The PHILCO Phorum

Hi all,

Not sure what happened to my original post. Bits and pieces were missing.

As I mentioned in past posts I am trying to learn this hobby slowly.  I have a 40-120 philco I have been tinkering with for a couple of months now.  I also bought 2 more off of eBay, one of which is in working order.  My goal was to take the schematic and study the way everything is connected while slowly re-wiring the one I had.  I managed to overcome several hiccups with bad RF coil and broken domino capacitor lead.   I have the original radio, I started with, wired identical, from what I can tell, to the radio that works perfectly.
All of the original wax tubular capacitors have been replaced with brand new capacitors from Antique Radio Supply that are equal in microfarads as well as voltages. If the voltage size was not available I chose the next size up, never lower. All resistors were replaced with brand new resistors of the exact Ohms and wattage. I have tested all the new capacitors to make sure their capacitance is still true and so far all the new caps have been reading very close to spot on. The only resistors that haven't been replaced are number 34 which is the long filament resistor and it reads correctly. Also number 27 which is a 130 ohm, 1/2 watt resistor connected on the bottom of the 35A5 tube. It tests as 135 ohms.  

I am at the point of power on which I have done with a Variac and my Cristy A2 dim bulb tester.  Pilot light comes on slightly bright and then dims as the tubes warm up. All I hear is the exact same sound as when you have a bad filter cap.  The volume control does work at decreasing and increasing the volume of the hum.  I have checked the new electrolytic cap for microfarads, correct polarity, and basic leakage with the Cristy A2 Tester which can also test for leakage.  Both sides are perfect.  All other capacitors, which are all brand new from Antique Radio Supply, test correct as well.  I have swapped out the tubes with the set that works perfect into the set I am working with and get the same result. 
I have been busy watching Mr. Carlson and Glasslinger on YouTube for ideas.  So far the closest thing I have seen is a video about a Ground loop, but I cannot find anything like that.  
What might my next step be?   If someone could give me an idea on what to check next it would be much appreciated.

>>All other capacitors test correct as well.


What do you mean by it? The original caps that are in the radio? Or have you replaced them?
What about the #8 cap?

You might want to check the resistance from B- the chassis. Should be very high. If low that can cause your hum issue.

Hi T32,

Did you have to replace any wiring on the set also?  Recheck wiring.

We call these sets "hot chassis" but they are not quite so.  Looking at the schematic, the B- and filament string "cold" end is not to the chassis but is a series of connections that include the output tube cathode resistor, the 7C6 First Audio cathode and 1 filament leg, the power switch, one end of the volume control and other points.  This is usually referred to as the B- Bus.  The chassis connects to B- through #8, a capacitor and coil pair., with the cap in series with the coil.

With power off and set unplugged, connect one end of the meter to the power switch contact that is connected not to the power cord but to one leg of the volume control and wiring that goes to another B- connection.  Then check the following points.  All should give zero Ohms:

• The "ground" (B-) end of the volume control.
  
• The "ground" (B-) end of the output tube cathode resistor # 27 (as opposed to the 35A5 cathode end).
  
• The "ground" (B-) end of resistors # 28 (Output Tube grid leak), 23 (1st AF Tube grid leak) and R5 (1st RF and Convertor cathode resistor).
  
• The 1st AF Tube and IF Tube cathodes (Pin 7) and the 1st AF Tube.  
  
• The "-" connections of filter capacitors # 33
  
• The "ground" (B-) end of capacitor # 9 (AVC Capacitor).
  
• Note that the tuning cap (and push button caps)
  

          connect to the chassis and not the B- Bus.  the connection between chassis and bus is through capacitor #8, a 0.25uF cap.  The coil is not                  highly critical; it is merely several turns of wire around the capacitor.

As RadioRoslyn (Terry) stated, there should be a high or infinite resistance between the B- Bus and the chassis.

Hope this helps!

Defiantly had to redo the wiring. That rubber wiring was was absolutely disgusting nasty! It was a slow process, I have big hands and this chassis is very tight. I have since read somewhere that if you are trying to learn this hobby to never start with a Philco 40-120. :-) I am determined to finish and make it work because it is a radio that has been in the family since it was made. Practice makes perfect and I have 3 total to fix.

The hardest thing for me has been identifying a closed circuit on the schematic. It finally clicked after reading through the book titled "How to Use Diagrams in Radio Servicing". On page 3 there is an illustration showing Ground at the bottom, a coil and a capacitor hooked to that ground, and the capacitor and coil hooked to a grid / anode pin on a tube at the top. I kept visualizing the whole time that current was flowing in a circle between the Capacitor and the Coil and that the pin on the tube was in series with the coil and the capacitor. It didn't dawn on me that the ground is on one end shared by the 2 components and that the tube pin is feeding current to both of the components then to ground.

Having fun and learning at the same time,,slowly.

Thanks for all the helpful replies.

Ah, so blood IS thicker than solder!!

And rejoice when you realize the AC signal is also doing it's thing in the same circuit.

Hello t32above,
Well, yes and it sounds like you are learning some great stuff as you work on this set.

Sincerely Richard