[morzh]

This is not the first one of this sort I've seen. The first time it was here at this Phorum I was told what it was. I honestly thought it was a cap.

[sdradioman]

Boy, this really got off topic. Still don’t explain why all 4 mica caps aren’t in the radio with no signs of them ever being there…

[MrFixr55]

Hi SD Radioman,

Getting off topic is what we do best.

If the caps are not there, it won't harm anything. However, do check the wiring of the 2 diodes in the detector / 1st AF amp. In the schematic, one diode connects to the secondary of the 2nd IF transformer, the other diode connects to the first diode via a 100pF cap. The alternatives are that the diodes are merely connected to each other and to the transformer or only one diode is connected to the transformer, and the other one is grounded.

Unfortunately, in the world of electronics and especially for antique radios, changes happen within runs that are not documented. However, you can either add the caps per the published schematic or not. If you find the sound too harsh and the issue is not the Audio section, add the caps per the schematic. (Add a "Radio / Aux switch at the input of the volume control to evaluate the amplifier section and to and enjoy Old Time Radio from your computer without the distortion that modulating / demodulating causes.) If the diodes are connected together or one diode is grounded, don't bother changing that part of the circuit.

Hope this helps.

[RodB]

I think the caps are there for rf bypass and won't affect the sound but without them certain noise and interference may cause the circuit to produce oscillation. This is a typical setup I've seen in other sets and shouldn't have any detrimental issues. You, however, may have issues with installing at least two of them.

[sdradioman]

So would it hurt anything if I added them? If the diodes are wired the way you say( haven’t checked them yet) and I wire in the caps. Will it affect anything?

[RodB]

Yes, go ahead. I meant physically when I said you might have an issue. They're not in a very accessible area. You might want to tack them in first then see if there is any significant difference.

[Arran]

Here is another thought or two, the service info mentions that the diagram is also for a 37-620 as well as a 37-630, the latter has the shadow meter, the prior does not. Maybe the 37-620s needed those caps because of the lack of a shadow meter but the 37-630 needed them? It might be educational to compare this example to other chassis out of this series, I think it was earlier this month that someone had a 38-7 (I think) with a few mystery parts that were left out of later production sets, including an odd looking RF choke, I thought it was a bodge but someone found a thread on the ARF with the same parts.
Philco was usually good about documenting revisions, though they sometimes have errors in the diagram that were missed, showing wires connected together where there wasn't, or at least not clearly defining them as NOT being connected. There have been exceptions with not denoting changes, especially on earlier models.
Regards
Arran

[sdradioman]

So I got back on the 37-630 tonight. I was really studying the schematic and found that capacitor 42, between pins 4 and 5 of the 6Q7 is there. It is a micamold type O 3 dot capacitor with an arrow. The color order starting at the tail of the arrow are yellow-brown-blue. That checks out to a 41,000,000 pico capacitor? Does this seem correct? Did they maybe put this high value in to make up for the other missing caps? That cap is connected to the same circuit as missing caps 33 and 35, albeit through the 2nd IF can and a resistor. It could potentially connect to cap 46 through the volume control pot. Could this explain why they are missing? Or am I out in the weeds on this one… and wouldnt a 110 pico cap have dots of brown-brown-brown?

[MrFixr55]

Hi Radioman,

The cap couples the detector diode to the AVC Diode and according to several different schematics, it should be 100pF (or 100 mmF). 41,000,000 pF is a more than a little unbelievable That would be 4.1 uF. During this time frame, that would be an electrolytic can about the size of the electrolytics in the set. Check the other side of the cap or if you see another row of dots along the front, then the dots at the bottom are for voltage, vibration tolerance and precision tolerance. If you see brown-black-brown, that would be a 100 MMF or 100 pF cap. If there are dots on the other side, then these dots indicate voltage, vibration tolerance and precision.

Bottom line, does it work? Does it sound OK? Thes are only to filter RF energy from the audio circuit.

[RodB]

I don't believe you will gain much by removing the cap just to read the code. The one in my set is buried in the same spot, under the tube socket pins, and the potential damage to both the tube socket and the cap far exceeds the possibility that this is a 4 mfd cap. Some early mica caps had a separate coding because manufacturers were pushing to have their parts become the standard. If you have the test equipment you can try measuring the cap in circuit. I'm showing you photos of my set so you can use them as a reference to install your caps. First is cap 42(code 125 schematic):

   

Now this is 33 and 35 at the right end of the terminal strip:

   

Then 46 connected to the center of the volume control:

   

Like I said, cap 35 is some sort of tubular and the colors are not decipherable. My set is not recapped but I replaced the ecaps and the audio coupling cap and the radio works. I'm hoping this puts the cap on this issue.

[morzh]

My rules about the mica caps: do not touch unless absolutly have to.

[sdradioman]

The items you show for 33 and 35 in mine are resistors. The yellowish one is a 490K resistor (32) and the mica mold is the 400 ohm resistor(29), that used to be a 700 ohm, but was changed in the service notes, that is shown piggybacking the .25 MFD cap in the component location/schematic. So it looks like your 125 might be missing 2 of the 110 pico caps too. The only one I dont have that your 125 has is the mica cap from the center tap of the volume control to ground…

[RodB]

Ah! You are correct. We both have a chassis wired on a Friday afternoon. I was going by the pictorial layout and assumed that's where they are. And the color code is correct for the resistor, yellow-white-yellow (BED) 490k.

Hey Fred, we're running low on those 110mmf micas.

Maybe when you get it recapped and running you can tack in some caps and see if there's a difference. It'll be a while before I get to mine. I'm working on a Zenith chairside veneer.

[sdradioman]

Sounds like a plan. I will try and find a cap close, I think I have a 120 pico, and wire it from the center tap to ground on the volume control…

[Arran]

The Micamold resistor is the cathode resistor for the 6K7G tube, I wonder if the change in value from 700 Ohm to 400 Ohm was in aid of eliminating the need for extra mica caps. By changing that resistor they made the effective grid bias of the 6K7G slightly less negative by making the cathode less positive with respect to B-.
Regards
Arran