Is there any way the picture tube can cause a tv to lose the video, and, have no contrast control action?
Brightness works normally.
I still have full audio reception.
If not, what could be causing this?
It's my beloved?? RCA Victor 6T74.
Thanks.
Bill Cahill

Is there any way the picture tube can cause a tv to lose the video, and, have no contrast control action?


Could be a cathode heater short, if the video signal is injected into the picture tube via the cathode.

Some failure in the video amplifier circuitry, rather than the CRT, hopefully...

Do you have any other CRT's to test it with?
I also have the feeling it's something wrong in the video amp.

With the brightness working correctly, its unlikely its the tube. My vote is video amp. put a scope on the contrast control wiper and see whats going on. Check scope pictures around the video amp.

Sadly, I don't have a working scope, and, don't know how to use one.
I've checked all resistors in video amp, and, except for a few slightly off ones, they check good.
Bill Cahill
I may put my test tube on it.
Could failure of 3rd, or, 4th video IF cause this problem, and, still have sound?
Of course, my contrast control is having NO affect on it at all.

try putting a composite video signal at the grid of the video amp. Isolate to before or after the video detector. Narrow down the problem that way.

If you don't have a suitable source for composite video, you can also just use a normal function generator. If you inject a nice big (several volts amplitude) audio tone (1000 Hz is a good choice) into the grid of the video amp, you'll see some nice bars (not synched) across the screen that will tell you the video amp and CRT are working. Then start backing up into the video IF, using audio modulated RF (at the IF frequency).

You can also inject an audio signal right into the CRT cathode or grid, but here it has to be pretty high amplitude (10 V amplitude works OK). Not every function generator will produce a signal that big.

:no: Well, it looks like I'm in big trouble here. I don't have any of the equipment you mentionned.
I think I'll test pix tube. If no go there, and, it checks good, how about the video cable from a dvd player?
Do I need a capacitor there? What value?
Is leighth crittical?
Jumpers o.k.?
I know ground has to be connected directly to chassis.
Just for the heck of it, I'll try something else simple. I'll try two different video I F tubes, just to eliminate them......
I'll have to put this back together tommorrow. Thursday, that is....
Thanks for putting up with me, guys.
Maybe you guys can help me find some needed test equipment at an affordable price?
I'm on a fixed income, and, I'm afraid that right now, I'm stretched.
Bill Cahill

I use a DVD for the video composite. Yes a cap is a good idea, just in case you get on something that has potiential. I dont know about value, but I have used a .1, too small and you will get some smearing. The main thing is to protect the DVD from that potiential, I smoked a DVD once when I accidentally hooked up to a B+ stake that was right next to a video test point, and did NOT have a cap on it.

Another check is to just get a mutimeter on the point before hooking up, that would have save me on the B+. I guess the problem with the cap is with a large value you could still get a pretty good jolt as it charged up if you got to something like a B+ as I did.

I aways do lots of test with a multimeter, looking for hot chassis, B+ etc.. so I check the ground lead of the test equip to the chassis I am working on with a mutlimeter 1st, then check the test point to the chassis (looking for the B+).

I suppose some amps may have a +v on the grid depending on what the cathode voltage is, seen that on audio stages where the tube is used as a voltage divider. I think most of the video amps are going to be 0v on the grid. I have hooked up with out the cap but its just one more safty step.

I know on my B&K1076 there is a big 10uf NP cap on the video out.

I just eliminated one possibility. I put my B K tester on pix. tube. Tube checks wonderful, and, according to the chart there are NO shorts in pix tube. So, I believe this eliminates the pix tube.
More when I know more.
Bill Cahill

Oh. I love new pix tubes. This one goes over 800, and, takes 30 seconds to go to 0 on life test.
In fact, it took over 10 sec for needle to even start moving down....
:thmbsp:

As some of you from ARF may remember, this has been a very long, drawn out project. My world's worst nightmare.
I bought this set on ebay a little over 3-4 years ago. She actually shilled me for sixty bucks, and, I was the only bidder. She was using a friend in NY who collected purses, lady friend, that is.
I paid sixty bucks for this gem.
Cabinet: O.K. Dirty, missing some finnish.
Set looked good, but, dirty, smokey.Not cigarettes, as it turned out. It was from all the disastors with this set.

