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Philco 91 Mystery - Printable Version

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Philco 91 Mystery - Mark H - 02-10-2012

This is my first time posting and am grateful to be part of this forum ! I recently started working on a Philco 91 that I had acquired several years ago and am trying to figure out what is a mystery to me? I am told that Ron is the Philco expert, so I am very curious to see what he says ! A little background...When I opened this radio I discovered that two sections of the original BC resistor (900/2700 ohm) had apparently become very hot and at sometime had been replaced with two ohmite type ceramic resistors...one of them had deteriorated as well. The only other work that I could see done was two older dry electolytic caps had been wired in to replace the two originals. Anyway, I got the schematic and all the info I could from several sites and began to restore the radio to original specs. I replaced all the caps, rewound one coil etc etc. When i slowly powered the radio up I could not receive stations well....I took tube voltage readings and noticed the SG voltage on the RF/44-Det/36-IF/44 were way out of spec...155 volts instead of 50/80/85...I also noticed that the new BC resistor I had assembled out of 25 watt ohmites was getting pretty hot in the 2.7K ohm section....after several posts on another site, another guy with a Philco 91 opened up his set and found the same voltages ? The tubes are only rated for 90 volts max on the SG, so how could this be ? I tried replacing the 900 ohm section with a 5K ohm resistor and the SG voltages dropped to around 77 volts and the radio plays beautifully now all across the dial. All the info I have says the BC resistor was as I found it (900/2700/95/205). I am happy with the repair. I am just wondering how Philco built this radio the way it did ? I don't see how the Screen Grids on those tubes could be what they call for with that 900 ohm resistor ? Were there lot's of problems with Philco 91's ? Anyway, I am curious to see what others have to say.

Thanks for any input !

Mark H

Re: Philco 91 Mystery - LASJayhawk - 02-10-2012

Just so nobody tries to reinvent the wheel, his thread from arf is here:

http://antiqueradios.com/forums/viewtopic.php?f=6&t=185407

Re: Philco 91 Mystery - Ron Ramirez - 02-11-2012

Mark came here for the answer, let's try to give him an answer here if we can.

OK Mark, I've finally found a little time to sit down and examine this situation. I have three 91 sets, but none of them are working at this point.

Which version of the 91 do you have? AM only or AM and "police" band?

I have looked in a 1933 Philco parts catalog in addition to studying the schematic, and I found a discrepancy. Philco service Bulletin 129 for the early 91 gives the resistances for the "B-C" resistor as 900, 2700, 95 and 205 ohms...but the 1933 parts catalog specifies 1200, 2400, 95 and 205 ohms. The overall resistance is the same, just distributed differently.

Complicating matters is the fact that Bulletin 129-B, covering the late Model 91 and the early Model 14, does not bother to give the resistances of the "B-C" resistor sections.

I've been doing some voltage/resistance/power calculations, and they do not make sense. I need to pull one of my 91 chassis and confirm the resistance readings myself. I see Norm Leal has already done that, but as I have two of the late chassis, I should pull one and see if the late version is the same or not.

The circuitry is basically the same as the late version 90 with two 47 outputs. I used to have one of those - but no more, it was sold several years ago. It worked very well with the same "B-C" resistor.

Comparing the late 91 and early 14 with the later 14, I see Philco changed the resistors used to drop B+ voltage to the screen grids of the RF and IF amplifier tubes - from 900 to 32,000, and from 2700 to 70,000 ohms. The B+ fed to the 32K resistor is taken directly from the output filter capacitor, not dropped by a large value field coil as in the 91 and early 14.

Don't know if I'll have time to check one of my 91 chassis this weekend - my weekend is pretty well booked up with things to do - but I'll see what I can do.

But also keep in mind that with today's higher line voltages, and modern voltmeters being far more accurate than the old 1000 ohms/volt meters that were considered "state of the art" in 1932, voltage readings are going to be higher than published values. The only way to really get voltage readings that agree with the Service Bulletins is to drop your line voltage to 115 (by use of a Variac), and to use a 1000 ohms/volt analog voltmeter for voltage measurements. These could be purchased new as recently as the 1970s; a manufacturer named Calrad (which specialized in cheap instruments back then) comes to mind for these.

Re: Philco 91 Mystery - Mark H - 02-11-2012

Thanks for you reply Ron, I was very anxious to hear what you had to say. The Philco 91 that I have is just the AM band model. Norm and Curtis were very helpful to me on ARF. I was very reluctant to make that kind of change to my radio without back up from expets. I am not an expert by any means. I was not able to find anything about Philco changing the BC resistor, so again, thanks for all the info...it really helps explain things for me. The radio is working wonderfully now.

Thanks,
Mark H