The PHILCO Phorum

I'm working on a 1934 Packard radio. Philco PA Packard but Chuck is sending me paperwork for a Philco Model "A" Receiver. Riders stuff isn't too good but I'll post the link anyway.
http://www.nostalgiaair.org/PagesByModel...014113.pdf
Here's a picture of the can.



The Riders paperwork calls this out as part #48 which just says condenser but with a part# of 7774. The part number on the can is 7440-X.

The schematic also shows this capacitor as being part of a biasing circuit for the 1st Audio #37 tube. This is not what was originally in my radio. Just #38, 2.5K resistor was biasing the cathode of the 37 and this #7440-X can was connected to the cathode of the 2nd Detector #85 tube along with a 5K resistor. And a switch that appears to be some kind of "Sensitivity" switch.

So I'm trying to locat exactly what value of capacitor was in this can. One end connects to the cathode of the 85, other end to chassis ground. Across the leads of the can it reads about 135pfd right now and each lead reads about 45-55pfd to the can itself. So the cap is dead.

Any ideas of what was in a can with the Philco part #7440-X or #7774 on it? I'm gonna guess a .05 or .1ufd.

Pepperoni

According to my Philco 80 schematic which shows a 7440 cap it is a 10uf DRY electrolytic cap.

Thanks Keith! Philco and their numbers. Yours is correct being a 10ufd (probably 450V) filter cap on the B+ line with the 300 something ohm wirewound #35 I think, resistor. But this is a car radio and all of the filter caps are in a can on the dynamotor housing. No filter caps on this chassis. I've identified all cap can values (less this one) and none of them are lytics.



And there would be no reason for a 10ufd lytic filter cap on the cathode of the 85 2nd detector in this radio. Maybe a 1ufd or 2ufd @ 25 volts. I'm just not sure. My Riders paperwork calls it out as part #7774. What is the asterisk next to the part number on your 10ufd for? Possibly some note?

Like I said... Philco and their numbers. I'm hoping the paperwork from Chuck S. will shed more light on this. Maybe later someone else will chime in and know for sure.

Thanks,
Pepperoni

The * is indeed a note that says this cap was eliminated is production runs over a certain number.

Keith, Here's a model 80 schematic that must have been a later run because that #37 10ufd cap isn't on the schematic. Maybe replaced by the 8ufd and 4ufd.
http://www.philcoradio.com/tech/images/80.jpg

I have some 1 & 2ufd tubular non lytic caps and I'm tempted to put one of those in that can. Still waitin on the paperwork from Chuck to see if it helps any.

Pepperoni

That 10 uF dry electrolytic in the early model 80 was a low voltage cap, placed across the bias resistor (#36) in series with the power transformer center tap to ground. Evidently it never needed to be so big because it was replaced with a .015 uF paper cap (#32 on the schematic) in later production.

I have a feeling the same thing happened with your receiver. Philco found the electrolytic was not necessary and eliminated if or replaced it with a smaller paper cap.

I am sure a 1 or 2 uF would work fine as a cathode bypass.

Thanks to the both of you for your help. I believe I'll go with the 1ufd or 2ufd just because the size of the can dictates a larger size capacitor. A smaller capacitor would have been put into a mica block or just a mica cap itself installed.

Now I have to figure out what's going on with this "Micamold" cap that reads .006ufd but the parts list calls it out as a .06ufd. #42. It goes between the plate, pin 2 and whatever pin 5 is on the 79 output tube. Any ideas on what I should trust? The reading of the Micamold or the parts list? I've read that if you see a Micamold just throw it away.



Pepperoni

That cap is your parasitic oscillation supressor cap. Seeing as the tone control is also across the same points, a .06 seems quite high, and would affect the high end frequency response. My gut tells me that the .06 is a typo, but others here may be more familiar with the circuit.

Still in all, good idea to replace the Micamold, since it's just another paper cap in disguise.

Thanks BrendaAnnD! Glad you brought up the tone control because it's not on this chassis. It must be on the speaker housing itself which I have but haven't looked at yet. This radio is out of a 1934 Packard and there are several pieces that fit under and in the dash of the car that all have to come together. And all that parasitic oscillation supressor stuff I'll have to do some reading up on. Never even heard that first word used in antique radio talk before. But I've only been doing this for about 7 years.

Lets see if someone else chimes in on it but at this point I can only trust that you know what you're talking about. And with you mentioning the tone control I can see on the schematic where this cap would affect the high end frequency. There's no reference to the 3 caps in the tone control circuit so I'd better take a look at that now.

Pepperoni

I also agree with Brenda that the parts list is wrong and the cap in question across the plates of the 79 tube is a .006 uF.

Keep in mind that although Micamold did at one time make paper caps which looked somewhat like micas and are troublesome, when your radio was produced they primarily made mica caps which are of good quality. So don't condemn the cap just because it says Micamold.

From its appearance I believe your cap is really a mica not paper, but if you feel more comfortable changing it, use a high voltage cap (at least 600V or higher) since it is across the full center tapped secondary winding. This also goes for the caps in the tone control which will also see the output transformer inductive voltage spikes. These tone control caps are definitely paper and need to be replaced.

I put a .006ufd 630V cap in place of the Micamold.

Below is a picture of the capacitor that was in the can that originally started this post. It's #48 on the parts list but no reference at all to its value. Parts list has its part number as 7774. Part number on the can was 7440-X. It reads about 68pfd now and read about 135pfd when in the can and was shorted at both ends to the can. Irrelevant by all means because it's fried. So short of putting an electrolytic in it I decided to go with a 2.2ufd 630V. Sure hope this works.

Any questions about where it goes in the radio please refer to the beginning of this post. It wasn't wired in where the schematic said it should be.



Pepperoni