This set was a real factory lemon. It has had everything wrong with it.
Let me go over a list of parts I've replaced, so far, not in order.....
Electrolytics
Capacitors
Micas
Resistors
Pix tube
All other tubes
Tuner
Speaker
Flyback
Yoke
Vert. osc. transformer
Cabinet power on light The wires cracked, and, were brittle. Socket was rusty
Removed incorrect 10A hv fuse, and, installed correct .25A fuse.
Hv cage. Old one had some rust.
A number of screws.
Tube shields.
Volume, and, tone controls
Vertical hold, and, horizontal hold controls. Dammaged by UPS
Height control dammaged by a short while I was working on it.
Vert. lin. control. Open.
Width control. Changed value.
Many resistors
Picture tube socket. Wires were brittle, and, breaking.
What a mess!
Oh, yeh. Power cord
Back cover as original had chemical stains, and, labels ripped off.
I have correct model cover on it.
I am deeply into nostalgia.I am crazy. I got in too deep, and, can't quit now.
But, I wish I had two years ago.
Be it as it may, how much more can go wrong?
Did I mention horiz. osc. coil?
That wasn't working at all right.
Now, I have no video.
What more can go wrong?
True, it no longer blows up, pops, smokes, or, catches on fire.
Speaker, by the way, voice coil fell apart.
This set has been my worst nightmare! :no::thumbsdn::sigh::worried::screwy::saywhat::dam mit:
Please help me finnish this set, guys. I can't quit now..
Oh, I forgot. 2 video if transformers, one dammaged by me when set fell off bench. Other by UPS. One aud. det. trans. also dammaged by UPS.
I'm going crazy!
I want to finnish this set. I can't quit now!
Besides, I have friends supporting me, and, enemies harrassing me....
I must finnish this set!
Vertical problem is fixed!
For a brief while, looked like it was going to have a decent picture.
Last year, though not correctly, I actually had it playing for two weeks.
HELP!!!!!!!
Bill Cahill Thank You......

Very unlikely on old B&W tubes, but that's a common problem with 1990s Zenith color tubes.

Charles

At what point do you just give up, and look for something in better condition?!?!? Bill, your thread over on ARF is over 40... YES 40 pages long! I realise that this set has sentimental value for you, but sooner or later you need to come to terms that you're fighting a losing bettle. Find another one, and start over.

Im sure that Mr. Cahill's problems are nearly over. I sure wouldn't give up on this now, since I have a feeling that he is so close to getting it right.

It will be a good set. Getting smashed by UPS didnt help anything.

That's the way I kind of feel. In the past I've given up on things, stripped, them, then, found the reason they wouldn't work.
But, this set has certainly been a tough dog....I guess I'll just have to keep pushing along.....
Bill Cahil

Bill I have to say you have been an inspiration to this newbie. Not only have your posts on ARF been informative for me but without them, I would be up a creek without a paddle sort a speak. I'm trying to get a 1950 RCA TA129 going(very similar set) and have never even attempted anything remotely this complicated in the vintage electronics arena. I have started running into alot of the problems you have run into and right now I am fighting a lack of resistance on my 5U4 tubes. I am assuming its a short somewhere down the line and not a transformer because this is an intermittent thing. Just knowing the tenacity that you have shown on your set keeps me fighting to keep going as well. I know its not a democracy but my vote is to tough this thing out and "Get R Done". I know if you can finish it up, I will also. Even if you eventually do figure to give this thing up your actions so far will still keep me persevering on my own project.
Perhaps a search throughout the country for the same set and finding it may give you the ability to finish this off with the least amount of pain. Perhaps having a "parts" set sitting next to you to compare, switch out parts etc. etc. would be an option. I know you are on a fixed income but if this is something you'd like to pursue I'm sure with the help of this forum and ARF we could make it work for you. Count me in for sure.
Again thank you for being so dang stubborn, its inspirational to us all.

Here is one thing you can do.... making sure where everything connects first, one at a time, disconnect B+ feeder line from each major circuit until you have found the shorted area. When resistance goes up, then, you have found the circuit with the short.
That's what I had to do alot of.
The set you are on isn't like mine.
Yours has floating B-, and, +90 volts going to chassis.
That is part of the negative bias.
The KCS 47, which is mine, is a much simplified version.
Do you have the schematic on yours?
I know it's in RCA, and, may also be in Sam's. I know it's definately in Ryder's.
Thanks. I've been under the weather today, so, have been in bed alot.
Hopefully by next week, I'll be better, and, back on the set.
Bill Cahill
:yes:

Not to hijack Bill's thread, but Pugs, if your set has (especially) wafer type tube sockets, sometimes arcing can occur between the two layers and the resulting carbonization can cause a short, especially in rectifier or other high power tubes. The fix is to replace the socket with a molded type, or ceramic, even better. Worth a look.

