WG K7400 random shutdowns & a blown HOT

mecha

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this is a monitor I capped last July and was never able to get a dead-on B+ adjustment with it. the monitor would still go into shutdown (reason I capped in the first place) every so often. it's also worth noting that when I was readjusting the colors, brightness, focus and contrast, the monitor would also shut off when you got the focus where it needed to be.

so where we're at is just yesterday, the monitor didn't turn on at all, and once I was able to power the cab up with none of the other games on at work, I heard tick-tick-tick. the HOT was shorted, so I put another good one in and replaced the flyback for added measure (the focus thing I found suspect).

monitor works again, and because the monitor still periodically went into shutdown after I capped it last year anyway, I decided to fine tune the B+ once more. this is where it gets interesting: Randy Fromm's flowchart says the B+ should be 130VDC, modessitt says 117VDC, and quite certain Chad Entriger says 118VDC. I just would like to know once and for all where these are supposed to be set at.

the B+ walks between a volt, which I'm sure is normal depending on if the screen is bright/dark, but I don't remember which one you want to set it against. I set it for 117VDC one time, the voltage dropped to like 116.4 and that's when the monitor went into shutdown again.

Fromm's flowchart told me to test some diodes too in instance of shorted HOT, and they all checked out fine. the caps he lists though I wasn't able to find at all, they're nothing from the cap kit, they must be the other kind of caps.

so specifically I need to know:
1) why the HOT shorted in the first place.
2) what the B+ should be set at.
3) why it still randomly shuts down. (I would lean towards bad shutdown pot, but the shorted HOT thing is alarming and shows that maybe a resistor somewhere's out of whack).

I don't do schematics, unfortunately, so if there's any particular parts I should be targeting, please let me know.

thanks.
 
1) Probably due to a flyback issue
2) 117.20vdc. When in doubt, check WG's page for the voltage checkpoints of the chassis, and TP202 (which is the B+ checkpoint) is listed at 117.20. 118 is fine, too.
3) Could be several things. What does the B+ go to when it shuts down?
 
1) Probably due to a flyback issue
2) 117.20vdc. When in doubt, check WG's page for the voltage checkpoints of the chassis, and TP202 (which is the B+ checkpoint) is listed at 117.20. 118 is fine, too.
3) Could be several things. What does the B+ go to when it shuts down?

1) yeah, figured, that's why I changed it.
2) this can be kind of tricky since it's a vertical mount, I do have another game that takes a K7400, swapping chassis to test shouldn't matter right?
3) I honestly don't remember exactly, but I'm thinking it went 0.0, usually when I turn the game off it dips to like 20 and then gradually decreases. that should make a huge difference right? it's so random I don't know if I'll get it to do it again without cycling power seemingly 100 times.
 
Put an alligator jumper on the test point when you install it. Makes it easy to check it when it's installed and running...
 
I know thats exactly what I did, just saying with the monitor sideways its gonna be hard to get at everything.
 
does the 7400 have that little transistor either on the same heatsink as thr hot ot mounted remotely? MAbye im thinking u5000 but i had a hot eater and that little mosfet was shorted and overworking the hot.
 
does the 7400 have that little transistor either on the same heatsink as thr hot ot mounted remotely? MAbye im thinking u5000 but i had a hot eater and that little mosfet was shorted and overworking the hot.

I've only ever seen those on the K7500. this one FWIW has it mounted to the black heatsink.

why exactly is that transistor moved to its own heatsink and connected remotely anyway? never made sense to me, seemed like it just introduced more points of failure.. and then of course the risk of connecting it backwards too lol

just wanted to report I haven't had to "reboot" the game since I did work on it. I constantly check that sucker for a blank screen lol
 
I've only ever seen those on the K7500. this one FWIW has it mounted to the black heatsink.

why exactly is that transistor moved to its own heatsink and connected remotely anyway? never made sense to me, seemed like it just introduced more points of failure.. and then of course the risk of connecting it backwards too lol

The HOT puts out a lot of heat, especially on the med-res K7500. Moving that transistor off the board to the frame keeps it from failing more often from the heat...
 
Good news.. came into work and it was in shutdown! Grabbed the meter and alligator clip and get this, the B+ was 164.3V.

So now what should I look at?
 
Check C107, and inspect for bad solder/traces. Check the voltage at U701 pin 10. It should be about 9 volts. If not, report back...
 
those go bad frequently? I had a U5000 I was messing with a couple weeks ago that had U701 socketed, I'm guessing it went bad and whoever replaced it put the socket to make it easier to change out. does running that socketed affect its purpose at all?
 
Socketed is what we do when we're not sure if it's bad or if something else is frying it. Easy to swap out other chips with the socket, rather than desoldering and resoldering every time. Still works fine in the socket.

No, they don't go bad frequently, and it might not be bad, but if the voltage there is bad, other parts could be the problem. If the voltage is good, then we go a different direction...
 
if it helps any, I replaced C107 already, same symptom both before and after the cap kit. I haven't had a chance to test U701 yet.. downside to me being the jack of all trades, I'm kinda needed everywhere lol
 
I figured you replaced it already, but you need to inspect it for a bad trace that separates as it warms up...
 
I figured you replaced it already, but you need to inspect it for a bad trace that separates as it warms up...

there were no bad traces on this one at all if I remember right, I don't think I had to do any hacks. probably the most durable of the garbage era WG chassis I've ever worked on (always had at least 1 solder pad burn up whenever I did a U5000 or K7500), this thing was mint and never messed with before me, but I'll give it another look for sure. thanks for clarifying.. I couldn't find that cap when I was looking at one of my extra K7500s, but a cap map over at Bob's showed it's right next to the width coil... the cap's importance makes a lot more sense now
 
as per mod and the Fromm flowchart, I am supposed to test Q708, Z705, and Z702. This is because my tested voltage on pin 10 of U701 is not 9 volts, but 12.12 volts.

IMAG0839.jpg


Observe, I have neither Q708 or Z705! So, now what?
 
The flowchart is wrong. Those are optional parts, and Pin 10 on the schematic says it should be 12vdc, so it looks like yours is okay.

What is the B+ again? Have you tried connecting a lamp to the B+ to see if the problem is in the PS or the deflection circuits? Connect one prong of the plug to the frame and the other prong to TP202. If the bulb lights, you're B+ will probably register properly...
 
B+ is fine, when I have raster/neck glow. I set it between 117-118vdc. When it happens to go into shutdown the B+ goes up to 164.3vdc when I tested it. So something causes it to sometimes skyrocket and shut off.
 
B+ is fine, when I have raster/neck glow. I set it between 117-118vdc. When it happens to go into shutdown the B+ goes up to 164.3vdc when I tested it. So something causes it to sometimes skyrocket and shut off.

Other way around. The shutdown causes the B+ to go up because there's now no load on the power supply.

I have a 7400 I need to get around to fixing. I think I'll stick around and watch.
 
Ken says the epoxy WG used on the pots wound up damaging them over time. What if its just a bad shutdown pot? I dont know anything else otherwise.
 
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