Midway Space Invaders

ArcadeTechGW

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Hi all,

If you recall, I'm working on a Midway Space Invaders. At first I didn't get video, but on inspection, the ground on the tube was not installed properly. I removed it, cleaned and burnished the connectors, bent the bracket so it would contact the tube, and hooked it back into position. It is now in good contact with the tube, where it should be.

I pulled the A frame and cleaned all the connectors. Then I took a hard look at the board.

There are a few issues.

The diode next to (to the right of) the 7474 is broken - one leg is sheared off. I can't seem to find a parts list for the motherboard - does anyone know what the right diode is for this application?

Also, one of the chips looks like an ePROM - half the cover, including the middle is missing. I didn't know which one, and didn't find any "chip pieces" in the bottom of the cabinet, so it's likely been like this for some time.

So, two questions:

1. Does anyone have a parts list for the mother board?
2. Does anyone have experience with running a chip with "the top down?"

Please advise.
 
The diode next to (to the right of) the 7474 is broken - one leg is sheared off. I can't seem to find a parts list for the motherboard - does anyone know what the right diode is for this application?

I just looked at two SI main boards. I didn't find any diode near any of the 7474 chips. In fact, I didn't find a diode anywhere on the motherboard at all. Perhaps it's a capacitor?
 
Space Invaders

Thanks. It is definately a diode - it's glass. I've never had a glass cap.

On the schematic for the Midway Space Invaders game, it shows a diode, but doesn't call it out. This is at board coordinates 3 - E on the board and diagram.

I have found a schematic as well as the Midway diagram, which says you can find parts locally. I'll probably have to de-solder it, and try to read the writing. I was hoping to replace it first.

I'll have to bring out my drawing tonight, and figure out which chip is the "convertible". I know ePROMs run with their covers off, so I'm not adverse to seeing if this works, but generally, when a chip top is missing, it is because it faulted out (and blew the top). I don't see any cracks or heat damage, so it could be that the darn chip has been installed since 1979, or got bumped at some point. It was one of the bigger chips on the board, not an 18 pin version, but one of the larger ones.

Thoughts? Ideas?
 
Zener Diode and Chip number

I pulled the motherboard, and clipped out the diode. It's a Zener.

From what I can read without a magnifier, it has 104Z stamped on the barrel, as well as A03.

The "convertible chip" is a 2107B.

Does anyone have a line on where to find either of these?
 
I've never had a glass cap.

You have now.

On the schematic for the Midway Space Invaders game, it shows a diode, but doesn't call it out.

Can you post a clip of the schematic showing a diode? I looked at them, and see no diode in the schems at all.

I pulled the motherboard, and clipped out the diode. It's a Zener.

From what I can read without a magnifier, it has 104Z stamped on the barrel, as well as A03.

Why do you think that's a zener? It's not a zener part number I've ever heard of. Sounds more like a 0.1uF cap to me...

The "convertible chip" is a 2107B.

Does anyone have a line on where to find either of these?

I have no idea what you mean by "convertible," but 2107B is the RAM used on the Midway 8080 boards. arcadechips.com and Twistywristarcade both sell it, or appropriate substitutes.
 
When I got my Midway space invaders, it didnt play right and I was about to replace a few chips. But I tried the Braze tech space invaders multigame and it fixed all of my issues. Saved a ton of work and headaches.
 
A photo of the convertible (top down) chip

This might help - This is a picture of the motherboard (with an iPhone, sorry for the crappy quality) and the chip with the missing top.

As to the Midway schematic (for the diode / cap), it looks like an oval with a dash in it. I haven't run into this in the past - it sure is different.

Anybody have experience with this? It looks like a Zener to me. I've been doing electronics for 30 years - not that I've seen everything (I did see a Crowbar circuit though), but a glass cap would be a first.
 
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This is a capacitor found on Carnival boards and many others from the 80's. Does your part look like it?
 

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104z is code for a cap value. 104= .1uF, z= value tolerance (-20/+80%). It is a bypass cap which you will find *all over* any board with logic gates on it. They are there as local power rail filtering. Anytime a logic gate changes states, it puts a little high frequency trash on the power rail. These little caps take that noise and chuck it straight to gnd.
 
