[Radioroslyn]

The two that are fixed to the chassis are Philco issue. The third can isn't and just has two leads, a red & black. These equal the + and - connections. There is no connection to the outside part of the can. On the Philco parts there is a connection to the can.
The - connection on the Philco caps is done thru the metal clip that is fitted between it and the cardboard insulator. B- is not at chassis ground as use to provide - 15v or so to the control grids of the 42 tubes. That's also why that 10uf cap looks like it's in backwards. It's filtering - voltage or the - side of the power supply. We can dig into that later if you like.
 
What looks confusing is on 77 is one of the center terminals is connected HV and the terminal right next to it is connected to chassis ground. You rarely see any of the center connections connected to chassis ground. Typically electrolytic cap in a can the - are all connected to the outside shell.
All of the + are terminated at the bottom and have makings to tell which is which. There just about all the time they are at a + voltage. Back to the 3650 the outside of the can (shell) would be at -25vdc or so both 77 & 78 caps.

[MrFixr55]

Hi Hamilton,

My $0.02 worth-

The 2 capacitors in the upper left of your pic appear to be installed correctly, and matches Terry's drawing. It appears that the Cap with the green line indicating a positive terminal connected to chassis ground is 77 and the cap above it with the 2 positive terminals tied together is 78. If these are the originals, Philco likely tied these together at the plant because the correct single capacitor was not in stock when the radio was built. Otherwise they are replacements and the best that the repair shop had available.

I agree with Terry that you should remove that Aerovox capacitor. It appears to be paralleled with 78 and was likely added to cut down on hum. (I sure hope that the can on that Aerovox was insulated from the capacitor inside!)

This power supply can be confusing to anyone who worked on radios from the 1940s and later but have not worked on many radios from the late 1920s to mid 1930s. The common negative terminals for C77 and C78 do not go to ground. They connect to the center tap of the Power Transformer Secondary per your schematic and Terry's drawing. The Schematic is further confusing because they don't use the usual "curved line" indicating the negative pole of an electrolytic cap (capacitor).

So why does a Positive terminal of a cap go to ground? This is because the Power Supply is actually set up to supply not only a "B+", but to also supply a "C-" (Think of battery sets)! The cathodes of the '42 Output tubes are at chassis ground potential (B- / C+), but the 42s need a grid bias of somewhere around -18V if Class A Pentodes or -35V if Class AB Triodes, like your radio is. This bias is obtained because the center tap of the secondary of the driver transformer (Output Tube grids) is connected to the most negative section of the Power Supply, the HV Secondary Center Tap. Resistor Pack 66 (250 Ohm total) acts as a voltage divider between chassis ground (B- / C+) and the center tap of the HV Secondary of the Power Transformer (C-). If you look at many non-Philco radios from the early to mid 1930s (particularly RCA, Crosley, etc.), as well as Philco radios of the era, you will find a similar setup. Radios from the 1940s and later usually have the HV Secondary Center Tap and the negative of all of the electrolytics at chassis potential and have cathode resistors between this point and the cathodes of those tubes that require bias (usually the Audio Output). These resistors are usually bypassed by a capacitor, and in the case of the Audio Output, an electrolytic capacitor.

If that Audio Output circuit is in fact wired for Class AB Triodes with 350V B+, then this circuit is good for about 15W! If there is no Phono Input, add one! At high volume, this radio will ROCK any external input (until the speaker comes apart)! I fixed a Crosley 02CP for a friend and added an external input and he actually plays a guitar through it on occasion for that great Tube sound! (But insufficient sensitivity to overdrive it.

Good Luck,

John, MrFixr55

[Hamilton]

Thanks very much John and Terry. I now have a clearer understanding of these caps.

However, I do have two questions. 

The first is regarding the small green wire in the enclosed photo connected to the metal clip. The one end of this green wire does connect to both of the - connection metal clips and then goes to the CT of the transformer but it’s the other end that confuses me. The blue arrows show its start point and its finishing point.  

This green wire starts at the same - connection metal clip and meanders all the way to the other end of the chassis, loops around bakelite block 31A and then heads back and is connected to the end bakelite block #19 with resister #9 100K. It isn’t connected to anywhere else along this journey. What is this connection for and do I reconnect it after I replace #78 cap. I was going to use this clip again for the - connection of the new cap after hollowing out the old one.

The 2nd question was, after comparing the caps and resistors on my radio with the schematic I noticed Part #71A, an 8 mfd cap. It’s not on my radio and it’s not on the parts list. I couldn’t find any reference to it in the Service Note Changes that I have. Any advice regarding this part would be greatly appreciated.

[Radioroslyn]

This is how the - or bias side of the power supply works. If the center tap of the hv winding of the power transformer was connected to the chassis there would be no bias volt thru the power supply. But it isn't. It sneaks up to 66 which connects it to ground well kind of. It introduces 250 ohms in series to ground. This is where - voltage is developed across 66. In a way it's a little bit like a shell game. If you have 300v total B+ with the proper resistor to deliver -15v then the 300v drops to 285v.
For the most part Philco uses grid biasing in their audio amplifiers. There's another way to bias a tube and that is to use a resistor in the cathode to move it above ground electrically.

Anyway back to the questions of hand. Perhaps this drawing will make things a bit clearer. [Image: https://philcoradio.com/phorum/images/sm..._crazy.gif]  The two tabs that at the edge of each capacitor (77 & 78 ) is the common -

I didn't see 71A either. I cut them open and clean the innards put new cap inside. It's simple enough to replace the 1uf w/a 10uf. The 8 in parallel with a 1 uf. You could replace the others with 4.7uf@450v to make things easy.

