K7000 blown caps cause

Deverezieaux

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I just picked up a blind NARC that has a K7000 (specifically a k7935) in it with some badly cracked and leaking caps (C56, C57, and C50 are all cracked and have oozed) and some serious blackening around the traces. On desoldering some are lifting away, but I can address that when I do the cap kit. I checked the HOT off the board and it's reading within range, and the flyback physically appears okay, though I'll be swapping those as well anyway. Before I go about this I'm trying to figure out what could cause those caps to fail so bad so I don't end up just frying everything again. All of the equipment appears original, though I can't seem to locate an isolation transformer, nor a place where one would've been mounted. I'm wondering if that's the issue. Anyone know where the iso would be in this cabinet? I'm assuming it's supposed to have one. I checked the schematic and don't see it. As an aside I found a property tag on this NARC from the arcade I worked at when I finished high school, it was one I worked on 22 years ago.
 

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more than likely the caps are not leaking. that is the glue they use for the larger caps and pots that was probably spilled or on the larger caps was put there intentionally to keep them upright while going through the wave table. usually old caps fail on their own accord than are taken out but either way is possible. i would cap it, flyback, test HOT and VR and go from there and if you need parts i have all in stock.

BTW: is the fuse tested with a meter good or bad?
 
Hey security, it's funny you write, I have all the stuff from your eBay store waiting in my cart since yesterday. I haven't seen glue like that before, but most of my monitors are g07s. This is only the second WG I'll be capping. You were right though, of course, when I pulled the caps just now and scraped at what looked like cracks down the side, they turned out to just be sloppy glue work where they had left strands of glue. There are still some burns and lifted traces, but they may be unrelated. Perhaps sloppy soldering? The fuse was blown, but I had replacements on hand, so it's currently good. Do you have the HOT for this in your store? I didn't happen to see it.
 
Hey security, it's funny you write, I have all the stuff from your eBay store waiting in my cart since yesterday. I haven't seen glue like that before, but most of my monitors are g07s. This is only the second WG I'll be capping. You were right though, of course, when I pulled the caps just now and scraped at what looked like cracks down the side, they turned out to just be sloppy glue work where they had left strands of glue. There are still some burns and lifted traces, but they may be unrelated. Perhaps sloppy soldering? The fuse was blown, but I had replacements on hand, so it's currently good. Do you have the HOT for this in your store? I didn't happen to see it.

yes i have the HOT in stock on my site. BTW: if you buy from my site its will save you alittle money over my ebay store plus i have about 20 times more parts on our site.

https://www.arcadepartsandrepair.com/
 
Found it, thanks, and just put the order in. After this I have Hantarex Polo in OCP to work on.
 
I agree that it's likely the glue you see but as for burnt or lifted traces, that's pretty common for the entire k7000 lineup.
It was a piss poor design to begin with and then they decided to make a medium resolution chassis out of it which only made the problem worse. Yours being medium rez likely looks like all the others so do a very thorough rebuild and add a fan to blow across your monitor. These things cooked themselves to death so air movement really helps.
 
the resistor network that carries power from the filter capacitor across to the voltage regulator is where you'll find the most issues on these. R101 and R89 in particular; the flowchart almost identifies the suspects for you.

the problem is these monitors have reached a point in their service life where the factory solder is breaking down and the heat produced from the resistors simply doing their job is breaking them down to the point that they literally just break. you'll probably find halos around pins on those resistors, the flyback, the yoke header, and the signal header (the latter 2 crack just from unplugging/plugging stress)

the heat on the resistors will make the traces lift or solder pads break off or burn up. so the legs you cut off when doing your cap kit will be beneficial for bridging all those breaks back together. once you've reflowed all the areas I outlined though your monitor should be solid.

I did one of these out of a NARC almost 2 years ago. despite the medium res K7000s reputation for being difficult to work on, it isn't. you use the same approach as you would a regular K7000, you'll just probably have to patch up more spots because it throws even more heat.

good luck
 
NARC's up and running and looking good: https://youtu.be/O2Xc79xG2I0. Thanks to everyone for the info. Once the cap kit and flyback were done I realized I forgot to check the VR which had a short between the first and second legs. I may throw a fan in once I figure out the best spot to cut in from. Now I've got to fix the Hantarex Polo in my Primal Rage, for another thread.
 
from my experiences with 2 (total) medium res K7000s, the power resistors in the middle of the chassis are going to be the most problematic. you can't simply replace caps on these, you have to ensure that those are like Fort Knox. that means if any solder pads lifted or burned off you ideally patch them all back together with the cap legs you cut off.

resistors by design just attract a ton of heat, it's from the power they're blocking, if you work on a lot of K7000s chances are you'll find a few where the solder joints are either soft or have broken off from the pin entirely. you can physically watch them move when you wiggle parts. sometimes the heat damage is so much it separates the pad from the trace.

I've seen the yoke headers separate, flyback pins will halo and crack, and the signal header often cracks as well. any part that you plug/unplug will be stressed from trying to unplug them. flow your solder through very well and it shouldn't be a problem again.

the factory solder I'm guessing is just enough to get the job done, so when we reflow them, we're making them better than factory quality. :)
 
Another set of eyes

Hey gang, I finally got around to bringing NARC in to the arcade after dealing with a control issue, and wanted to leave it on for a while to make sure it was good. Sure enough after about 4 hours being on the monitor died. I did a total rebuild on it back in April (HOT, Flyback, Cap Kit, reflows). I think the new flyback failed. I don't see any super obvious cracks or leakage, but there does appear to be a little right around the base by the shield. I tried to get in close for the pic which made it a bit blurry, not sure you can actually make out what I think I'm seeing. I wanted to get a second set of eyes on it before I go ahead and replace it.
 

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nope, pretty sure that is the glue that is used to glue the ferrite core in.
 
Thanks for that. I need to meter this, haven't had a chance.

EDIT: just metered the HOT and VR (both newer) and they checked out.
 
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I gave the solder side a complete check over and don't see anything obvious. It was still pretty messy even though I cleaned it when I rebuilt it, so I gave it another quick cleaning to make sure there were no bad joints. To be fair it's old and beat up in spots, but still looks okay.

I measured B+ and got 123 on one lead and 160 on the other. This is a 19" so the 123 seems fine, but the 160 seems high.

Also, R103 measures fine at 2.7 and I'm getting neck glow.

Not certain how meaningful it is but I don't hear the characteristic hum or static crackle when powering up either. That's probably a given since there's no picture. Also, I discharge it every time I pull the board out, but I haven't noticed any noise when doing so, though I've found sometimes I hear something sometimes I don't when discharging anyway.

Tried reflowing the flyback, most of the caps, the coils, no change.
 
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Just a follow up. It looks like it was the flyback. I couldn't tell until I pulled it. Replaced and all is good. Old one on the left, new one on the right.
 

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