K7500 No video after 5 minutes.

travisbmartin

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I'm not a monitor guy, so I'm probably going to get a bunch of eye rolling from this.


I'm working on a WG 7500 in a dedicated Gauntlet: Dark Legacy. I capped it a few months back because we'd occasionally lose green. Then I went back into it and changed out the Color Drive transistors on the neckboard, which actually solved our color problem.

But I have a feeling I fried the Flyback though. I decided to fiddle with the screen control to help out on the brightness control and it caused me to lose video. I'd turn it off, and when I turn it back on after a while I get video back, but it's gone in about five minutes.

I see no visible cracks or 'goo' coming out of the flyback. But, I don't trust the screen control knob anymore. It can be moved around in the casing and looks like it's leaking around the edge.

I tested the HOT using the diode test on my meter, and it checks out. But I sort of have doubts about it being good still.

So, I'm basically asking for confirmation of if this sounds like a bad flyback. And if I should just go ahead and replace the HOT, does anyone know where to get one? Bob Roberts doesn't seem to have one listed, and Mouser.com doesn't have them either.
 
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if the flyback is original, chances are you have a problem with the actual screen pot (usual symptoms on these are the brightness will change on its own, or the monitor will go in and out of focus, meaning the pots inside the flyback are failing). I've never heard of it before, but I'm sure it's no different than when a power supply gets a dirty +5 pot and it goes into shutdown.

the HOT I matched up to NTE part # NTE2354. however, mouser DOES NOT have them in stock, which I find insane. I'll dig some more to see if I can find them anywhere else.

I would replace the flyback and HOT (assuming we can find one). I've bought flybacks from either Happ or Bob Roberts. also more than likely, your color transistors were probably all good and you "fixed" them by resoldering them. those neckboards commonly barbecue the solder pads to those, or they'll develop cold solder.

I think it's time I make another guide. I've been answering the question about neckboards and colors on these monitors for waaaaaay too long now.
 
I thought it was odd that Mouser didn't have the HOTs too.

I actually just went ahead and replaced the color transistors. The green would go in and out randomly, or just make green horizontal lines on the screen, so I said screw it and switched them out.

I appreciate the help. This game's been a pain ever since we got it. I'll probably have to replace the hard drive soon too. But, when it's working, the game gets played A LOT in the arcade.
 
It would be unlikely for the flyback pots to fail. My guess would be that an internal component of the flyback is failing when it gets warm.
 
Make sure you replaced ALL the caps in the power supply section (left side of the board). It has nothing to do with your HOT they don't go in and out of working, they're either good or bad. What's happening is your B+ is rising to the point where the monitor is going into high voltage shutdown, it's possible that's being caused by the flyback but it could also be about anything in the power supply section (resistor getting too hot and changing value, cap you forgot to replace, etc.)

Good luck!
 
I'm very sure. I capped it a few months back, and aside from the green dropping in and out it was fine. Changing out the color transistors fixed my green issue.

I'll probably have a dead day at work today, so I may just recap it again anyway.
 
Hey mecha, I found the HOTs on MCM Electronics website. They also list them on Frys website.

Of course none of the sites that I can find sell both the flyback and the HOT. Figures.
 
I sell both the flyback and hot..
hots on sale for $4.00 each shipped.
flyback is $33.00 shipped :)
hope i am able to be of help.
thanks.
chad.
 
I had a similar issue with a 25" monitor and would have bet the house that it was a bad flyback or some issue with the chassis. The screen brightness would go up and down and in several occasions the screen would go completely dark for several minutes at a time as if the monitor was off (neck glow was still present). The monitor otherwise would have a nice picture.

Before I placed the order for the flyback, I decided to rejuvenate the tube and to my surprise, it had big shorts. I zapped the shorts several times until the shorts light on my rejuvenator was not lit anymore. I went ahead and did a clean/rebalance.....and the rest as they say...is history.

The monitor no longer had brightness issues nor did it go dark. It's been working great for the past month with a beautiful picture. I had never seen a short cause these symptons before. In case you wonder, I have a B&K 490 model.

Hope this helps.



Hey mecha, I found the HOTs on MCM Electronics website. They also list them on Frys website.

Of course none of the sites that I can find sell both the flyback and the HOT. Figures.
 
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I just put an order for a flyback, because I really don't trust the one that's in there (and from what I can read about the K7500s, no one else does either).

The ironic thing here is that i have a rejuvinator. I actually bought one to fix this monitor and a couple others. When I went to plug my CR70 into the neck, one of the neck pins was bent just slightly, and it ended up crushing the connector for that pin. So until I can find a replacement adapter I'm sunk. Or at least until I can figure out how to use the universal adapter that came in the set.

