Need help on K7400 - Lightbulb test works; still clicks. No B+

Emby6201

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Need help on K7400 - Lightbulb test works; still clicks. No B+

Recently got a capcom big blue with a k7400 (sticker reads 2574 or something similar i think) in it.

When I first tried it, got a chirping/clicking noise. Flyback did not show any visual signs (cracking/leak).

I checked the HOT, which read shorted out of circuit. Replaced the HOT, flyback, and did a cap kit. Also checked ZD701 (per the bulletin on this model) - it does not check shorted.

Still getting a chirping/clicking noise.

So I try the "lightbulb test" - lights up steady and stays on.

I tried reading TP202/3 - gets something like ~2.3v or something like that.

I'm kinda clueless at this point - any pointers?
 
Looking around the chassis - I saw one of the caps - C116 - near U101 ( PWM IC) has this bulge on it; not sure if that is purely cosmetic.

I was thinking this is still around the power supply, so I thought if it was a problem, I would not get my lightbulb lit up, and the problem is elsewhere.

Maybe this is a problem?
 

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the damper transistor in the middle goes hand in hand with the HOT I'm told. perhaps that's at fault?
 
the damper transistor in the middle goes hand in hand with the HOT I'm told. perhaps that's at fault?

That's D707, right? Checks like a diode? If so - tests good.

If not - what is the location of the damper transistor?
 
If the HOT tests shorted in circuit and fine out of circuit you have a short on the B+ rail you need to find.

Andrew

Unfortunately, I'm still not sure where to start looking. I usually shot-gun stuff and am not so good on reading a schematic.
 
even with my newly acquired schematical wisdom, that K7400 schematic is absolutely terrible to understand lol

but I think you just follow the HOT backwards. since it's a dead short I'm guessing it's going to be a diode of some sort.. but that should blow the fuse I would think.
 
Sounds a lot like the problem I had. Here's what ended up fixing it. Give this a check and report back what you find out...

https://forums.arcade-museum.com/showthread.php?t=371057

I forgot about this. I wonder if that same part exists/fails on the U2000/U5000. I got that prototype War Gods with a U5000 that spontaneously died in storage spanning about 20 years.. so it has to be something stupid like that right?
 
Sounds a lot like the problem I had. Here's what ended up fixing it. Give this a check and report back what you find out...

https://forums.arcade-museum.com/showthread.php?t=371057

I'm getting different readings than you are, so I'm not sure if its the same.

I have two other k7400's and the cap reads similar out of circuit to the one that works.

But now I have TWO with this same problem of HOT shorted; replaced, still chirping.
 
Check these 3 parts. They are in the horizontal drive circuit. Any one of these can cause your problem.

R727 (3.9K ohm 1 Watt), R728 (1K ohm 2 Watt) or Q703 (2SC2482). Pay particular attention to R728 and Q703.

Ed
 
Check these 3 parts. They are in the horizontal drive circuit. Any one of these can cause your problem.

R727 (3.9K ohm 1 Watt), R728 (1K ohm 2 Watt) or Q703 (2SC2482). Pay particular attention to R728 and Q703.

Ed

Thanks for the info! Ill be checking these tonight.
 
Check these 3 parts. They are in the horizontal drive circuit. Any one of these can cause your problem.

R727 (3.9K ohm 1 Watt), R728 (1K ohm 2 Watt) or Q703 (2SC2482). Pay particular attention to R728 and Q703.

Ed

Had a chance to check - resistors check within range; q703 does not appear shorted.
 
Try replacing C121 it could be bad. I haven't had to replace one, however others have and that fixed this problem too.

ed

I saw that post about c121, I've checked the ones I have in my bad chassis and they both check around 77pf (printed on there is 100pf), so my thought was these were okay. I also checked the one in my good chassis, and its around the same.

I can try just swapping it out of my good chassis, but I figured with those readings they were alright.
 
I saw that post about c121, I've checked the ones I have in my bad chassis and they both check around 77pf (printed on there is 100pf), so my thought was these were okay. I also checked the one in my good chassis, and its around the same.

I can try just swapping it out of my good chassis, but I figured with those readings they were alright.

most of those type of caps have a 5% or 10% tolerance and your measuring almost 25% so it may not be your issue but i would replace caps that far out of spec.
 
most of those type of caps have a 5% or 10% tolerance and your measuring almost 25% so it may not be your issue but i would replace caps that far out of spec.

I Must be reading them wrong, or these ceramic disc caps have more tolerance; I've checked the ones in my working cab and they all read about the same.
 
I Must be reading them wrong, or these ceramic disc caps have more tolerance; I've checked the ones in my working cab and they all read about the same.

post a picture of the cap and the writing on it.
 
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