TAB BOOKS NO 496
RCA COLOR TV SERVICE MANUAL
BY CARL BABCOKE.

Timmy you MUST buy this book, often on ebay for $5-10$

it explains CRT bias and HV regulation the best I have ever read.

I was reading that book, the low side of the brightness pot should be -14 the high side +12. Check the peaking coils right by the 12BY7, the -14 is pretty close to what you have the high side seems too high (+20v not +12).

this could be an open peaking coil or a weak 2nd video amp.

check L12 and R61. What value do you have for R64?

ok ill check those resistors. and the 2nd video tube would be the 6gm6 at that cap that burned but i put another one in there. then there is the 1st & 2nd videotube which i also changed so i would think that rules the tubes out at least for now until i check the other components. ill check again but i have checked r64 several times in the past to find it was good. ill get back to you on the others.

ok this is what got here, L12 tests at 6.8 ohms compared to 6 ohms, c39 reads infinity, other .001 at the 6gm6 tube tests good so i left it in there, L13 however didnt test so good , 6.5 ohms down from 2200 ohms, ( 2.2k ) so i know now i need this coil 2200 ohms 120 uh . r 61 1185k ohms sams shows 2200 ohms , r 64 .732 meg looks good sams shows 680k

the L13 is fine they are giving you the value of the resistor not the coil another of sams errors of omission. I was taking a long shot trying to see if there is any reason the hot side of the briteness control was so high. I dont think there is a problem. More likely just some tube issue (weak) in the 2nd video amp.

that book I mentioned goes over much of this in great detail. I did not see one listed on ebay currently but if you check back I am sure there will be one, they come up all the time, remember it has to be the 1st volume by carl babcoke on rca color tv's.

so L13 measuring this is ok, ill put it back in, bty there is only 2 legs on the L13 so how could anyone tell if its bad. and the brightness pot measures 242k .

fyi for future ref, you generally do not need to pull peaking coils out just check them in circuit, you should see low ohms.

ok, good to know as these boards are so brittle and i hate messing around them, traces break, ect...

the 3rd video tube 6ej7 on my tester has the grid emission needle jump up in to the area of rejection but only when i press the button it then goes back down and also the neon light that indicates shorts flashes when the needle jumps. would the 3rd video tube possibly be responsible for the 20+ volts on the video out tube ?

nope dont think so.
I will be doing a video later to see what it normal

Hi Guys, sorry I was all tied up for the last few days.... I've read all the fun
since I was here last, and it seems to me you guys have not resolved the odd
voltages on the 12BY7A....? If that is correct, I was wondering if you checked
the following resistors.... R146, R148, and R139 See attached images so we are
all thinking of the same parts....

Just in case it's not an open feed to the plate, but biased to that 20V grid take a
look at C729, when you follow the path through R133 and L708 it runs over to
a 17V point on the Sync Sep 6AK8 and AGC control 6K 2W, that's got to have
a very influential amount of current.... So if'n C729 is being bad, that could
take the grid strongly to 20V..... Wadda-ya think....?

Heater and other stuff tied me up at wife's parents house, snow might do it
to me for tomorrow, let me know if you got that 12BY7A under control....'OK'

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The .1 in the grid circuit could cause a prob, but none of the ones I have ever worked on have been bad (leaky). I hesitated bring that one up for that reason AND the fact that its in a kinda busy area of the circuit board.

I am pretty sure he says he has checked those power resistors around the setup switch. one easy thing to check is to see if the plate voltage goes away from the 12By7 in the service mode. It should not, but if it does one of those resistors is bad.

Hopefully the problem will be resolved with as little need to remove parts as possible to test.

I am going to be making some videos later, and will be checking voltages both a the CRT pins and the video out.

what was the plate of the 12by7, #8 or 7 . ill check to see if the voltage goes away in service mode but yes i did check those resistor around the service switch all but one i didnt cut from the circuit because being very close to the service switch, but ill have to check it for sure. i did check it but all and all i may have got a feed back reading in circuit. the.1 cap is this the cap at the grid of the shunt tube ? im sorry im editing again, i guess the .1 you talk of is the one next to the 12by7 i check in my simple way by using the dvom and it went to infinity .

Anything short of a vintage eye type cap checker such as the Heathkit C3, is a lousy way to test capacitors. Most caps will fail from minute leakage currents that are too small to measure on a modern low voltage device. Caps need to be checked for leakage at full rated voltage for a test to be meaningful.

