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Author Topic: WG K4900 bad C702? - Updated  (Read 1045 times)

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howitzer

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WG K4900 bad C702? - Updated
« on: July 09, 2005, 12:50:33 am »
Hi guys,  I've been troubleshooting a k4900 monitor from a pacman machine I bought a couple weeks back.  The chassis has just been re-capped, and the game worked great for a day or so.

After a while, the top of the screen became brighter than the rest, along with some degradation of the picture (see pic).  Later, the picture dsappeared completely, leaving only a white band at the top of the screen (see earlier post at: http://forum.arcadecontrols.com/index.php/topic,38787.msg348832.html#msg348832)

I've pulled the chassis a couple of times, replaced one cap that was missed when the cap kit was initially done, and re-soldered some joints that looked bad, but no luck.  All I get now is the white band at the top of the screen.

The troubleshooting flowchart mentions cap 702 as a possible cause of the initial bright screen problem I was having.  I havent yet re-done the cap kit, but may try that next.  It seems strange that a cap would go bad so quick though.

If anyone has seen this type of problem, I'd appreciate any advice as to whether I'm on the right (or wrong) track. 

Thanks
« Last Edit: July 22, 2005, 04:47:19 pm by howitzer »

Ken Layton

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Re: WG K4900 bad C702?
« Reply #1 on: July 09, 2005, 01:25:52 am »
Capacitors, just like any electronic component can go bad at any time for any reason. Is c702 connected with the correct polarity? Is it soldered well?

howitzer

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Re: WG K4900 bad C702?
« Reply #2 on: July 10, 2005, 01:14:21 am »
Crap..  Just re-did the capkit.  Same dang thing.  Running out of ideas here.

Ken Layton

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Re: WG K4900 bad C702?
« Reply #3 on: July 10, 2005, 11:55:44 am »
It's got to be in the vicinity of that capacitor. Probably a crack in the foil. Maybe an old cap c702. Are all the caps polarity in correctly? Could you have one in backwards?

howitzer

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Re: WG K4900 bad C702?
« Reply #4 on: July 12, 2005, 09:57:42 pm »
I'm pretty sure the polarity is correct on them all.  I double checked each of them after soldering & before clipping the leads.

I think it must be something other than the caps since this was the second capkit with no improvement.  I'll check the foil for cracks around 702 - that makes sense.

Could the problem be with the tube?  Is there any way I could excude the tube as the source of the trouble?

Thanks for the help.

howitzer

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Re: WG K4900 bad C702? - Updated
« Reply #5 on: July 22, 2005, 04:51:46 pm »
Just a quick update for the archive in case anyone else runs into a similar problem..  I've cleared up most of the issue by replacing the horizontal output transistor.  Everything is back to normal with one exception.  No Red on the screen.  I had this problem intermittently before the horizontal deflection went out, so it may be unrelated (possibly on the game board?).  If anyone has any thoughts, please let me know.

Thanks,
John

Ken Layton

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Re: WG K4900 bad C702? - Updated
« Reply #6 on: July 22, 2005, 06:05:25 pm »
No red: resolder the video input connector pins. Resolder the transistors and the large resistors on the neckboard.

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Re: WG K4900 bad C702? - Updated
« Reply #7 on: August 02, 2005, 09:48:17 pm »
Dead on my man!  Cold solder joint on the red input pin.

Hell, a month or so ago I had no idea what a cold solder joint was.  Thanks for all your help.  Working like a charm now.

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Re: WG K4900 bad C702? - Updated
« Reply #8 on: August 03, 2005, 01:19:23 am »
Many monitors develop bad solder on the video input connectors.

Glad your monitor is now working. :)

  
 

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