Thanx for the tips! My mistake on the similiarities in our sets. Yes I do have the schematic and it has helped immensly. I have done some of the B+ ellimination but it is time to do it in earnest. I will definitely keep in mind the possibility of arcing but being so new to this I'll still bet it was a screwup on my part when replacing caps or a bad solder joint.

I've found the problem!
I've FOUND the problem!!
Almost positive, anyway....

I had a brand new old stock GE 12AU7 in video socket.
At the time, it checked per fect!
I tested it again tonight. One section was DEAD!
I then checked the one I had tried in set. Same section was DEAD.
I checked two others, and, they tested good, both sections.
So, I went to put good tube in video socket.
Guess what I found!
A broken tube pin in socket.
The other tube had broken the pin when I installed the tube, never had that happen before.
New GE? Maybe an open cathode?
I'm going to check other tubes to be on safe side, again. Then, I'll put the set back together, and, try it out.
Wish me good luck.
Bill Cahill

good luck, hope it works out.

I really believe this should fix my video problem. Then, I can get back to adjustments....
Thanks...
Bill Cahill

Haven't been able to get back on set to finnish checking tubes tonight. However, it looks prommissing... :banana:
Bill Cahill

Just finnished checking some other tubes. I had a weak 6AU6 2nd video I F tube. Quite weak.
It's gone.
Well, it was used.....
Anyway, when I'm next on it, a little chassis neatenning, then, I put it back together, and, try it.
It's gotten dusty on top, and, I want to do a minor cleanning.
Also, one of my pix tube wires is too long, and, I wish to shorten it slightly.
Bill Cahill

:tears::thumbsdn:
It's a NO Go!
Tried set today. Still no go. :(
Just a raster, and, now, I notice sound is a little weak, and, has some hiss.
Is problem in I F's, or agc circuit???
I'm at a loss........

Bill Cahill

I wish I knew enuff about this stuff to offer a suggestion....Or I could come to yr QTH & stand there, wring my hands, knock my knees together, bite my fingernails, moan, "Oh, woe is me !" & do yr worrying for you...(grin)

I don't have a schematic for your set, but if it's like the 6T72 then it could be anything from the 3rd video IF on. I don't think it would be an AGC issue.

But, maybe it's something really simple. It looks like you could still have a raster and no effect from the contrast control if the CRT socket was making a bad connection to G1 of the CRT. That would be the video output line. I'd check for that. It could even be a problem with that pin of the CRT needing re-soldered.

Since you had a broken pin in the the socket of the video output tube, 12AU7, then there's a chance that the socket was damaged, perhaps that socket was spread wide and isn't making good contact now.

Try measuring the DC level of the video output going to the CRT. On the wire going to the CRT it should be a negative value. On the 6T72 it appears it would be -1.8VDC.

John

For grins, I scanned in the video output circuit of the KCS47A... hope it shows up ok. Contrast control is on the 1st video amp, Plus it has a Dc restore/AGC/Sync separator on the video output.
jr

Thank you both very much. Your ideas and, from my friend Mr. Detrola at ARF
coincide, and, give me an excellent starting point on what to do.
As soon as possible, I ntend to impliment those ideas.
I'll let you know the results.
INterestingly, the agc switch is also having no affect, so, that shows that for some reason, the signal just plain isn't getting to the picture tube.

I have a further question...
I checked the tube on my BK 465 tester, and, it says tube is wonderful!
Would it still check good if that grid wasn't making connection?
Not sure where the tester gets its' data. Cathode, grid, or, what?
Bill Cahill

If your BK 465 is showing the cutoff as being good, then the base connection must be o.k.

John

Thank you both very much. Your ideas and, from my friend Mr. Detrola at ARF
coincide, and, give me an excellent starting point on what to do.
As soon as possible, I ntend to impliment those ideas.
I'll let you know the results.
INterestingly, the agc switch is also having no affect, so, that shows that for some reason, the signal just plain isn't getting to the picture tube.

I have a further question...
I checked the tube on my BK 465 tester, and, it says tube is wonderful!
Would it still check good if that grid wasn't making connection?
Not sure where the tester gets its' data. Cathode, grid, or, what?
Bill Cahill

did you try puting the DVD composite signal at test point E (think that it, the testpoint after the dual diode tube). that would isolate the problem to before or after the IF section.

No, but, I tightenned pins on pix tube socket. I then checked grid voltage at pix tube. It should have approx. +8 volts, but, only had 1.255-1.575 volt.

I got to thinking it might be a video I F problem, but, I checked some resistors with no luck.