Did you change out that tube with one from your other set....?
You should not leave any item overlooked with this set, as it already
looks like no one here has yet to see a similar problem like the
ones you are having.... And it took quite a long time to make the
progress already made....

There has to be a reason that tube grid has +20 on it..... I would
focus on that first.... It means that tube is on - hard on.... And the
plate voltage should be low..... First cause is that +20.

I agree on those Heathkit Cap checkers.... I got a IT-11 I think it is....
smaller one than the 3.....

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yes i changed that tube for sure. and yes the focus now is this new problem with the voltages on the 12by7 and i dont want to overlook anything at this point at all.

L13 shows 2200 ohms in paralle with the coil so how is anyone able to check this if its inside the coil , it measures 6.5 ohms ?

right now no voltages go away from the 12by7 with the service switch in service mode. so now i am going to lift one leg on each of these 4 resistors, r- 60, 61 ,62 , 63 , and check them. why would a resistor show it as 150k but under that it shows 270k,are they saying it could be either one ?

ok r-60- 10k =12.40 k 61-2200 ohm =2335 ohm 62 150k = 156k 63 330k =.330 meg i guess they are all good, something is bad here, where the hell is it, lol,lol.....

any possibilty that the L22,24,26 coils may have anything to do with anything ? i ask because i once took one of these off the board to repair one wire inside thinner then hair and i was lucky i had a magnafying glass. who knows what was done to the set befor i got it maybe things were shorted out like the .001 cap and the focus coil got hot, im just grabbing at really small straws.

Hi Timmy, See posting #291, That capacitor C729, take one leg out of the circuit, or remove
it completely and turn the set on and see what the voltage is on the grid of 12BY7A, if
it's still around 20V then the problem is in another part.... Check that first please...

I don't have a Sams for this tv, just the ctc-12 schematic off the internet, so you
have to use the images I put up to see what parts I'm talking about..... And I can
only follow you if you refer back to those part items the same way....
Also you have to understand tube function.... -7V on G1 turns off the tube.
Anything above that begins to turn it on.... +20V It's ON really ON, so it is to
be expected that the plate voltage will be low because the tube is conducting.....
The only other thing I can see checking is R720, a 220 Ohm resistor in the cathode
of 12BY7A. to be sure it's not open....

On resistors in parallel, There is a formula to figure out exact value, but as a
general rule:

30 ohms || 4,000 ohms you will read close to 29 ohms
10 ohms || 20,000 ohms you will read close to 10 ohms.
200 ohms || 200 ohms you will read close to 100 ohms...

Coils, for the most part are going to be low ohms, 30 ohms or less....
a lot of times they are going to be less than 10 ohms....

A lot of times you can check some items in circuit, but it takes time to learn
what you can, and can't and what to expect. Capacitors block DC and pass
an AC signal, they charge and discharge. In the case of this problem, I am
looking for a reason for the grid to be close to 20V, I can see a path to
a 20V source IF that one capacitor is shorted.... So it is wise to check it and
get that possibility out of the way....


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ok i have in this circuit a .1 but the rca shows a 0.1 so ill lift one leg and see what happens. then ill look into that 220 ohm resistor off pin 1 .

in circuit the 220ohm resistor measures 216 ohms and lifting one leg of the .1 cap made no difference. dont forget i have the chassis out not plugged up to the yoke. there are 2 disc caps 390mmf in between that resistor , dont know if this may be of any concern. it is a royal pain in the !##$@#$$ ass but if i have to left legs out all over to find this problem well then i have too.

Ok, coming off pin 2 R759 do you have 20V on both sides of that resistor...?
What voltage is on center pin of brightness control..?
What is on R733, either side...?

You are running the set with what no HV...?

No one is asking you to lift every cap, just ones that may lead to an answer...
Sorry you are the one stuck doing it all..... :)

Found cool little website while looking up 12by7a to be sure pin numbers are
right.... Pretty neat..... http://www.intio.or.jp/jf10zl/index.html


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r733 is the 2.2k 38v one side and 25.5 other. no hv hot out. r759 close to the same. bright max 30.32 volts low 23.04 volts. i have a funny feeling this excess voltage is coming from somewhere else further down the pike .

Not quite what I expected.......

