I have an important question

[Bill Cahill]

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

[wa2ise]

Originally Posted by 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.

[electroking]

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

[zenithfan1]

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

[Username1]

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.

[Bill Cahill]

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.

[DaveWM]

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.

[Tom Albrecht]

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.

[Bill Cahill]

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

[DaveWM]

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.

[Bill Cahill]

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....

[Bill Cahill]

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! :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......

[kx250rider]

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

Charles

[miniman82]

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.

[holmesuser01]

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.