I then found one of my brand new 2W metal Oxide resistors has gone bad. I thought that was it, but, I think it's feeding horiz. osc., so, probably not.
I don't have any more of that value, so, I'll have to order them.
I think I'll connect the test tube, and, start checking voltages.
Bill Cahill

Hey that sounds "poetic".
It shows the fustration you are having in a poetic way.
Reminds me of the Book of Job where nothing could go right with that feller Job.
Just messing with you Bill -- hope you get it all sorted out.

Something to make you feel better -- I had a long saga with my Zenith Porthole and now its working. But, I miss working on it. Enjoy it now, you might miss working on that TV :-)

Carl



As some of you from ARF may remember, this has been a very long, drawn out project. My world's worst nightmare.
I bought this set on ebay a little over 3-4 years ago. She actually shilled me for sixty bucks, and, I was the only bidder. She was using a friend in NY who collected purses, lady friend, that is.
I paid sixty bucks for this gem.
Cabinet: O.K. Dirty, missing some finnish.
Set looked good, but, dirty, smokey.Not cigarettes, as it turned out. It was from all the disastors with this set.

This set was a real factory lemon. It has had everything wrong with it.
Let me go over a list of parts I've replaced, so far, not in order.....
Electrolytics
Capacitors
Micas
Resistors
Pix tube
All other tubes
Tuner
Speaker
Flyback
Yoke
Vert. osc. transformer
Cabinet power on light The wires cracked, and, were brittle. Socket was rusty
Removed incorrect 10A hv fuse, and, installed correct .25A fuse.
Hv cage. Old one had some rust.
A number of screws.
Tube shields.
Volume, and, tone controls
Vertical hold, and, horizontal hold controls. Dammaged by UPS
Height control dammaged by a short while I was working on it.
Vert. lin. control. Open.
Width control. Changed value.
Many resistors
Picture tube socket. Wires were brittle, and, breaking.
What a mess!
Oh, yeh. Power cord
Back cover as original had chemical stains, and, labels ripped off.
I have correct model cover on it.
I am deeply into nostalgia.I am crazy. I got in too deep, and, can't quit now.
But, I wish I had two years ago.
Be it as it may, how much more can go wrong?
Did I mention horiz. osc. coil?
That wasn't working at all right.
Now, I have no video.
What more can go wrong?
True, it no longer blows up, pops, smokes, or, catches on fire.
Speaker, by the way, voice coil fell apart.
This set has been my worst nightmare! :no::thumbsdn::sigh::worried::screwy::saywhat::dam mit:
Please help me finnish this set, guys. I can't quit now..
Oh, I forgot. 2 video if transformers, one dammaged by me when set fell off bench. Other by UPS. One aud. det. trans. also dammaged by UPS.
I'm going crazy!
I want to finnish this set. I can't quit now!
Besides, I have friends supporting me, and, enemies harrassing me....
I must finnish this set!
Vertical problem is fixed!
For a brief while, looked like it was going to have a decent picture.
Last year, though not correctly, I actually had it playing for two weeks.
HELP!!!!!!!
Bill Cahill Thank You......

Forgive me, guys, while I rant..
BOY< AM I A DUMB POLLOCK!
Yes, I'm half Polish.

I found my problem..
IT'S ALL MY FAULT!! :nono::sigh::thumbsdn:

There is a 6800 ohm resistor coming off the 370 volt line feeding a 10K resistor, and, choke in parallel, feeding voltage to 2nd half of video amp, and, feeding some voltage to agc circuit.
Well, I needed to replace that 6.8 k resistor.
I did.
WITH A 68k RESISTOR!
Dumkoof, you fool!
I read the resistor wrong!

Now, you ask, why was he temporarilly getting video?
Easy.
The connedtion is right next to a B+ connection feeding horizontal oscilator.
My second mistake is I also had my new resistor dressed wrong, so, it was temporarilly making connection, so, I was getting video, but, horiz. oscilator was severely messed up!

Now, the bad news. The clolsest I have is 2 15k resistors. I tried putting them in parallel, but, I'm getting over 7500 ohms.
I may try it anyway.....
Bill Cahill

I think 7500 ohms would be close enough, it's very close to 10% tolerance.

Hey that sounds "poetic".
It shows the fustration you are having in a poetic way.
Reminds me of the Book of Job where nothing could go right with that feller Job.
Just messing with you Bill -- hope you get it all sorted out.

