K7000 kicking my butt

ELutz

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I've got a late era WG 19K7000 (7901 to be exact) kicking my butt. It's dated 1994....I didn't know they were making 7000 at that time. It's got a hum bar from hell. It was in a Galaga. I moved the monitor from the game to my bench....problem remained (with a different motherboard and different power supply). The problem is definately the monitor. The B+ is right at 124VDC. I swapped the filter cap....no change. I swapped the regulator IC...no change. It looked as if it had just recently been capped....I re-capped it anyway, just to see.....no change. All solder joints look good. The hum bar is crazy bad.....but slowly gets better if the monitor is left on. At approximately 5 minutes, it's gone. The B+ never changes, rock stable. This screams of a cap, but it's not. I'm at a loss....anyone ever seen this issue? Thanks!

Edward
 
How's the flyback?

You know, it's got a wierd flyback I've never seen. It looks like the basic K7000 flyback, except there's something inline with the focus wire. Focus wire exits the flyback, goes into some stick rectifier looking thing (I don't really know, it's covered in heat shrink....looks factory....I didn't want to cut it open. Two wires exit this thing.....One goes to the focus assembly/neck socket....the other is a ground and is tied to the chassis frame/ground.

Edward
 
you said you swapped the filter cap.....I'm guessing you mean the big 560ufd cap....did you check or test C56 or C57, both are filter caps on the regulator...i'd suspect the 47ufd cap could be the problem.
 
you said you swapped the filter cap.....I'm guessing you mean the big 560ufd cap....did you check or test C56 or C57, both are filter caps on the regulator...i'd suspect the 47ufd cap could be the problem.

Yup, changed those.......actually changed all electrolytics. Also, unplugged degausing coil.....no change. Checked the anode voltage with a high voltage probe.....voltage was solid from a cold start (with hum bar) through warm up (and after hum bar dispated).

Edward
 
Post a pic... theres a cap or two, fromt eh rear looking foward it would be behind the fly to the right a little. Replace those again, one of those is notorious for causing jailbar issues.. ( i want to say c56 but not sure off the top of my head)
 
Post a pic... theres a cap or two, fromt eh rear looking foward it would be behind the fly to the right a little. Replace those again, one of those is notorious for causing jailbar issues.. ( i want to say c56 but not sure off the top of my head)


Hey cadillacman.......Hum bar, not jail bars. Think the worst Pac-Man hum bar you're ever seen.....and you're getting close. It's crazy....it'll slowly clear up once it warms up....maybe 3 minutes and it looks great. Acts just like an electrolytic cap issue, but it's not.....I've already replaced them all.......TWICE!

Edward
 
Just for giggles, unplug the degauser coil to see if the hum goes away.

Already did that, too....no change. I'm swapping the flyback today.....just to see what happens. All voltages look good, B+ is good, HV is good and stable......I'm stumped! Thanks!

Edward
 
have you tried replacing the BIG filter cap?

edit" nevermind man im a tard sometimes..

post a video of the issue if you get some time..
 
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What about replacing IC4 (VR)? Just because the B+ is good, doesn't necessarily mean anything. Another thing to think about would be IC2. If your hula is on the vertical, IC3 may have a problem with the pulse width adj, too.

Now, since the filtration thru C55 (and to a lesser extent C56 and C57) should be getting that ripple out - and it's not - then logic would seem to point towards something bypassing it, perhaps R301 being bad, or a problem with R84/R85/R86. Perhaps even some stupid broken trace there leading to or from the VR...
 
What about replacing IC4 (VR)? Just because the B+ is good, doesn't necessarily mean anything. Another thing to think about would be IC2. If your hula is on the vertical, IC3 may have a problem with the pulse width adj, too.

Now, since the filtration thru C55 (and to a lesser extent C56 and C57) should be getting that ripple out - and it's not - then logic would seem to point towards something bypassing it, perhaps R301 being bad, or a problem with R84/R85/R86. Perhaps even some stupid broken trace there leading to or from the VR...

Thanks Mod, you gave me another angle to think about. I did swap IC4, with no change.

Edward
 
post a pic of this "weired thing off of the flyback" you speak of...

I pulled this flyback...and it had hairline cracks in it. I replaced it with a "regular" K7000 flyback, no change with the hum bar. I'll snap a picture of it today and post. Since it had cracks....I cut the heat shrink open. I still don't know what the damn thing is. I found three more K7000 chassis with this in my "junk" chassis box. The flyback isn't the normal K7000 flyback.....it's a KFS-60717.

Edward
 
If you unplug the monitor does the humbar go away ?
:)

I moved the monitor from the game to my bench....problem remained (with a different motherboard and different power supply).

Is the machine and test bench on the same house circuit?
Curious if it is a ground loop issue...i.e. air conditioner running or something goofy like that.

Just trying to think outside the box, I've never had a humbar on a K7000 personally.
I'll be watching to see how this turns out for sure.
 
The K7000 manual shows 'scope waveforms at 16 different test points. You might try comparing yours to those & hopefully find the area in the circuit that's wonked...
 
If you unplug the monitor does the humbar go away ?
:)

Dammit, it sure does.....sometimes you overloook the obvious!;)

Different house circuits....

The annoying part is it goes away after a few minutes....totally like an electrolytic issue (but it's not). I guess I could let it warn up and start shooting stuff with the freeze spray.....see if I can make it reappear.

Edward
 
post a pic of this "weired thing off of the flyback" you speak of...

Here you go.....It looks like a big resistor, but I don't think it is. I've got three chassis like this, and none of them....I get any kind of reading through this thing with my meter. Of my three chassis, one works fine...though, I see hairline cracks in the flyback body. The text on this thing...three lines...

HMC
470M 20%
HV 100

The focus wire runs from the flyback to one side of this thing, then from the same side...the wire runs on to the neck socket. the wire on the other side runs to the chassis frame/ground.

Edward
 

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