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

[rokgpsman]

That's curious about C903 & C904 being one value and C905 being a different value. Wish we knew for sure that the schematic is the right one. I think it is real close but it may have different designators for the various parts.

I don't think crt monitors are made with a self-discharging method. The charge inside the crt is electrostatic so it will decay over time but the ones I've messed with still had some charge even a day after they were last powered up. Just to be safe always take safe precautions and assume it does have a charge until you have manually discharged it. As soon as you figure it will be ok to take a shortcut is when you get bit by it.

Hoping you will find something in the vertical circuit that fixes the tearing problem.

[shortrackskater]

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.

I can't find Q702, only Q701. I did find D702 and it checks good with my meter on diode setting. I found IC701 but have no idea how to check it. R720 measures 195 ohms... I think, if I'm reading my meter correctly. I'm not really sure what else to do here. Shot two pictures. Q701 looks either like it's been hot, or it's just the PC around it being heated by R774

[rokgpsman]

I don't think there is a Q702 on the schematic, did I typo about it somewhere?  Q701 is the one to worry about.

R720 is 140 ohms on the schematic, does it have any writing or markings on it that you can read to make sure what value it is supposed to be? Also, did you lift one end of R720 to measure it, that's the only real way to measure it truly. If it is reading 195 ohms in circuit then you know it will measure even higher resistance out of circuit. If R720 has increased in value a lot then it could be due to too much current going thru it and thru Q701, over heating them both and changing the value of R720, which now changes the way the circuit works, possibly causing the tearing image problem. Of course, this is just a theory....

The darkening of the board around Q701 makes me think it is from Q701 getting hot, not so much from the 82 ohm R774 nearby. So if Q701 got that hot I'd be suspect of it, might be breaking down during part of the vertical signal and causing the tearing. Maybe you can replace it. And also check the other parts around Q701 like R729, R774 and pot R773. R774 is a 2 watt resistor so that means there is some heat in it and current thru it and Q701 normally. That circuit part labeled  YV1 and YV2, is that the yoke windings for vertical deflection? You are at the point of just checking things in that area and replacing anything you aren't sure of, that's the only way to eliminate it as a possibility. Resistors can be measured, but caps and IC's you really don't know until they are replaced.

Take a look at that picture I posted a few comments back (post #49) of the screen tearing at the top and compare it to yours. That guy said he replaced R720 and C714 and it fixed it. He also said a leaky D702 can cause problems. Only you can decide how far you want to go with this, I know you have other stuff going on. But you might be right there on top of the problem and just need to change a few more parts to fix it. Based on that other guy's repair I'd think about changing R720 and C714 and maybe Q701 before ending the effort on your monitor.

But whatever you decide to do will be ok. You can probably get a replacement monitor, even a used one and get things back up & running. A lot of people would choose that way rather than give repair a try.

[shortrackskater]

Thank you, again.
I think I'll give this a try. It's already apart again. I can't see the ID on the transistor. Is it C1213? I don't remember anything on parts anymore. I only knew these numbers a zillion years ago when I was at Rockwell.

[rokgpsman]

The schematic shows Q701 as a C1213 transistor. If you can't find anything on it we can check a cross-reference. There may be a place that sells electronic parts in your town, also ebay, Jameco, Mouser, etc.

I just checked the cross-reference, a C1213 is also known as a NTE289 transistor. Did you check for bad resistors in the vertical circuit, especially ones around Q701? If a resistor has opened or increased in value a lot then that could cause a new Q701 to overheat. You can lift one leg of a resistor from the board, measure it with ohmmeter, then resolder it if ok.

Looks like the new NTE289 transistors sell for about $2-$3, then a shipping charge.

[shortrackskater]

I was able to buy c714 which I hope is a .10mfd 100v cap (I took part of the schematic to the parts place) and D702 which says 10df4 on the schmetic. The replacement diode says NTE574 D041 which replaces ECG574.
I have no clue on these numbers.
They didn't carry the transistor. I'm looking on ebay and see some that say C1213, or C1213a, and NTE289 as well.
I'm wondering if I should replace the cap and diode I have now (if they're correct!) and see what happens or find the transistor and do all three?