Something to make you feel better -- I had a long saga with my Zenith Porthole and now its working. But, I miss working on it. Enjoy it now, you might miss working on that TV :-)

Carl

Don't bet on it. I have two of the cheaper chassis to work on, and, a possible 3rd tv coming. Believe me, I WON'T miss it!
Bill Cahill

Well, replacement resistors hadn't arrived yet, today, so, I jury rigged 2 15K resistors in parallel.
Close second.
While checking out final wiring, I found that the shield under the 4th video I F had broken loose from center pin on tube socket, so, I soldered it back on.
Anybody know what that might have caused?
I'll be trying set maybe as early as tonight. :yes:
Bill Cahill

Tried set today. Had my video back. However, bringing variac up, an old hv problem happenned again. Raster kept flashing on, and, off, and, I was getting alot of static noise in speaker. I heard sizzling in chassis.
I shut it down. Smelled like something burning.
This happenned twice befor with shorts on horiz. out. socket.
Tonight, reluctantly, I took set back apart, and, put a very strong light on output socket.
I found something!
Factory problem Number 586.
On pin 5, a resistor, there is a tiny piece of the resistor wire sticking sidewayl
Wait a min.!
That's pin 4 and, 3. Pin 4 is a tie point.
Well, that short little piece of lead had a tendency to intermittently short to pin 3.
I moved the two pins away from each-other.
Then, a 47 ohm resistor going to adjustable cap off p5 broke loose from cap due to poor solder connection.
So, tommorrow, I have to fix that.
Then, I'm going to give that circuit one real good going over again. If I see no more problems, I'll put it aback together, and, try it again.
This set has truly been this technician's nighmare!
If I never run into another problem set like this again it will be too soon!
How many times, I wonder, did this set keep breaking down?
It must have been a constant problem to keep it running.
This set should have been torn down at the factory, and, re built.
It was a major lemon!!
Now, this may have actually been the cause for it blowing high voltage fuses for original owner in 70's.
This set had a 10A fuse in it. Flyback was cooked.
For a year I kept getting intermittent shorts on horiz. out. socket, but, every time I tried to find the problem, it went mysteriously away.
Today, it had returned..
This set is very frustrating.
I hope that outside of final adjustments, this will finally finnish this saga once, and, for all....
Bill Cahill

I will be trying set soon. I recieved help on horizontal output socket. It was a problem before, and, I couldn't find the short. Now, someone with better eyes found the intermittent shorts, and, I believt that problem is fixed once, and, for all.
I have given up posting at ARF, and, will post there on this set no more.
I will finnish posting on this set here, and, at my own forums.
I'm tired of the sniping.
If some people don't like the posting, just don't read iot!
I did it as a service to others. It sure hasn't done me any good.
I wanted to help others in servicing these sets.
Bill Cahill
I just am fed up with the sniping, and, flame baiting there. PERIOD!

I will finnish posting on this set here, and, at my own forums. I'm tired of the sniping. If some people don't like the posting, just don't read it! I did it as a service to others. It sure hasn't done me any good. I wanted to help others in servicing these sets. I just am fed up with the sniping, and, flame baiting there. PERIOD!



Hate to be the one to point out the obvious, but most of the people there are also on here. :naughty:

Also, when threads are 60+ pages long they become valuable to no one, and annoying to everyone. You said you did it to help others- who's gonna search through a 60 page thread? I'd be very surprised if this thread here is allowed to get much farther than 10 pages, or the same thing would likely happen.

If you want my honest opinion, your thread over there reads more like a blog than an informative fact-based electronics restoration. I mean, just look at this post from the other day:

"We also went out to lunch for some of the best chicken around anymore. he agreed. His comment to them was, "This is the second best tasting chicken I've ever had, and, I haven't found the best yet." It is just great chicken!
He was only going to eat one piece, and, ate most of it...Yum!!!!"

WTF does that have to do with a TV?!? That stuff belongs in a general forum, not a tech forum. My advice is do not post in this thread unless there's actual SUBSTANCE, for example: you fixing your TV.

Hate to tell you this, but, I'm aware many people here are also there.
If you read those 60 plus pages, you'd find alot of valuable information.
Evidently,, you've missed alot.
As for anything else, it's a moot point.
My answer is set is playing.
I still have some picture issues, but, I have a picture, and, sound.
Now, is there anything else you want to know that will help?
:scratch2:

Just a fast note of information.
I adjusted the set most of the night.
At this stage, I have a picture, sound, vertical is full, but, both sides of horizontal are short.
I also have a very slight picture pull at the right outer edge.
It's either got to be tubes, or, maybe electrolytic problems.
I'll try the tubes first.
Other than that, set was on for over 4 hours, and, is stable.
Bill Cahill