertiary(?) winding has connection to the secondary winding. Another funny thing, there is no sound at all with c314 in place, and it has been replaced. I've just bought an EICO sweep marker and I want to see what the fm bandpass looks like. Philco complicated things by choosing 9.1 mhertz as the if. I already have a Heath sweep marker but it is for 10.7mhz not 9.1. The EICO has variable marker (I believe) so I should be able to adjust it to the Philco i.f. We'll see how that goes. I keep wondering if there should be a cap across the secondary of the ratio transformer. The schematic does not show one but similar schematics for other radios do. What do you think?

RE: model 86 fm detector problems - Don Lind - 06-27-2012

That discriminator coil was a trouble spot for those radios. Look for a broken slug. There should be some internal caps as well, but less likely to be bad.

RE: model 86 fm detector problems - daddydonn - 06-27-2012

Thanks Don. Guess I'm going to have to open it up. I already opened up the first fm if xfmr. It had an open wire, which I fixed. I know I was sweating bullets, pretty well sure I'd never find a replacement if I went to far. I'll post my results when I'm done. Thanks again

RE: model 86 fm detector problems - daddydonn - 07-05-2012

Thanks to Don Lind for his suggestions re- the ratio detector coil. I took it apart and found that the slugs were intact and did move up and down. I checked for an internal cap and there was one on the secondary only. It showed no capacitance whatsoever with the coil unsoldered. I cut it out of the circuit and carefully reassembled the can. I had no idea what value of cap to use in the secondary circuit. I dug into my stock of small mica caps and came up with a 47pf unit which I soldered in. I was now able to do a proper alignment and the secondary adjustment does not show a null. The radio now plays distortion free. Thanks again for everyone's suggestions.

RE: model 86 fm detector problems - daddydonn - 07-05-2012

sorry for the typo in the last message. It should have read that the secondary does NOW produce a null and all distortion has gone away!