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Technical help with Space Invaders PCB

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Paulieboy View Drop Down
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    Posted: 21 Sep 2019 at 2:57pm
OK - just a bit confused really got several SI PCB and slowly working my way through them, but puzzled by one which has cost me a lot of time.
NO PICTURE WHATSOEVER!
Checked C5 and replaced it with one from a working board, infact I have socketed every chip on this dam thing and still no picture. I don't just mean the ram I mean every chip, and yes about two weeks on and off it was the answer to "how long did that take?"
Most of the chips that have been re-seated on the board are new or have been removed from one of my working Board Sets.
Any Ideas oh Great Ones out there?
I will try any suggestion going even if it seems simple, as I may have overlooked the easiest of fixes, so don't be frightened to suggest something worst I can say is "yep tried that - still no picture"
I await any replies with extreme eagerness, but I do not have a scope hence replacing all the chips but I do have a probe and also a soldering iron.
Lets hope one of the Masters will come forth with the answer to my quest
9/13 boards have been successfully fixed with either braze kit or test rom or reading through the midway 8080 test procedure but this one has got me well and truly stumped after changing everything!
Thanks in advance!
The Space Invader Museum
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ace` Quote  Post ReplyReply Direct Link To This Post Posted: 21 Sep 2019 at 3:17pm
Have you checked the crystal is good and that the clock is making to the cpu? I had a SI motherboard once where the crystal was installed in a type of socket (which looked factory original) it caused an intermittent connection until I soldered it in properly.

I'd also check the cpu socket and the edge connector that the sound board plugs in to.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Paulieboy Quote  Post ReplyReply Direct Link To This Post Posted: 21 Sep 2019 at 3:49pm
Several different crystals all working ones, although not soldered one in to the original socket just pushed in to the factory socket.
I've tried wiggling the socket and moving the edge connector on the mother board but nothing brings anything to life on screen.
That was one of the next things to change on the board after all I've changed everything else including my socks 14 times since picking up this board LOL!
There is 12v at the caps so C5 should be ruled out especially after fitting a working one in new socket

How would I check the clock to cpu?
I do have a scope Paul but never used it, as you know I buy and sell quite a lot of stuff and picked this up cheap thinking it might come in useful one day.

Going to check the cap near motherboard socket as I haven't already done so.
Thanks for suggestions so far.

A hint with the Scope might be nice love playing with stuff I've bought that might be useful
Thanks
The Space Invader Museum
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ace` Quote  Post ReplyReply Direct Link To This Post Posted: 21 Sep 2019 at 5:23pm
Clock pin of the 8080 cpu is pin 19, some times called sync. The scope is a good tool and will quickly show you if there's an issue. Personally I don't fire up the scope much as I have to look up at the screen wait for it to update and then look at the signal. The logic probe is much faster and has a audio feature so I don't even have to look at it half the time. Smile

Also check the reset pin 21 while you're at it, to make sure it's not stuck.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Paulieboy Quote  Post ReplyReply Direct Link To This Post Posted: 21 Sep 2019 at 6:27pm
Both pins 19 and pin 21 are low should they be pulsing?
I've not got a working board set to hand now, suppose I should have but all need ram only got about a 20  removable working ram for about 12 board sets maybe its time to lift off some more ram and get a fully working one up and running before going any further
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ace` Quote  Post ReplyReply Direct Link To This Post Posted: 21 Sep 2019 at 6:46pm
19 should be pulsing, the board won't do anything without a clock signal.


Here's the schematics, you can either start in the top left corner or start at the cpu pin 19 and trace back.
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