Working on a RCA KCS72M

[Electronic M]

Hi all. I've been working on a 17" metal table model RCA KCS72M I got from miniman a few years back.



I've been working on it without a schematic as an exercise in refining my troubleshooting skills.
After about 90% recap I've hit a snag...I think the yoke is bad.

My first clue was when unplugging the yoke brought the HV up from 0 to 10KV.
My second clue is that if I use the yoke and CRT of my Telematic test jig I get about 12KV of HV and a decent picture (as long as I can keep it in synch).



The original ohms out to about 23.5ohms and the test jig yoke ohms out at around 14 ohms on the horizontal windings.

The numbers on the original yoke are 971385-2 274243 c14. Anyone know the numbers of any valid subs for the original yoke.

Also I've never encountered a bad yoke before so tell me if my logic seems wrong.

[kramden66]

you should check the doorknob cap too just to be sure, if it is showing a picture with the test jig I would jump to the same conclusion you did

[Eric H]

Your theory makes sense however there may be some caps inside the yoke, it's possible (tho unlikely) one of those could be shorted and killing the HV.

[bandersen]

If it is a bad yoke, maybe you can swing by Rockford, IL and pick up this one for parts ?

http://rockford.craigslist.org/atq/4148231134.html

[Kevin Kuehn]

971385-2 crosses to Thordarson Y-42.

[Electronic M]

I'm fairly sure that there is no door knob cap in my set as I don't recall seeing one in the HV cage and this set uses a glass CRT.

I already checked the yoke for a cap and the only one in there, a pico-farad range ceramic tube, measured open on the 2M scale of my DMM...I could always hit it with 450V from my heathkit C-3 and see what happens.

For a while I was able to get some HV and a ~1" tall horizontal line on the set by plugging the yoke in such that there was loud arcing in the socket (ie yoke almost unplugged).

I'll have to consider both options as I don't want another cabinet to have to dispose of, but then again the CRT in mine is below the good range on my tester without bumping up the filament voltage so a better CRT might be worth while.

[Tim]

971385-2 crosses to Merit MDF-89 and RCA P/N 77697.

Playthings of the Past lists the RCA.
9998-905 TELEVISN RCA 77697 YOKE WITH OCTAL PLUG, RCA 971385-5, 2746350-3, LOOKS NEW.. 1 0 0/0 0 0/0 0 0/0 9.00

http://www.oldradioparts.com/2a23ffl.txt

They are closed until 1/12

[dieseljeep]

Originally Posted by Electronic M

I'm fairly sure that there is no door knob cap in my set as I don't recall seeing one in the HV cage and this set uses a glass CRT.

I already checked the yoke for a cap and the only one in there, a pico-farad range ceramic tube, measured open on the 2M scale of my DMM...I could always hit it with 450V from my heathkit C-3 and see what happens.

For a while I was able to get some HV and a ~1" tall horizontal line on the set by plugging the yoke in such that there was loud arcing in the socket (ie yoke almost unplugged).

I'll have to consider both options as I don't want another cabinet to have to dispose of, but then again the CRT in mine is below the good range on my tester without bumping up the filament voltage so a better CRT might be worth while.

The yoke plug has a jumper between pins 6&7. It's used to supply B+ to the chassis.
How do you get the chassis to produce HV, without the yoke plugged in?

[Electronic M]

That jumper circuit was open EVEN with the yoke connected so I've had it jumpered since early on in my work.

I need to find a schematic for it eventually as I want to investigate using the RCA video jack on the back as a composite video input.

[Penthode]

Before condemning the yoke, have you done some basic visual and DC resistance checks around it?

I am not a betting man but if I was I'd wager that with a bit of examination you'll find what is wrong with the yoke.

[Tubejunke]

I have not seen capacitors inside of yokes, but I have seen resistors hidden underneath the cover.

Originally Posted by Eric H

Your theory makes sense however there may be some caps inside the yoke, it's possible (tho unlikely) one of those could be shorted and killing the HV.

[Electronic M]

Most of the stuff around it looks to be in good shape. I presently do not have a schematic so even if I did resistance checks I likely would miss any problem encountered as I don't know what the nominal expected resistances between points are.

Dave G. has offered to send me the schematic so until it arrives or I get a new yoke I'm leaving this one on the side so I can focus on other projects.

[bandersen]

Quite often there is a mica cap inside the yoke across the horizontal coils. This set has a 91pF in series with a 1,000 ohm resistor.

Here's the schematic from Riders vol 10. There are several chassis revisions and this is the earliest. http://www.flickr.com/photos/70039806@N00/11598231834/

[danny]

Hi if the b+ interlock has failed it might be worth checking the plug and socket ! disconnect it and hard wire it. Also have you done a ring test on the yoke to see if it has shorted turnes . If it has shorted turnes then i would dismantle it to find the offending short and re insulate it. Ive done this in the past with total succsess.
Danny

[Dave A]

banderson...Cherry Valley! My home town. Population 950. I was just there last Monday but the set would not fit in the overhead back to Phila. I hope that set was original to Cherry Valley picking up Rockford tv in the 50's. We had a Radio Craftsman set...the first in the Valley to pick up Chicago before Rockford came on the air.