As to the Midway schematic (for the diode / cap), it looks like an oval with a dash in it. I haven't run into this in the past - it sure is different.

I assume you're refering to the attached diagram. If so, two things:

1) That's not a schematic. It's a PCB layout, or component, diagram.

2) As you can see, when it's rotated right-side-up, that's not an "oval with a dash in it," it says ".1", as in 0.1 micro-Farads, the capacatance of the component in that location. Those caps are not explicitly shown in any of the SI schematics (actual schematics) I've ever seen.
 

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104Z mystery - Solved.

Some more information - thanks to a colleague with a strong magnifying glass:

104Z
50V
U (with an underline)

Now the tricky part - anyone know of a source for these puppies?
 
The diagram

Indeed that is the one. I have to admit, these Midway boards leave a lot to be desired. No chip numbers, no component numbers, and the schematic - well, it is non-standard in my experience (which includes a lot of Williams, Bally and Gottlieb stuff, but not much Midway apparently.

Thank you for your help in figuring out this puzzle. Now all I need to do is find one to replace this with (so it can chuck the garbage in the trash).

Or can I just go with a Tantalum?
 
Any 0.1uF cap should do fine. Tantalum... ceramic... whatever.
Pretty good chance it'd work fine without the cap at all.
Oh, and make sure you get the polarity right if you go tantalum.

Regarding your broken RAM. Hard to tell from the low quality pic, but I'd say the top half of the ceramic housing is broken. Being ceramic, they're pretty brittle, and prone to fracture. The case is there mostly for protection. It's possible that it broke off w/o damaging anything internal that matters (i.e. the tiny wires that connect the legs to the die). But I'd replace it anyhow, so it doesn't look hoopty.
 
Caps and Ram

Re: the cap - thanks. I figured as much. I'd never seen a glass cap, and bounced it off another engineer I work with - he'd never seen one either. Go figure.

I have a whole drawer of caps - I'll see if I have one with the right rating. (You know, the more I keep spare stuff, the less it ever matches up with what I really need.)

Re: the RAM - I'll probably try to fire it up, and see if I can get into Self-Test first. The first time I powered this game up, I didn't get any video and smelled something "like chips" burning, so I powered it down, and moved on to the next target. Now I have that one up and running, so I'm back to Space Invaders. I fixed the ground plane on the tube for the monitor, so I figure that might have been what was causing my no video problems. I'll replace the cord (ground lead busted off) and power it up, check the power supply out and see if I get any light from the monitor. If I do, Game On! If I don't, I'll probably send the boards off to Elektron Forge for refurb.

I need a better desoldering tool. Neither the braid or the solder vac work worth a darn on these boards. I don't want to lift a pad.

Can anyone recommend a good desoldering system?
 
Ha! Finally, years of keeping parts pays off. I found 2 Monolithic Ceramic capacitors in my stockpile - 0.1 mfd, 50 VDC, with legs long enough to reach the attach points!

Huzzah!

All I need to do is find around 1000 more projects to finish up the rest of the resistors, capacitors, motors and connectors, and I'll be set. (or chuck the whole lot in the trash).
 
The WRX

Wow! I looked that one up, and the best price I could find was beyond my budget. I settled for the Hakko 808 for now - I've read the stories on upkeep, and it looks like it could work until I can find a WRX on eBay, or at the local pawn shops.

Slowly but surely, I'm building up my tool supply. All I have to do is find someone who can make back doors in the Omaha area, and I'll be in good shape. I need one for Space Invaders and one for Missile Command.
 
Re: the cap - thanks. I figured as much. I'd never seen a glass cap, and bounced it off another engineer I work with - he'd never seen one either. Go figure.

If you wanna get technical......it's a glass encapsulated ceramic capacitor. They are actually used a lot on video game boards......though, I don't know exactly why. You can always substitute a "regular" ceramic cap in it's place.

http://www.greatplainselectronics.com/Category-27.asp
They sell them.....about half-way down the page.

Edward
 
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