[FStephenMasek]

The schematics are here: http://www.nostalgiaair.org/Resources/118/M0013118.htm
The mystery involves what capacitor values seem reasonable in specific circuit locations.  Why?  The schematics in Riders Volume two have two different number schemes for the capacitor block (can)! 
If you look at the bottom diagram on Philco Page 2-36, you will see the top row, from left to right is 5, 3, 1 and  the bottom row is 6, 4, 2.  If you then go to Page Philco 2-38 you will see that the top row is 1, 2, 3 and the bottom row is unlabeled ground, 5, 6.  
This leaves us with a need to study the circuit and decide which values seem most reasonable in the various locations.  That is where I need your help.  Specifically:
1) For the capacitor from the low end of the volume control to ground, is 0.5 or 0.15 more reasonable?
2) For the capacitor from after the field coil to ground, does it seem more reasonable that it would be 1.0 or 0.5?
3) For the capacitor from the second (screen) grid of the last '24A tube to ground (across a resistor), does it seem best if it is 0.5 or 1.0?
4) The capacitor from the B+ feed to the primary of the audio push-pull driver transformer is 0.5 either way.
5) For the capacitor connected from the high voltage winding center tap to ground, does it seem more reasonable that the value would be 0.015 or that it would be 0.5 for the 60 cycle radios and 1.5 for the 25 cycle radios?

[Ron Ramirez]

Hi Stephen,

You want to look at the schematics and service data for the Model 112 above serial number 174001 (which is the 2-47 version). I don't have access to any of my stuff as I'm at work on lunch, but I believe the data on Rider's Philco page 2-38 is correct for your set. Almost everything above that is for the early 112 with 2-45 tubes, below S/N 174001 - the exception being, as you mentioned, the under-chassis drawing at the bottom of Philco page 2-36.

It's hard to make out in the scan, but you're right - the numbering scheme of the terminals are different in the 2-36 drawing than the 2-38 drawing. However, I am pretty sure that the capacitor layout in 2-38 is correct.

I will look at my notes when I get home to confirm, and to be able to properly answer your questions.

[Ron Ramirez]

[Image: http://www.philcoradio.com/images/phorum/schem/112b.jpg]

Click here to see the full-size version.

This drawing is correct. I once owned a late version 112 (112X to be exact) so I am somewhat familiar with it even though it has been several years since I let it go.

On a somewhat different subject but on topic with the late version 112, please note resistor (51). This is rated at 15K, 2 watts. It was running at its design maximum in 1931, and with today's higher line voltage, it is really going to be stressed out. I recommend that you replace it with a 5 or 7 watt resistor to give it a greater margin of safety and to keep it from burning out prematurely on today's higher line voltage.

[FStephenMasek]

Thanks! 

I wonder why they needed (wanted) such a large capacitor (1 MFD) from the screen grid of the last '24 tube to ground?

The center tapped wire wound resistor between the center tap and ground would carry all of what goes through the resistor you mentioned, plus other loads, so I wonder if it is adequate?

[Ron Ramirez]

1 uF does seem a bit overkill, but that's one of those questions you would have to ask a dead guy (someone on Philco's engineering team). And dead men tell no tales.

Actually, someone like Mike (morzh) could probably answer that question. What say you, Mike, if you're watching?

Now regarding resistor (69), the dual 70 ohm resistor with the center tap. It is connected between B- and ground, where the potential between high voltage center tap (B-) and ground is low. Low voltage, low resistance (70 + 70 = 140 ohms), low current.

Resistor (51), on the other hand, is connected between B+ and ground. A lot of Philco chassis used a resistor between B+ and ground in this manner, but in other Philcos the wattage of this resistor is much greater. In fact, the others (including 111, early 112, 16, 17, 18, and others) used a very large tubular resistor which was probably at least a 20 or 25 watt resistor. I don't know why Philco used such a small resistor in the late 112, unless the bean counters were trying to cut costs. They should have used something more sturdy than a 2 watt resistor in this position. If that resistor is even still in the ballpark (and it is highly doubtful), it will run red hot in operation. Replacement with a 5, 7 or even 10 watt resistor will result in a resistor that will still generate heat but will last much longer.

[FStephenMasek]

I had already obtained a 10 watt 15,000 ohm resistor to use in that position. The one in the chassis when I got it is much larger than 2 watts. I do not know the size, but I have some measurement data for the old resistors and it is at least twice,as long and twice the diameter of a two watt resistor. I have no way to know if it is original.

[Ron Ramirez]

Philco may have changed that resistor to 3 watts, still too small for that application.

10 watts should do quite well.

[FStephenMasek]

It is playing, so it seems the values work.

[FStephenMasek]

The secondary of the 3rd IF was open. A surprising amount of signal was still was making it through.