BOOM! says the K4900...

modessitt

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So, I started with a working K4900. Installed a cap kit. Powered it up and about 5 seconds later - BOOM - C702 literally explodes! Nothing but a bunch of shredded stuff and a little black piece still attached to the two legs. Although it's not something I do often, I wonder if perhaps I put it in backwards during the rebuild. I replace it - making sure it is in properly - and power it up and nothing explodes this time, making me think I must have put it in backwards.

Except now I have a rolling pic that won't lock in. I find that the 175v I should have on the positive side of C702 is only 111v. I find the R702 (2.2-ohm 1/2-watt) is bad. I replace it with a 1-watt version I have handy, power it up - and 5 seconds later - BOOM! C702 is again blown to smithereens.

When I was checking voltages before I replaced R702, I found I had proper +130 coming out of the flyback.

So, anyone have this happen before? According to my manual and what was in there, C702 should be a 10uF 100v cap, right?
 
gimme a sec i should have a unmlosted 4900 chassis on my parts shelf


yup c702 10 uf 100v +105 c on the 4901 i have thats never been caped
 
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I've had C702 die repeatedly (not explode....but the vents on it's top would bulge). My fix was a bad D702.

Edward
 
Well, just for fun, I swapped out the flyback, put in another 10uf 100v cab, turned it on, waited 5 seconds - BOOM!

This time I put in a 10uF 350v cap. I wanted to see if it still blows up or if something else would go. Turned it on - and it's not blowing up - but my picture is still all out of sync. I now have my +175. Also have my +15 and +130.

Checked the voltages at Q301 (sync separator) and found that they are off:

Should be:

B - 4.7
C - 15.1
E - 4.2

Actual:

B - 1.9
C - 15.1
D - 1.7

Before I replaced R702, they measured just fine.

Checked the voltages at IC301, and they all seem to be pretty close except for pin 8. It should be 1.9 and it's measuring 8.9. Since this is the output of the Sync Separator, I'm guessing this is my problem.

So, I check my inputs, and pin 10 is a bit low. Just for grins, I decide to take off the iCade board and hook up my Wheel of Fortune board - and the pic is perfect. Hmm. Could it be a problem with the iCade, even though it worked with it before the rebuild? I hook up my Blitz 99 board, and it's messed up again. Still works with the WoF.

I hate monitors that won't keep the same problem between game PCB's....
 
I've had C702 die repeatedly (not explode....but the vents on it's top would bulge). My fix was a bad D702.

Edward

I'm talking the outer casing flies straight up like a rocket, the fiber inside shoots everywhere, lots of magic smoke, and two legs with the black bottom still attached....
 
BTW - I was thinking about replacing D702 even though it seems to measure just fine, but then why will the board work with the WoF PCB? Different frequency?
 
BTW - but then why will the board work with the WoF PCB? Different frequency?


You got me on that one....it is kinda crazy! For the record, my C702 was a slow death. Monitor would start out looking great, after a few hours it would be a little too bright.....I would re-adjust, and after a few hours it would be too bright. If I kept doing this, the cycle would continue until I had full white raster....like if the screen pot on the flyback was turned all the way up, when in actuality.....I had it turned all the way down!....along with all the other brightness/black level pots. When I pulled the chassis, C702's vents were cracked open with it's magic ooze seeping out. I did the same as you....kept upping the voltage of the cap:) After several days (and four caps later)....I was about to give up. Looking at the schematics, I kept coming back to D702. It tested good with my meter (I'd checked it early on in my troubleshooting). I decided to replace it "for the hell of it".....problem solved!

Edward
 
Well, let's make it even weirder. Last night I hooked up my Blitz 99 PCB to see if I could get a cross hatch for convergence adjustment, and it was all scrambled. So I came in to the computer to update on what i was dealing with. After an hour or so I went in to shut a bunch of lights off so I could go to bed, and realized I'd left the monitor and PCB on - and the picture was perfect. I turned the monitor off and on a few times and it stays good. I plugged the iCade board back into it and it's a good pic.

