Tatung VS-14428 monitor issue ... horiz lines at top

[shortrackskater]

Well that confirms what should be there.
And, what I pulled out was the value for C906!
Interesting.
And thank you!
I wonder if I should just do the neck board and see what happens?

[rokgpsman]

I wonder if I should just do the neck board and see what happens?

That's a thought, I wouldn't think it would hurt anything.

here's a picture that came up on google search, supposed to be the equivalent neck board, it is used on a 14" Tatung. Does it look like yours? The photo was not too sharp, so is hard to read the component writing on the board.

[shortrackskater]

Here's mine... looks like it.

[rokgpsman]

It sure does look like the same board. I'm still trying to find schematic for neck board. I've seen a lot of requests for it, the answer is always a referral to contact a company called Merit, I think they own Tatung or do their service work. But this gives me another neck board part number to search for schematic.

Since the main problem you have with your monitor was the screen image tearing at the top then the problem may be on the main board, in the vertical (or horiz?) section. So just doing the neck board might not fix that. But you can try it after completing the neck board part of it to see what happens.

[shortrackskater]

Well it didn't explode! But the original problem is still there.
So I'm on to the main board. I think I'll save that for Friday though...
THANK YOU for all the help and support, so far.
Still that's strange on that cap being the wrong value. I wonder if it was put in that way originally when, whoever did the assembly, they just put in the value for C906?

[rokgpsman]

Still that's strange on that cap being the wrong value. I wonder if it was put in that way originally when, whoever did the assembly, they just put in the value for C906?

You're probably right, someone at the factory saw the C906 written on bottom of board and looked at their list, inserted the 100uF cap.

Hard to say if it would make a difference without seeing the schematic. Just so I am clear, what value caps did you install for C905 and C906 and did it match what the cap kit list said? The guy you got the kit from may have schematics of the monitor, including neck board. Or maybe not. Wouldn't hurt to send him an email to see, while you take a break from soldering.

I did find a schematic of the neck board - I think - but it is just about unreadable. Looks like the worst scan ever. And the original drawing was made back in the day when the writing was done manually by hand, so it really gets blurry when enlarged. Check it out:

[shortrackskater]

C905 is now a 4.7uf 50v and C906 is a 100uf 25v

Oh my! Impossible to read. It looks like cuneiform. 

[rokgpsman]

Whoa! I think yours is closer to making sense.  I'll check back in on your next part of it.

[shortrackskater]

 

[rokgpsman]

C905 is now a 4.7uf 50v and C906 is a 100uf 25v

well, I had too much caffeine, so continued to look into this, here's more info.

I found a readable Tatung neck board schematic that looks similar to yours (I think). It does show that C905 is 4.7uf and C906 is 100uf, which matches what you have. So maybe you can use this schematic as a guide even if it isn't exactly for your neck board it may be pretty close. It shows that C905 is just a coupling cap, blocking DC but allowing the BLUE video signal to pass thru, so it may not have mattered much if it was 100uF instead of 4.7uF.

Next I found a repair blog where someone had a screen image with horiz tearing at the top, similar to yours.  They had done the recapping but it didn't fix it. It was suggested to them to carefully check the resistors in the vertical circuit around IC701 LA7837, especially C714 and R720, circled in the center of the attached schematic. They also said a bad or leaky diode D702 has caused this problem. On your main board these parts may have different designators but hopefully they will be similar parts. Anyway, the guy said replacing C714 fixed his screen tearing problem. He said it was a Mylar cap, 100v. I don't know if C714 is in your cap kit, but after you are done recapping if the tearing problem is still there this might be something to look into.

[shortrackskater]

Just finished the re-cap. I'll check those other things now...

Where the hell did this insulating washer drop from???

Update: I didn't see anything obvious, but during my recap, I did see a LOT of bad solder joints, which I repaired. It appears this has been recapped somewhat... some I replaced but others that weren't part of the kit I got.

[shortrackskater]

Well crap... now I have the same tearing with a nice RED screen. I tried adjusting the RGB in the back separately (since I have to pull the CRT out) but didn't get any change, unless it's a combination of all three causing this. I need an extension cable.

