Interesting Centipede PCB Problem

Alan173

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I have read before people referring to a 'snowing' or 'interference' or 'artifact' problem being displayed in the background of a monitor while a game is being played. Well...last night I had my repair technicians repair two Centipede PCB's (see attached picture of them hard at work). Well...they repaired both boards but the second board has an interesting issue. I have three boards and all of them play fine without this problem...so I have confined it to this particular PCB. I told my repair technicians "Nice try...but for what I am paying you (1 whole cookie each)...I expect these two boards to be 100%". They asked me to put up this post on KLOV to get them out of a jam. The video quality is very crummy....but the screen is razor sharp and the colors perfect. Please see this video and let me know what you think:

 

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Bill - you working with bondo tonight? Open a window!

My guess is the 27LS01 @ C8 (bi-polar RAM)....or something along that circuit. I'm hoping sombody will chime in to save my two little repair technicians a ton of extra work because they have not seen this problem before.

Of course....all the obvious...brightness control doesn't help...etc...it is 100% pcb related...
 
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I'm mentally insane from the boys!!! I mean, they are killing my braincells!

Juno First Image... ;)

junofirst-4.png


If your having issues with both boards you sure it isn't something with the power supply? We can test it in my father's Centipede if you want.
 
I'm mentally insane from the boys!!! I mean, they are killing my braincells!

Juno First Image... ;)

junofirst-4.png


If your having issues with both boards you sure it isn't something with the power supply? We can test it in my father's Centipede if you want.

I'm 100% positive it is coming from the video output circuit...on the PCB. Other boards in the same game run just fine. There are really two main chips in that circuit that to me would create this issue....my bet is C8. I'm hoping sombody will chime in to confirm this or give me a way to check a few of these chips for this issue..
 
Interesting. As I'm searching on the internet...it seems quite a few folks have seen this on their Centipede boards...and just like this video I found shows...the high pitched whine is coming from the PCB...same thing on mine. No solutions are listed...and to me the brightness adjust does not clear it up 100%...

 
Replaced the RAM 74S289 @ C8 and replaced 74LS175 @ A8. Both seem to be major players in the video output circuit....but I still have the white dots! :(

Any ideas welcome!!!
 
That's something you need to track down with a good, high quality oscilloscope. Just about anything could cause that.

I had a Galaga with strange random dot streaks on the screen that looked like bad video RAM but it ended up being a buffer in the clock circuit. It was a bitch to track down too.
 
Solved!!!

Well...a great big shout out to Brian (LUCASNOV77) for pointing me in a new direction. He told me to yank the EAROM and see if the dots went away....and sure enough they did! He said the -30vdc line running that EAROM had caused a similar problem on his board. So...that focused me on the -30vdc circuit and not the video output circuit. I pulled my notes on this board and realized when I got it I rebuilt the -30vdc line because the voltage was cut in half. New caps @ C86 & C84 (1uf)....new diodes at CR4 & CR5...new timer at A11 and the voltage was a solid -28vdc. Fixed...or so I thought as I never checked it with a scope. Well...now heading back to the -30vdc line I noticed I didn't replace a cap @ C85 (10uf)...at the time of my repair I was out of them. Well...I just got in a new order and flipped a new one in there.

Guess what?

NO MORE DOTS!!!

Whooooooohooooooooooooooo!

This is an interesting repair to take note of...and at least for my case...rebuilding that -30vdc generation did the trick.

Last repair of 2010!.....looking forward to 2011!

Thanks again Brian...:)

Alan
 
That's something you need to track down with a good, high quality oscilloscope. Just about anything could cause that.

I had a Galaga with strange random dot streaks on the screen that looked like bad video RAM but it ended up being a buffer in the clock circuit. It was a bitch to track down too.

Funny...I replaced the buffer as well on this board thinkging the same thing...and no love there for this particular repair problem...
 
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