So - either my cat fixed it for me while I was out of the room, or something weird is going on. It'll be interesting to see if it still works tonight when i get home....
 
wow

I would just like to add that i wish I could hangout with you guys. I have a computer science degree and feel like Im reading greek. How in the heck do you guys learn this stuff???????
 
How in the heck do you guys learn this stuff???????

Well, part of the learning comes from working with these machines, reading the manuals and schematics, and finding and fixing faults.

And part of the learning also comes from having components explode in a puff of smoke.

-Ian
 
So, I started with a working K4900. Installed a cap kit. Powered it up and about 5 seconds later - BOOM - C702 literally explodes! Nothing but a bunch of shredded stuff and a little black piece still attached to the two legs.

Not even going to lie. My heart skipped 2 beats when I read this.. ;)


Well, part of the learning comes from working with these machines, reading the manuals and schematics, and finding and fixing faults.

And part of the learning also comes from a having components explode in a puff of smoke.

+1. I've been in computers for almost 30 years. {trying hard not to sound like a jerk} People ask all the time, "How do you know how to do all this stuff?" and I tell 'em that it's a combination of time behind the wheel and the fact that I'm not afraid to break things.
 
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Well, let's make it even weirder. Last night I hooked up my Blitz 99 PCB to see if I could get a cross hatch for convergence adjustment, and it was all scrambled. So I came in to the computer to update on what i was dealing with. After an hour or so I went in to shut a bunch of lights off so I could go to bed, and realized I'd left the monitor and PCB on - and the picture was perfect. I turned the monitor off and on a few times and it stays good. I plugged the iCade board back into it and it's a good pic.

So - either my cat fixed it for me while I was out of the room, or something weird is going on. It'll be interesting to see if it still works tonight when i get home....

Sounds like it could be in the light switch..lol

Can you send that cat to GA to fix my monitors? I will pay the shipping...
 
Looking at the schematic, I would replace the diode. Nothing makes a capacitor into a oneshot noise emitting device faster than dropping AC through it. Had that recently with a power supply. The bridge tested good so I just replaced the caps (including one that had popped its case off). 5 seconds into the power on test the same cap ruptured very loudly and the +5V went off line. The bridge still tests perfect, even out of the circuit, until you start putting AC voltage to it. One of the internal diodes acts like a short and it pumps pure AC down the - wire. Electrolytics just don't like AC.

ken
 
Hi guys, I am also having the same problem with my k4915. The C702 pops its lid. Was wondering for the D702. Were is a good place to get that diode. Looked at the part page on the manual for the k4915 and it has rh-1v. But I don't see that on bob roberts. Let me know if you know the other part name to look for.

Thanks
 
Well, to close out this thread, it turns out I'm an idiot, but my mistake will help teach a lesson to everyone else.

Tonight I recapped a different K4900 that also worked prior, but was darker on one side than the other. When I powered it up, I blew C702 again. This time the shell landed nearby (last one I found across the room). I figured I must have a bad batch of caps or something. Went to pull one out of the drawer and throw it on my meter to see what value I was getting - when something caught my eye. It was not a 10uf 100v cap as I expected, but a 100uf 10v cap! :bigeyes::oops:

Did I have some of the wrong value mixed with my caps? Nope. All 100+ caps in that drawer were the wrong value. Apparently when I did my last bulk order, someone got dyslexic and put the 100uF 10v caps in a bag labeled 10uf 100v. I just dumped them in the drawer, and even though I always do a quick glance to make sure it's the right cap, the similarity between the u and the v caused me to misread it. This explains why the 10uf 350v cap worked just fine. Well, now I have no 10uf 100v caps, so I threw a couple 10uf 160v caps on and it's working just fine.

Moral: Check and Double-Check that the caps are the proper value when putting them in.

Anyone want some 100uf 10v caps?
 
Take heart, at least it makes some of us feel better to know that even a pro can make a simple mistake. If it's any consolation, think of the poor bastard who crashed a multi-million dollar Mars lander because he forgot to convert kilometers to miles.
 
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