Does it surprise anyone that the last month and a half I've been edging away from tinkering on our arcade games to work instead on pinballs?? lol.
 
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I'm not at the arcade at the moment, but I need to ask something incompetent.

Replaced the flyback, same trouble. On a whim before I had to run off, I checked the B+ at test point 203 and read 178V on my meter. If I'm reading the manual right, it's supposed to be around 152V. So the dumb question this time is my B+ too high?
 
Way too high. it's not supposed to be 152 either, more like 120 or so. I can't remember the exact number but it's similar to a K7000.

On a K7500 you can turn the B+ down, there's an adjustment knob. does your screen ever come on?
 
Yeah for a minute or two, then it dies. It's my day off tomorrow, but I think I'll run in, scrape the glue of the B+ adjust, and see what happens.
 
Look and see if you have any burnt up looking resistors on the left side of the board too just to make sure. You're probably right on the edge of where the B+ needs to be, and once it warms up it goes a little higher and falls off the cliff. Those things have a cut off (which by the way is adjustable too), and once it reaches that, it just pushes it up to 180 or whatever since the monitor shuts off all the load. So it works like:

If it's supposed to be 120, the shutdown might be set at 130... yours boots up cold at 127... warms up... rises to 131... goes into shutdown, the monitor turns off the load and the voltage jumps to 178. You measure it, and it's 178, but that's not the whole story, see what I mean?

I've been trying to figure the K7500's and 7400's and U2000's and U5000's out for years, I'm starting to get a little better at them.

And humour me, if you don't mind.. when you take the board out, make positive (again, lol) that you replace all the caps on the left side. Two or three times I've had a similar problem and it was because I missed a cap hiding somewhere, or the kit didn't have it, or whatever.
 
Cracked the epoxy of the B+ adjust and turned it way down. And it looks like I'm back in business. Thanks for everyone's help!
 
I'm not at the arcade at the moment, but I need to ask something incompetent.

Replaced the flyback, same trouble. On a whim before I had to run off, I checked the B+ at test point 203 and read 178V on my meter. If I'm reading the manual right, it's supposed to be around 152V. So the dumb question this time is my B+ too high?

Way too high. it's not supposed to be 152 either, more like 120 or so. I can't remember the exact number but it's similar to a K7000.

On a K7500 you can turn the B+ down, there's an adjustment knob. does your screen ever come on?

You're probably right on the edge of where the B+ needs to be, and once it warms up it goes a little higher and falls off the cliff. Those things have a cut off (which by the way is adjustable too), and once it reaches that, it just pushes it up to 180 or whatever since the monitor shuts off all the load. So it works like:

If it's supposed to be 120, the shutdown might be set at 130... yours boots up cold at 127... warms up... rises to 131... goes into shutdown, the monitor turns off the load and the voltage jumps to 178. You measure it, and it's 178, but that's not the whole story, see what I mean?
.

I know this is an older post, but I want to correct something in this thread just in case someone searching (like me) comes across it in the future.

The OP stated he got 172 at TP 203 when it was supposed to be 152 (approx).

The respondent told him it was way too high and was supposed to be 120 or so.

While the respondent is correct that the B+ is supposed to be about 120 (117.3 to be exact), the B+ is actually measured at TP 202. TP 203 is the measurement of the Vid B+ which shouldn't be confused with the regular B+. A reading of the Vid B+ at TP 203 should be in the 150's. 172 is a bit high, but is only off by about 10-12%, so it might not be enough to cause major problems. Your regular B+ is more important for functionality of the chassis...
 
I know this is an older post, but I want to correct something in this thread just in case someone searching (like me) comes across it in the future.

The OP stated he got 172 at TP 203 when it was supposed to be 152 (approx).

The respondent told him it was way too high and was supposed to be 120 or so.

While the respondent is correct that the B+ is supposed to be about 120 (117.3 to be exact), the B+ is actually measured at TP 202. TP 203 is the measurement of the Vid B+ which shouldn't be confused with the regular B+. A reading of the Vid B+ at TP 203 should be in the 150's. 172 is a bit high, but is only off by about 10-12%, so it might not be enough to cause major problems. Your regular B+ is more important for functionality of the chassis...

I'm glad you clarified it a bit. It's been over 6 months since I adjusted it, and it still works alright. The colors are a tiny bit off, but I'm color blind so I am just not going to obsess over it too much.

I can't remember if in the end I checked both TP203 and TP202 or not. All I know is that it's working for the moment.
 
that is a little high.

i would break away the glue and see if it will adjust.
if not just replace the pot.

Peace
Buffett
 
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