K7401 B+ was too high, now it's negative?

mecha

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got a 25K7401 out of a Ms. Pac/Galaga that was in shutdown, I had never capped it before, so I capped it. the B+ adjustment pot naturally was drenched in epoxy, so I tried busting that free first and succeeded.

when I threw the chassis back in the B+ was reading something like 144-150V with me trying my best to return the pot back to its original position. I had a lot of room to turn it down, lowest I had gotten it was about 121-122. now if I recall correctly, you want the B+ to be like 118V, so I tried turning it down even more and that's when it started showing a B+ of -6.5.

am I looking at a broken pot here? and why would the monitor still power on if it's doing that?? it made a whining sound that I could make out I think, I'm in a very loud game room.

also, it was a Happ kit, there were a couple caps it was calling for like C701 or C702 (can't remember exactly right now) that I couldn't find at all so I skipped it. it also wanted a C731, but the C731 was empty. in my dumbfuck lapse of judgment though, I may have actually installed that there, why I don't know. would that cause any of this? it was a pretty big cap.
 
Your first mistake was fucking with the B+ (HV shutdown) pot. High B+ on a 7400 is rarely due to a bad pot. I'd say you have an issue there..
 
thank you mod. I swapped another sealed/unmolested pot in and I'm getting the same thing, 144V. the -6.5 thing was a goof on my part, I was using the test point immediately next to the pot. I'm not mentally right these days, I'm running on fumes.

I texted cadillacman about it, he seems to think the voltage regulator might be bad. is there any way I can test that though? am I gonna have to do the light bulb trick to figure this out?
 
read the voltage at pin 10 on U701....you should have 12vdc....if not, check the resistors in this area and you'll probably find one is bad. Without the 12vdc, the horizontal section wont work and your B+ will be high. R 760 ...R 765 are known to go bad. Also test Q 706....another item that goes bad. hope this helps .....
 
Yep, if you're not ticking, check R760 first, then R765, the the transistor next to them. If they look okay, start inspecting the chassis for cold solder, broken trace, cracks, or whatever. I fixed one (well, almost - still ironing out some other issue) the other day that had B+ about 145 that I tracked down to a wire broken on T701...
 
I guess I was using the wrong test point again... did I want to use TP202 instead of TP203? TP202 looked like I had control at the B+ adj. pot and was within NORMAL operating range. when I was adjusting the focus, maybe by entire coincidence, the monitor did go into shutdown again, and stayed there until I lowered the G2 all the way, then it kicked back on again and I did more adjustments.

I let the machine run for about 30 minutes more and it didn't shut down again, so I'm guessing I have to tinker with the B+ some more.

I also went back to the original pot.

I'll check all these other areas tomorrow though. there was no cold solder anywhere on this chassis, it was in pretty immaculate condition visually, it's from 2004, so it's not that old compared to some of the other stuff I've worked with back in like 1997-1998 era.
 
Yes, TP202 is the spot to use for B+. TP203 runs a lot higher. If you have B+ around 117-118 and it's running good, it's probably working...
 
well, ok then, I could've avoided a lot of shit then if I used the right test point lol. I swear when I did that Carnevil K7500 last year I was told "TP203 by the back of the chassis". but oh well.

good news is the high voltage shutoff works... so my little men are intact still I guess.

is the flyback going bad though if I get the focus very sharp and it goes into shutdown then? I had to leave it slightly blurry just to keep it alive.
 
Sometimes that's the fault of the contrast circuit...

if you could, phrase that in terms I can understand, I can't read schematics and stuff.

is that something to do with resistance then, like a resistor going bad somewhere?
 
Well, the first thing to check is to see if the B+ goes up when you adjust the focus. If so, check the flyback knobs for surrounding cracks. If you find some, replace it. Or turn down the B+ slightly and see if focusing still causes it to shut down.

If not, check the contrast pot on the remote board. maybe swap in another remote board if you have one. I've seen other models shut down when adjusting the flyback and it was caused by a broken contrast pot. Usually, something is getting overdriven, but you'd have to check a bunch of test points to see what...
 
tube's immaculate, btw. I have all the bias pots turned down on the neckboard and have a pretty perfect white. I wish the convergence wasn't so loopy, but I'll take what I can get with it. it's Ms. Pac/Galaga, it'll get play.

I'll try it some other time. right now I got other things to deal with, and it worked fine all day today, so I think it's gonna be ok.
 
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