WGM2775 shutdown

[rzgrip]

Howdy! I have a WG K7500 series monitor with a 27"tube. On start-up the picture looks great, it is a medium resolution monitor. But after anywhere from 2 to 5 minutes, the picture goes to black. I can hear the game playing blind. I switched the game to standard rez and hooked it up to a normal monitor and the game played fine. Soooo, it is not the game, but the monitor. There is neck glow and I reflowed any suspect solder joints. I do not think it is a bad cap but I replaced most of them just to be sure. I am at a loss on what to check next. By the way, this is in a Gauntlet Dark Legacy game. I am not great on figuring out circuits but I follow directions well! Please, any ideas? My thanks in advance.

[SirPeale]

Is there still neck glow when the picture goes out?

[rzgrip]

Yep! That is part of what I do not understand.

[SirPeale]

The neck glow is just the heater.  Of course, without this you won't get anything.

Does it make any kind of noise, high pitched maybe, before it dies?

If you turn it off, wait a few seconds, and turn it back on does the picture resume?  Or do you have to wait considerably longer for it to cool before it resumes?

Have you measured the B+?

[rzgrip]

There is no noise when it goes to black. I restarted it after being off for about a minute and the picture came back on. Then it went to black again after another minute or so. I am not sure how to check the B+ voltage on this monitor. If it is the large ceramic resistor then it is 76v on one side and 242v on the other.

[SirPeale]

I don't have my notes in front of me, but that seems...high.  Let's see what I can dig up.

[SirPeale]

Does it make a subtle 'clicking' noise?

[rzgrip]

I do not hear anything unusual from the monitor. If I am not looking at the screen, I do not even know that the screen is black unless I look at it. I did turn the sound off so I could listen more closely.

[SirPeale]

My guess is it's going into a protect mode because the voltage is too high.

[rzgrip]

Thank's for the input Peale! It makes sense to me. If you or anyone else can tell me what to test or look for next I would be forever grateful.

[grantspain]

its gonna be one of two problems,the most likely is a cap on the ht smoothing or on the g2 input of the neck card
the other possibility is a breakdown in the lopti but this is unlikely
if you get tube heaters 9 out of 10 it means the monitor is running o.k,kinda rules out any fault in the frame or psu circuit
a common fault on nanao monitors is the electrolitic on the g2 neck card that causes this exact fault

[rzgrip]

Soooo, you are saying that it probably IS a bad cap? I replaced most of  them. But then got very confused as usual! I got a capkit from Arcadeshop that fits the K7400 and the K7500. I knew that I would probably have some left over caps. But when following their printout, I found several that had different values than the ones on the board. I know that you can install one with a higher voltage rating but the value was different. I checked Wells Gardner parts list and their specs matched the one from Arcadeshop! Should I follow what WG and the Arcade Shop says or stay with what is on the board? I cannot tell if anyone else has replaced any caps, but the board has been worked on. What can or will be damaged if a cap with the wrong value is installed? I did replace all of the caps on the neckboard. All of those matched WG and what was on there.

[grantspain]

if the monitor worked o.k before at some point then replace the caps with the values that are already on the chassis,please try one thing when the picture goes dark turn up the screen volts on the flyback to see if you have any picture

[rzgrip]

When I bought this game it allready had this problem. Now, The screen voltage control on the flyback was still factory sealed. I removed the blob of sealing gunk. After the monitor went to black I played with the voltage. I found out that if I turned the voltage way down and slowly increased it, the picture would come back on! However I could not get it to stay on. It would reapear and then then go back to black. After turning it back down and slowly increasing the screen voltage the cycle of reappearing and disappearing would repeat itself.

[Kevin Mullins]

If you just "wiggle" the SCREEN pot does it do this as well?
(with the screen pot turned at least half way up)

[rzgrip]

The screen goes to black before it gets to halfway. However, I just wiggled it while the picture was still on and when it turned to black. No change. However, For the first time I did notice that After the picture disappeared, It did not go to complete black. There was a very dim image on the screen. And the brightness control on the remote board had no effect on anything. Is the screen voltage contained solely in the flyback? If it is, then maybe replacing it will be my next step. By the way, my thanks to everyone for their input so far!

[grantspain]

how much is the flyback to replace?

[rzgrip]

Bob Roberts has them on sale for $19.00. Other places for $28.00 or $35.00

[grantspain]

a bad flyback can cause this problem,i have had ducksan monitors do the same thing.you see the difficult thing is to say 100 percent that this is your fault but gut feeling is leaning me towards a bad pot on the flyback,the only other possibilities are caps(which you have replaced) or maybe a resistor on the g2 circuit,difficult call this-if you go ahead and buy a new flyback and it ain't that then you're not gonna be best pleased so first maybe check the resistors on the g2 circuit(it should be inline with the g2 input wire from the flyback)

[Kevin Mullins]

I found out that if I turned the voltage way down and slowly increased it, the picture would come back on! However I could not get it to stay on. It would reapear and then then go back to black. After turning it back down and slowly increasing the screen voltage the cycle of reappearing and disappearing would repeat itself.

This is why I question the pots and or flyback.

Does the high voltage seem to turn on and off when the screen goes black and back on again? (You may or may not be able to notice this unless the crackle goes on and off.)
Have you Ohm'ed the brightness pot on the remote board just to be sure it is good and consistant when adjusting it?

Intermittant problems can be a bear sometimes and it can take some out of the ordinary checking and testing to really think about it. I was even starting to wonder if maybe the x-ray protect was kicking in for some reason after the picture came up and then it would kill the picture.