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RCA-76 cap question - Printable Version

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RCA-76 cap question - Eric - 04-06-2014

Hello Everyone,
Working on this GE J-105- C made by RCA using the R76 schematics. This has one of the huge can mounted on top with 15 caps inside. Most are non polarized so no problem but C36,C35 have me wondering if I have them connected properly.
I have the pos of 36 tied to the pos of 35 and the red lead tied there going to L3 and the neg of c35 going to the resistor block is this right?

The radio kinda works I get sound but it is very faint all tubes have tested good I've replaced a good portion of the dog bone resistors all others within 15%
http://www.nostalgiaair.org/PagesByModel/714/M0015714.pdf
Thanks Eric

RE: RCA-76 cap question - Eliot Ness - 04-06-2014

Quote:I have the pos of 36 tied to the pos of 35 and the red lead tied there going to L3 and the neg of c35 going to the resistor block is this right?

Maybe we're looking at different schematics but I don't see where you came up with L3. The + side of C35 & 36 goes to more than one place, and that includes one side of L16, L19, & L7, but I'm not sure how you came up with L3.

OK, I think I see what the problem is. I'm looking at the schematic and you're looking at the wiring diagram. In the wiring diagram it does look like red is going off to L3 (I think, but it is hard to read), probably as a tie point before it goes off to the other points. The same with black/red going to the resistor block, also as a tie point. So yes, your original assumption seems correct.

What you want to do is use both the wiring diagram and the schematic together. Actually measure with an ohm meter that each tie point is going to all the places it is supposed to. That is just-in-case you have accidentally removed or mis-routed a wire.

I also like to check this before removing components to make sure the factory didn't change something without updating the schematic. That, of course, is in the rare instance that a wiring diagram even exists. Seems like RCA and GE were better about doing this than most manufacturers.

RE: RCA-76 cap question - Eric - 04-06-2014

John
I have been using both the schematic and wiring diagrams. I'm trying to figure out if I had C35 &36 connected correctly the schematic shows the pos all tied together. Tomorrow I will take a set of voltage measurements and see if I can figure where I went wrong. There is a change noted on the schematic for tube 7.
Eric

RE: RCA-76 cap question - Eliot Ness - 04-06-2014

Yes, the + of C35 and 36 are tied together in both the schematic and wiring diagram. Then as you thought, in the wiring diagram it shows the + going off on the red wire and the - of C35 going off on the black/red wire.

Those capacitor blocks with several caps in them can be confusing when re-stuffing if you're not careful. There are a lot of wires and capacitor connections to keep track of.

Good luck!

RE: RCA-76 cap question - Eric - 04-07-2014

Well took some voltage readings per Riders page 3-35 They seem to be all over the place one Question I have at the top of the second column it says "Cathode or filament to control grid" do they mean the tube cap? Here are the numbers

1)RF 3.7 95 142 291
2)1st Det .5 47 122 294
3) OSC 3.6 0 --- 92
4) IF 3.6 50 134 282
5) AVC 0 0 --- 4.5
6) 2nd Det 0 .8 --- .8
7) 1st AF 14 14 --- 276
8) #46 396
9) #46 398
Got 2.5vac on all the heaters
Second detector seems to be the worse looks dead R-19 & 20 are good
just made some more checks before posting and it looks like L14 is open should read 2400ohms I removed it and will open the first layer to see if it's just a lead. Would it be considered a Choke coil?
Thanks

RE: RCA-76 cap question - Eric - 04-07-2014

Now the question is where can I find a close replacement all I have is that it is 2400 ohms since it is open I have no idea how many Henries my guess is 2-3 as I have an 11 henry and it is half or less in size. Any ideas???

RE: RCA-76 cap question - gregb - 04-07-2014

Just stick a 2400 ohm resistor in there or the closest you have for the time being and see if the radio works. It may work fine without it but if not then try fixing or replacing. Wont hurt anything to try.

Gregb

RE: RCA-76 cap question - Eric - 04-07-2014

Thanks I will give that a try

RE: RCA-76 cap question - Eric - 04-07-2014

Greg
Thanks that worked great I had a 2.2k 10 watt ceramic used jumpers and brought it up on the varac and it played fine next question should I stay with the 10 watt or can I bring it down to a 5w? I'm guessing I should stay with wire wound?
Eric

RE: RCA-76 cap question - gregb - 04-07-2014

The original coil I believe was there to block RF from the audio section. There is not any current to speak of so a 1/2 watt resistor would be fine but you should make an attempt to fix the original coil. All the ones I have ever seen were just a bad connection at the terminal.

Gregb

RE: RCA-76 cap question - Eric - 04-07-2014

I removed the coil and cut off the outer layer of insulation to expose the leads and they are attached. were talking some fine wire here maybe .003 or so. This will give me time to either track down something suitable or give a shot at rewinding it. Hammond has some listed and I could make up the difference in resistance with a resistor as I didn't see one for the 2400 ohms.

RE: RCA-76 cap question - Mondial - 04-07-2014

The best solution would be to repair the existing choke if possible. If not, any high inductance low current choke will do. There are only a few milliamps passing through it. The purpose of the choke is to remove AC ripple from the detector plate supply which is very sensitive to hum. With the resistor replacing the choke you may notice a increased hum level as a result

You can also try a much higher resistor as a replacement. The value is not critical and you might try values in the 10K to 27K range to provide increased filtering and lower hum levels.

RE: RCA-76 cap question - Eric - 04-08-2014

I do get some hum so I will try different values I will look for some chokes how high of a inductance should I look for 2h,5h??
Now it seems I have no volume control it appears to be at max is this related?

RE: RCA-76 cap question - Mondial - 04-08-2014

You will probably have a hard time finding a replacement choke, as its inductance is most likely in the 20 to 50 Henry range. It is wound with literally miles of very fine wire, which allows a very high inductance in a small physical size.

The most available replacement would be to use the primary winding of an interstage audio transformer as a substitute for the choke, leaving the secondary winding unconnected. It would have an inductance comparable to the original choke winding.

Another possibility would be to use the resistor instead of the choke and increase the value of C27 to 10 uF or more. This would compensate for the loss of filtering by the choke, and would closely approximate the type of filtering used on later more modern radios.

I doubt the volume problem is related to the open choke, now that you have bypassed it with the resistor. This radio uses a very complicated biasing scheme to provide AVC and volume control, so the loss in volume control could be caused by a problem anywhere in the bias or AVC circuits.

RE: RCA-76 cap question - Eric - 04-08-2014

Mondial
Right now I will try the 10uf for C27 and the resistor except what value resistor the 2.4k or higher?
I'm going to try and wind the coil but it will take me a few days to make a form then wind it.

I haven't lost volume it is wide open and the volume control makes no difference.