[Hamilton]

Thanks Terry. I’ll ignore the 71A for now unless I hear from other members of the forum. 

Least I’m making headway with the cabinet restoration. No electronics thinking required for me there…thank goodness! ?

[Hamilton]

I was away for a bit and back working on the radio and have a couple on issues.

I took the backplate off the output transformer, part #68, and noticed there is a green wire that may have possibly been connected at one time to the top right lug in the photo. It appears something was connected there at some point. Should that wire be connected there? It is supposed to be Philco #32-7078 but my transformer is marked 32-7079. Not sure if that makes a difference.

The other item was the electrolytic, part #71. 1, 2 and 3 mfd cap. Is the can itself the - connection? There is no cardboard insulator on the existing can. Can I put the - connections anywhere on the chassis? 

I still haven’t received any response regarding part #71A on the schematic, an 8 mfd cap that is not on my radio. Any assistance would be greatly appreciated.

[MrFixr55]

Hi Hamilton,

First off, fantastic job on the cabinet and on your chassis! Looks like the chassis was in rather good condition to begin with but your workmanship on it is great!

Regarding capacitor C71, yes, the can is the common negative for that "cap". I would definitely service the cap before powering up the radio; I'm sure that it is a dud. You can temporarily unsolder the connections and install new caps or stuff the can. Either way, any convenient chassis ground point is fine. Best to attach close to the original location of the cap can. Avoid tying it to a ground point that is being used as a "signal ground".

Regarding the output transformer connection, it appears that you have an issue there. It looks like one of the wires from the chassis is not connected to a transformer terminal. All 4 of the wires in that cable should have connections to the output transformer primary and field coil. Look carefully at the transformer and determine how many wires are coming out of the "spool" (the transformer windings). Correct wiring of the output transformer is critical.

Refer to your schematic. Your audio output stage is a "Push Pull" stage that requires a transformer with a center tapped primary. Per the RCA Receiving Tube Manual (Ver RC12), for "Triode connection", the Plate to Plate impedance is 8000 Ohms. The best thing I can tell you is to trace the wires. The diagram provided does not disclose the color of the wires, but lists the resistance (different from impedance, hence the big difference between the resistance and impedance numbers quoted here) of the primary as 350 Ohm Plate to Plate. Resistance of either plate connection to the center tap is roughly 175 Ohm. If you see slight difference between either plate connection and the center tap, this is normal. If the transformer does not have a center tapped primary with the above resistance readings or if the transformer has an open winding, then you need to obtain a replacement. Any transformer for 2 Push Pull 41s, 42s, 2A5s, 6F6s, 6V6s EL84 / 6BQ5, etc.) should work at least temporarily but one with proper primary impedance (7000- 8000 Ohm), proper secondary impedance (Experts?) to match the speaker and power handling capacity of 20W should work (I am thinking output transformer for combo guitar amp here)

In this radio, the plates of the '42s are connected to their screen grides (Triode connection) Pin 2 is the plate, Pin 3 is the screen. Pins 1 and 6 are the heaters and are the 2 larger diameter pins on the tube. Look at your chassis, identify the wires that connect to pins 2 and 3 of each of the 42s. I think that you will find that the green wire that is not connected at the transformer originates from the plate and screen of one of the '42s. In addition, identify the color of the wire that connects to the junction of 6uF capacitor 77, 0.3uF Cap 80 and Choke 79. This wire should go to the center tap of the output transformer primary and to one leg of the speaker field coil. The other leg of the speaker field coil connects to that wire that connects to the junction of 3uF Cap 71, R72, the driver transformer primary and several other B+ Points. Apologies, I do not have the Apps needed to mark up your schematic.

Please note that it is not a good idea to operate this radio with a broken connection between the transformer and the plate and of the 42 (or any other output tube on any other radio. If connected as a pentode and the plate circuit was open, the screen will overheat, burn out, fry the tube and possibly the driver transformer if the screen shorts to the grid.

I will attach pages from the RCA Receiving Tube Manual for the 42 and 2A5 as soon as i figure out how to. Other than heater voltages, these tubes have similar characteristics. In typical RCA fashion, the page for the '42 instructs the reader to refer to the 2A5 for all characteristics other than heater voltage and current.

[MrFixr55]

RCA 2A5 fr RCA Recv Tube Man RC12.pdf Size: 1.03 MB  Downloads: 77

  OK, I think I figured out how to post.  Here are the specs for a '42 Tube from the RCA Receiving Tube Manual.  The bulk of the data will be in the attachment labeled 2A5  I assume that they did not have word processors in 1934 (Thx Cap'n Obvious!) 

RCA 42 fr RCA Recv Tube Man RC12.pdf Size: 472.87 KB  Downloads: 84

when the manual was published and their fingers got tired typing.

[Radioroslyn]

The color code for the spkr wiring is the blue and green wires connect to each plate of 42's. The blue/ white wire connects to the field coil, supplies hv to some of the rf and IF circuits. The green/white wire connects to the ct of the output transformer, field coil and is fed power from the output of the power supply. Most of the time the white wire has turned orange from aging.

As mentioned do check the primary (plate side) of the output transformer. It's not uncommon to have a bad one, either one side of center tap or both.

[Hamilton]

Thanks very much gentlemen. You’re all a wealth of information and it’s much appreciated!

[Hamilton]

Traced the wiring that you mentioned and it all appears to be as you both indicated. I reconnected the one loose wire and took resistance readings that are shown in the photo. It a bit of a mess with all the arrows…If these readings are out of line please let me know.

I’m waiting for supplies from Justradios to replace other parts and will probably be back for more help.

I hope your cataract surgery from a while ago went well John.

Thanks again for all the advice and assistance.