Can you check the AGC pot, 6K 2W.... Be sure it's good....

And can you check the voltages around the sync sep. tube....

Images included....

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6ka8 pin 1 86.7 v pin 2 24.65 v 3 25.42 its high even by the sams i have here. agc pot 2601 ohms end to end wiper 6.18 varys.

Ok Timmy I got a funky project for ya-
I need you to find ground points on the PC boards where both of these tubes are, and
do 2 things.... With the tv turned on, measure voltage from the PC board ground to the
metal chassis ground, and see if the grounds on the PC boards where these two tubes
are have and voltage on them..... Second, that AGC pot, 2.6K is not the 6K my schematic
says it should be.... You may need to see what's going on there.....

For the 6KA8, cathode ground should be directly through the AGC pot...

For the 12BY7A, C750 - one side goes to ground... Now you have to use a part,
not the metal lugs that the circuit board is attached to, these are clearly grounded,
but just in case there is a trace broken, and it's a long loop far away from the
circuit board connections to the metal chassis.....

It's really strange for so many voltages to be off and the set still works - kind-of.
So we gotta look for some sneaky thing.... And high voltages like that,
especially on items that should be going low, kinda make me thing bad ground.....

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the agc on my sams says 6000 2 watt which is this set. the grounds of the 12by7 and the 6ka8 from chassis to the ground of the tubes with tubes in ?

Yah, don't take them out...

The tubes won't have a ground, you gotta find parts on the PC boards that go to
ground on one side, and just check to be sure electrically, they are really grounded.
One way is to check the grounded part, to the metal chassis for any voltage....
It sounds strange....

Take a look at C750, it's one example of a part with one side going to ground.

On that AGC pot, disconnect the two legs that do not go to ground to test it....

AGC pot, pins 1 to 3 should be 6K thats 6,000 ohms.... The wiper will
change as you turn it you know, from the wiper to either pin 1 or 3....

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on mine 2 legs tied together go to ground and has one wire on one tap.

yes I have it too.... Just be sure it maxes out at 6K, and trims down all the way to zero... as it is in the circuit....

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Ok, more goofy stuff for ya-

Please check the voltage at pin 7 of 6BZ6 V301. Schematic says it should be
around 35V.

Please check the voltages on both sides of R307...

Please check the voltages on both sides of R312, and pins 7 (165V)
and 2 (0V) of 6EJ7 V303....

Is that AGC pot mounted on the PC board or is it mounted on the metal chassis....?

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ok pin 7 of 6bz6 38.7 v both sides r307 38.2v and 0 v r312 4.17 and 0 v 6ej7 pin 7 155v pin 2 0v

Gotya - Are you sure - absolutely sure R 307 1500 ohms 1 Watt resistor has
38V on both sides of it.......?

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no the last post says 38.2 and 0 v .

Ok.... Dammit got excited for a minute....

Ok, So please check the voltage on pin 3, 6, 7, and 9 of 6KA8, V503A.

So Timmy, When you watch TV and the scene changes and goes from light to dark
and the picture blooms out, this is a problem you can easily replicate by turning
the AGC on a normal working tv.... Poorly adjusted AGC often make for a really
bad picture - especially makes them look like they have too much contrast, or
not enough.... It's possible, you have a AGC circuit problem here.... I have not
had that much experience in the AGC area.... Not sure entirely how the circuit
works, so outside of looking at known voltages I'll have to get out one of my
books and do some reading....

I was hoping for a bad ground, but there may be something funny going on
in AGC, and it's driving the 12by7a nuts too.... Gotta do some thinking...

Maybe someone with some smarts on this area has something to say....

So when you take the signal off this tv do you get snow....?

And is that AGC control mounted on the PC board, or metal chassis...?

.

well the agc appears to work properly and so does the contrast. 6ka8 pin 3 22.95v pin7 21.73v pin 6 .702v pin 9 36.2v

this whole thing is just nuts and besides im sure these voltages are dependent upon the chassis being all plugged into the convergence and yoke and the voltages im sure may depend on where a certain adjust pot is, like contrast ect. these voltages on the schematic , does this mean a working set, with all hooked up ? as crazy as these voltages are it still gave a watchable picture.

Does that pin 6 voltage match your schematic....?
Mine is not that clear for that pin.....

Yah, I'm not too crazy about the chassis being out and not hooked up to all it's support crew....


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