[rokgpsman]

Odd that the screen color would change. That was C905 that you found they had put in a 100uF instead of 4.7uF, and it is in the Blue video input. I wonder if when they aligned it they had to set it differently since they had the wrong cap in there for C905? Not sure, didn't think it would matter. You can always reinstall a 100uF back in for C905 (like they had it) just to see if it affects screen color. On the equivalent caps in the Red and Green video input (C903, C904), did they also get changed in the cap kit work? If not, are they 4.7uF or 100uF?

Are you saying that adjusting the Red, Blue and Green video input pots (R950, R952, R953) didn't affect the screen colors? Or is it the pots R936, R937, R938?

We may need to get someone more familiar with monitor alignment than I am to give an opinion.

[cowboygames]

Based on your picture you lost your green gun on the crt, blue and red are there. And quit messing with the crt board, it has absolutely no effect on picture size(vertical/horizontal)in any case. Those things are controlled through the yoke. Any stretching, shrinking, tearing, etc will be a product of cap, resistor, diode or IC problems in one of those circuits. If you can get your color straightened out I would find the pin out diagram for the vertical output IC and check all your voltages while it's running and go from there.

[shortrackskater]

The problem is that I've never worked on a monitor before! So I just bought the cap kit and replaced everything. I had no idea that the CRT board didn't affect picture quality...
Well, at least I learned how to tear a monitor completely apart and get it back together again.
I'll reinstall that different value cap and see if the color returns, then I'll report back.
Thanks cowboygames and rokgpsman.

[rokgpsman]

You could check to make sure all the connectors going to both boards fit properly, don't have a pin that got pushed back or bent over and causing the loss of the green signal. A cable or wire could have gotten tugged on or pulled while you had it apart. In particular check the cable carrying the red, blue and green signals from the main board to the crt board.

Also, you could check or replace that cap, diode and resistor I mentioned in an earlier comment (#49) that fixed the tearing on the other persons monitor from another repair blog. They are located in the vertical circuit.

[shortrackskater]

BUT... what I pulled out of the C905 hole was a 100uf cap and not the 4.7uf!
Would this have worked for however many years being the wrong value? 

Maybe it would have been fine if it was just a filter cap and 4.7uF was going to be good, the 100uF would have just been extra smoothing, not needed but didn't hurt anything. Depends on the circuit.

EDIT: turned out C905 was a DC blocker/coupling cap so value was not critical.

Okay I pulled out the cap in the mismarked C906/905 hole and put the 100uf back in. The color is back to normal. Now I'm back to the tearing at the top.
So this cap actually IS C905 i think... and it did make a color difference... I'm confused now with it being a DC blocker. On the neck board, this cap is at the top of a line up of caps C903, C904
Now I'll check things from the earlier comments...

[rokgpsman]

That schematic we are using may not be the exact one for your neck board, or there may have been changes and updates to your board after that schematic was made. For whatever reason it looks like the 100uF works better. Are the caps at C903 & C904 also 100uF? They are the similar caps in the other color signal inputs.

As cowboygames said, the tearing in likely to be caused by something in the vertical circuit on the main board. That repair blog I referred to earlier said to check or replace things in the C714, R720, D702 area of IC701. Also, Q701 is right there in the same circuit and it may be the vert output transistor cowboygames mentioned. 

[cowboygames]

If you're not familiar with circuit board numbering systems, in this case all vertical circuit components will be 700 series numbers. Makes it easier to pick them out of the jungle on the board versus following traces

[shortrackskater]

Caps C903 and 904 are 4.7uf
This board looks like it had some revisions/rework done as well since there one cap just soldered on the back of the main pc board.
Okay I'll take out the main pc board and check those areas.
Funny... I was (sort of) looking forward to hearing a ZAP when I discharge but, so far... no  sound. Do these particular monitors self-discharge when powered off? It only sat a little while and I got nothing. I'm making sure there's a good connection to chassis ground too.