02-11-2012, 09:01 AM
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.
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.
--
Ron Ramirez
Ferdinand IN
