Wild thing I just figured out about Midway 8080 pcb's

LyonsArcade

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So I'm working on a Tornado baseball, and on the schematics it says the board needs 15v. This is in addition to 22v (led V for the optocouplers), 5v, -5v, and 12v.

So I'm searching through trying to find why it needs it, because my power supply doesn't put out 15v, and there's no tornado baseball schematics for the power supply which is slightly different than the Space Invaders one.

So pin 6 should have 15v, and it's only used on 1 pin of the entire motherboard, pin 9 of the Intel 3245 clock chip. It's a weird chip that wants 5,12, and 15! So strangely enough, if you search for a data sheet on the 3245, you only find more modern ones like the d3245... which lists as one of it's features, that it only needs two voltages, 5 and 12. They leave pin 9 not connected, probably because they have it connected to 12 internally, and also so you can drop it in something that has 15 on pin 9.

So there must have been this whole thing in 1978 where the damn 3245 needs 1 weird voltage to work... and Midway designed a whole other little part of their power supply to provide it. My power board doesn't supply it.

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If you look at the 8080 Motherboard, mine is revision D, there are two traces that run way away from the 3245, over towards the edge connector where they both line up right next to each other and pass through the board from one side to another.... one of them is the line that goes to pin 9 (that wants 15), and the other is the line that runs to pin 1 (12v). Apparently ,they designed it like that so if you didn't have the power supply that had 15, you could put a jumper in that via, and connect the 12 to where the 15 needed to go.

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If you have a motherboard that is pulling down your 12v when you plug it in, the problem could be that you are using a motherboard with the 15 and 12 tied together, and you are using a power supply that outputs 15 volts (and 12) and it's causing problems. When I get a little father along I'm going to jump the two vias and see if that 3245 clock will run with the 12 volts at pin 9...
 
Pictures of what i'm talking about, this is Revision D
IMG_4823.JPG

3245 on the board (notice the weird traces around it, maybe already modified by revision D?)

IMG_4824.JPG

Trace on right goes to pin 9, which needs 15, trace on the left goes to pin 1 which is 12. Notice there appears to have been a jumper or something in it at one point.

IMG_4825.JPG

Unpopulated part of the power supply, which creates the 15v.


This power supply was in the cocktail cabinet i'm working on, I bought the boardset on KLOV long ago and it was from an upright.
 
So there must have been this whole thing in 1978 where the damn 3245 needs 1 weird voltage to work... and Midway designed a whole other little part of their power supply to provide it. My power board doesn't supply it.
3245 was probably an all-NMOS design so it needed 15V to pull up to 12V for the 8080.
3245a was in a later process.

(Same reason the Astrocade custom chips need 8.2V to drive 5V signals)
 
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