[Burchjburch]

Hello,
I'm working on a 1938 Delco r 1141 and am having trouble replacing the electrolytic capacitors. Someone replaced the original before me. Right now one positive side is connected to pin 6 of the 6k7 tube and the other one is connected to the terminal strip next to the power transformer. The negative ends are tied together and connected through a speaker field wire to B-. I get stations but very loud hum. I did replace the electrolytics. I'm struggling trying to follow the schematic. Are the positive ends of the electrolytic connected where they are supposed to be? 
Thanks
Jim

[morzh]

It would be nice to know which exact capacitor you are talking about. The two caps left and right of the Field Coil (29A and 29B) will have their minusses up (to the Field Coil) and plusses down (one of the filament pins of 5Y3G).
If you mean another one, let us know the number.

[Burchjburch]

I'm talking about 29A and 29B

[morzh]

well then, what I said before.

[MrFixr55]

+1 on Morzh's description. The schematic is a little confusing, as the negative end of the supply is usually toward the bottom of the schematic. From your description, I believe that the supply is miswired.

This supply is built in a manner common to some of the early and mid thirties radios but was not done this way in radios from the 40s and later. The + terminals of both caps go to the filament connection of the 5Y3 that is on pin 2 of the 5Y3's socket. Another wire from that same pin distributes B+ to the rest of the radio. Pin 6 of the 6K7 is blank but may be being used as a tie point. I think that it would be better to connect the + terminals of both caps to pin 2 of the 5Y3 socket. The B+ terminal of the audio output transformer, the 6K6 screen grid and B+ to other circuits should also connect here. Where the negative terminals of the caps is connected is very important. The negative terminal of Cap 29B, the 12uF cap should connect to the center tap of the high voltage secondary of the power transformer, Pin 1 of the speaker plug (Leg 1 of the field coil) and to one side of R54, an 800K resistor. The negative terminal of Cap 29A, the 8uF cap connects to Pin 2 of the speaker plug (Leg 2 of the field coil), and to one terminal each of R53, R51 and C60. In other words, the negative terminals of C29 A and B are not tied together, but coinnect to either side of teh field coil. The purpose of the confusing negative terminal connections is to consider the chassis to be B- / C+ and the Transformer secondary center tap to be C-, the bias for the 1st and 2nd AF tubes.

I generally like to up these caps to about 20 uF. Others may disagree, but the tolerance of electrolytics back in the day was +80 / -20% New caps nowadays are usually +/- 10 or 20%, so one cn go up a little. However, i think that your hum is due to the fact that the negative terminals of the 2 caps don't straddle the filed coil, and the + side of one of the caps may be connected to the wrong place, depending on what is connected to that terminal strip.

[MrFixr55]

This snippet of the schematic may be of some help:
   


The schematic is small but note the + and - signs. Hope this helps.

[RodB]

I like that term "straddle". It's really descriptive. Way to go MrFixr.

[Burchjburch]

Thanks to all that replied especially MrFixr55. The detailed explanation really helped me understand it better. I won't be able to get at it today but will report back as soon as I do.

[morzh]

I was trying to remember where I last saw a sch where the Field coil is also in the negative with the caps having common plus.

Well, it is the very R-73 I am working on now.

https://philcoradio.com/phorum/attachment.php?aid=27927

[Arran]

Mike;
 They did the same in some post war Addison radios, field coil in the negative leg, including the catalin ones, and those were AC/DC radios. The theory I heard was that it would reduce the current flowing through the field coil, plus they could use the field as part of the voltage divider. I also have sets where the speaker filed is connected directly to the chassis on one end, meaning that they were filtering nothing, (choke did the filtering) only connected that way to energize the coil. 
Regards
Arran

[Ron Ramirez]

Philco also used field coils in the negative leg of most (if not all) Tropic models which used a power transformer (i.e. not AC-DC).

[Burchjburch]

Found some time to work on the radio. I changed the 8uf to a 10uf (what I had on hand) and moved it to the filament of 5Y3. I changed the 12uf to a 20 uf but left it connected to pin 6 of 6K7. I connected each negative end as stated by MrFixr55. No more hum! Picks up stations up and down the dial on broadcast and shortwave with plenty of volume. Quick check of voltages looks good. I still have a couple wax capacitors to change out and check all resistors but I think it's looking good! Let it play for 30 minutes and power transformer was cool to the touch.

[morzh]

Good.


Arran,

Considering the choke goes to the tap which is the 100% return of all current, and only one resistor goes directly to it, past the field coil (and it is 800M value), the theory of reducing the current through the Field coil does not hold water.

As for the Field coil being a load and not filter, well, both 37-690 and 38-690 have the main speaker field as a load from B+ to GND.

[MrFixr55]

Hi Burch,

Glad it worked out.

I have seen the choke or field coil in the B Supply return in several radios including some AC-DC modls. My theory of why they put the field coil in the return is 2 fold.

First may be safety related. Assuming that the speaker is not physically connected to teh chassis, if the coil developed a short to the speaker frame, the speaker frame is somewhere between 250VDC and 360VDC above chassis potential and now a shock hazard (few transformer powered radios had backs in that time period.

Second, in addition, by having the field in the B supply return, the Field was helpful in developing the "C-" Bias voltages needed to bias the Audio Amp and Power Output tubes, as the cathodes are often connected to Chassis ground which served as B- / C+.

This all exists in later radios, but differently, by placing cathode bias resistors between chassis ground and the cathode of the tubes in question. It is actually simpler.

As to the 2 high value resistors that parallel the field in the Delco, they are merely a voltage divider to get the desired bias voltages.