25k7000 blowing voltage regulator, HOT, and R103 but not fuse

somn

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Howdy folks,

Got a 25" k7000 board that's really been giving me some trouble lately. It came to my bench with a blown HOT and voltage regulator, as well as some out-of-spec resistors (R89, R101, R96).

I replaced the HOT, the 30130 regulator, and those resistors and it powered on for about a second before going completely dark. I metered after and found the HOT and 30130 shorted, and R103 now open (it had been reading 3ohms before testing).

I've checked just about all of the regular suspects and everything checks out, so I'm beginning to wonder if the flyback has gone bad, even though it's only about a year old and shows no cracks or arcing burns.

Any ideas? I'm relatively new to monitor repairs so maybe I'm missing something obvious?

Edit: the 2A fuse is not blowing.
 
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If you're blowing fuses, you need to go through the power section completely. Mike has a ton of videos on this subject, but here is a really good one:


This is basically the procedure I go though on every K7000 that I work on. Don't skip any steps.
 
I love Mike's channel! Really great stuff. But no blown fuse, strangely. R104 is in-spec and all the rectifier diodes test good.
 
The other information on the power section is still pertinent, especially c36-38 c69 and HOT
 
The other information on the power section is still pertinent, especially c36-38 c69 and HOT
kind of weird the fuse isn't blowing. I had a K7000 years ago where C38 was shorted, and you know since horizontal deflection and high voltage go together, it was obliterating the fuse.

I love Mike's channel! Really great stuff. But no blown fuse, strangely. R104 is in-spec and all the rectifier diodes test good.
what is the amperage rating on the fuse? I only use 1A SB on those. with multimeter you can test C36/69 (if 4 leg cap you measure the legs kitty corner, not left/right) and C38 with diode check. they should just read open, maybe you'll get a high number, but if reads shorted or the meter beeps they're bad.
 
The other information on the power section is still pertinent, especially c36-38 c69 and HOT
Both c36 (this board has two caps instead of the four legged one) tested good as soon as I put in a fresh HOT. C38 is good too and I believe C37 read good but I'll have to double check when I get in to the arcade today. I'm guessing I've checked C69 since I was following some of Mike's 25" K7000 videos but it doesn't ring a bell so I'll have to check again.

what is the amperage rating on the fuse? I only use 1A SB on those. with multimeter you can test C36/69 (if 4 leg cap you measure the legs kitty corner, not left/right) and C38 with diode check. they should just read open, maybe you'll get a high number, but if reads shorted or the meter beeps they're bad.
The board I've got has 2A SB printed on it, so that's what I've been using. I can certainly swap in a lower rated fuse if it might save me some more broken components.

I'll go ahead and update after taking another look at those caps (C37, C69) to make sure they're still reading good. Unfortunately won't be able to do any testing until I can get a new R103 in. If relevant this board also has the vertical circuit mod at P7 but I kinda doubt that's gonna be an issue given all the components read good.
 
oh yes, I always forget about C37 because not all K7000s have them. I think it's just a connection to ground, the ones that don't have it just have jumpers instead. so maybe it being shorted theoretically shouldn't be a problem lol I've never analyzed that part of the circuit
 
If you suspect the flyback the just remove it for a test, or use Mike's lightbulb test procedure to isolate the problem to high voltage or regulated voltage.
 
oh yes, I always forget about C37 because not all K7000s have them. I think it's just a connection to ground, the ones that don't have it just have jumpers instead. so maybe it being shorted theoretically shouldn't be a problem lol I've never analyzed that part of the circuit
I believe I did have a shorted C37; *checks notes* I did have 2 different chassis with a shorted C37 and neither had a blown fuse so I would definitely check that.

To further elaborate, I keep complete notes on every chassis I rebuild so I can refer back to stuff like this. I also always check the Film caps C36-38 & 69 on the 25" as I have been bitten in the past by shorted C37 or 38 blowing up HOTs. It only takes a second to check them.

Another thought; you're using themal grease for the HOT and not CPU compound? The layer is conductive!
 
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I believe I did have a shorted C37; *checks notes* I did have 2 different chassis with a shorted C37 and neither had a blown fuse so I would definitely check that.

To further elaborate, I keep complete notes on every chassis I rebuild so I can refer back to stuff like this. I also always check the Film caps C36-38 & 69 on the 25" as I have been bitten in the past by shorted C37 or 38 blowing up HOTs. It only takes a second to check them.

Another thought; you're using themal grease for the HOT and not CPU compound? The layer is conductive!
yeah that's the one thing we kind of consistently overlook because... it's obvious and there's an expectation it shouldn't happen lol
 
Checked C36-38/C69 and all is reading good.
Another thought; you're using themal grease for the HOT and not CPU compound? The layer is conductive!
I checked for continuity between the chassis and HOT tab since I've been bitten by a cracked insulator before. Looks good.

I'm thinking the lightbulb test is my next step but I'll be waiting on that replacement R103.
 
well, if R103 was bad the voltage regulator wouldn't get 160~V
To be a bit clearer, I ran my last test with a brand new regulator, HOT, and a borrowed R103 from another chassis. All three were broken after being on for a few seconds.
 
you have a short somewhere in the AC rectification stage. AC goes in, passes through the bridge rectifier, then the filter cap, then R103, then it goes to the regulator. or, there's a short in horizontal deflection and it's backwashing.

you may be better off sending it to someone else to fix. when you tested the bridge rectifier diodes you lifted ends out, right?
 
Yes, the rectifier diodes and R104 all check out, in and out of circuit. Filter cap isn't shorted and reads ~430uF out of circuit despite the label being 560uF.

you may be better off sending it to someone else to fix. when you tested the bridge rectifier diodes you lifted ends out, right?
Not an option for us; money isn't there.


or, there's a short in horizontal deflection and it's backwashing.
If this is the case, will the lightbulb test still be good or am I risking some more components?
 
if you remove the HOT then the chassis is only using the power supply section, so basically the parts mecha described. Can something blow up? Sure but there aren't many parts involved
 
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