[rokgpsman]

ECG was the standard cross-reference for semi-conductor parts for a long time and a lot of places still use it but NTE is replacing it. Either one should work fine for getting the right parts. I did a quick search on internet and diode 10dF4 does come up as NTE574. Be sure to install the diode the correct way, with the diode leg nearest the band on diode body in same solder hole as the old diode or use the marking on the board. (you already know this, just a reminder)

Since you have these 2 parts I'd be tempted to install them if it was me doing the work. Just keep in mind this is a bit of a gamble, don't leave the monitor on too long if you still see the tearing on the screen. That old Q701 probably isn't completely bad since you do get a fullsize vertical picture but it may be running hotter than it should. I'm surprised the store didn't have the transistor but I guess they can't carry everything. If you try this and the screen still has the tearing I'd try replacing Q701 next.

I think the C1213a is just a higher power (or voltage) spec'd version of the C1213, probably would work fine. The guys selling the parts should be able to give advice by comparing the specs listed in their catalog/database. They get questions like that on parts they sell.

Good Luck!

[cowboygames]

If only the IC is bad you'll be ok doing that. If you have a resistor breaking down under load or a bad diode then it will trash the new vertical output IC. I would put a meter on the diodes in that circuit and see if you have a bad one. A bad resistor may or may not check bad without a load on it. Signs of excess heat around resistors isn't uncommon, but it will be common to all similar boards in the same area if they just run hot in that area for whatever reason. It's not a sure sign of a problem. Best bet would be if you had the pin outs for the vertical output IC, LA####, and could check the voltages on it with the monitor running.

[shortrackskater]

I did get the transistor ... it's an NTE 289a.
At this point, I can replace the three parts mentioned. I don't have any extension cables to check running voltages.
Should I just replace the parts and see what happens? I'll remove the resistor and check it out of the circuit. The diode tested good on my meter but it was still in the circuit.
Gah!

[cowboygames]

Diodes will check good or bad in or out of circuit, but if it checks bad I'll pull one leg up to make sure another component isn't the cause. Resistors check the same in or out of circuit, but can open under load, that's why it's better to check the voltage on both sides while it's running. RCA had a huge problem with resistors that checked good but broke down under load. Ultimately, if shotgunning the circuit is your only good option then ya gotta do what ya gotta do. If it gets to frustrating or expensive, send it to one of the companies that rebuilds boards. Seems like they've been mentioned on here before, but that might have been for LCD's

[shortrackskater]

All the diodes tested good. I went ahead and replaced C714 and Q701.
I got this...seems to be a working monitor!

[cowboygames]

Good job

[rokgpsman]

Looks good from here!   

[shortrackskater]

Thank both of you for holding my hands through this one!
I'm wondering if it was the cap or the transistor?

[rokgpsman]

Thank both of you for holding my hands through this one!
I'm wondering if it was the cap or the transistor?

I am too. There are transistor checkers and cap checkers but you'd have to locate one. The cap or transistor could have been breaking down during the vertical retrace and that carried over for a few lines down the top of screen. If either part shows physical change like cracked or overheated that could be a clue. In the shop a lot of times we'd change 2 or 3 parts at the same time in order to do the repair faster, since the parts cost less than what the time spent going one by one would cost. But never know for sure which part was the actual cause of problem. My guess would be the transistor but it's just a guess.

You could put one of the parts back in and see if it still works or begins tearing again.  NOT! - That's a Bad idea, you got it working, so I'd leave it alone. 

Was your adventure with this worthwhile for the experience? I'd say yes but your time spent may have taken you away from other things you'd rather been doing. But it is good for one's confidence, tackling something new and having success at it. That's a big plus I think.

[shortrackskater]

I actually thought about putting the old transistor back in! But nope, not gonna do it!! Perhaps if I had better test facilities, rather than the garage bench, and constantly moving laundry around to clear more space.
I really want to make up some extension cables for IGT and Bally too...that's on my list.
As for the time and trouble spent here, I would do it all again in a heartbeat. Thanks to you and cowboygames, I was able to rid my fear of CRT's. I still know next to nothing on them but at least know, I know a little more...and I can tear them down, discharge, and put 'em back together too. 


Note: I changed the topic name to reflect the actual item issue. There's way too many mismatched topics on NLG. I wish we could do a "clean up" day here sometime.