Midway Space Invaders has me stumped

Nama

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Hi all,
Been poking around on and off with this one for a while now.
I've got it to the point where it can recognise the test ROM.

The test ROM identified a few RAM chips that need to be replace...last RAM to get replaced was RAM #7, however it still says that RAM #7 needs replacing! I've moved the RAM around, but always #7 is said to be bad.

On the test ROM screen there are a repeating pattern of thin lines. if a RAM is bad, a line will be missing or double thickness. The pattern of lines looks perfect and complete on my system, suggesting that the RAM is actually ok. However next to the number '7' seems to be what looks kind of like a 'c' shape. I don't know what this is exactly, and I haven't seen anything similar in my internet searches. If I temporarily remove other RAM chips (the ones that I've socketed), the bad RAM number will change, but the 'c' shape remains.

Finally I have some observations from probing the circuit with a logic probe.
With he test ROM installed, address lines A11, A14 and A15 (measured from the 8080A CPU) do not seem to be pulsing and are low.
For the 74LS174 @ D7, the input data lines seem to all be pulsing, but the outputs on pins 2, 7, and 12 are all low. Only the outputs on pins 5, 10 and 15 are pulsing. However Pin 10 seems to be pulsing at regular intervals, like a clock. I've looked at this pin with my (very old) scope, and it does seem to be pulsing regularly.

Anyway, hoping someone may have an insight into my issue. I really thing I'm very close to getting this all working, and if I can solve this last issue I'll be up and running.

Much thanks in advance

Phil

***UPDATE*** Just wanted to add that I have also tried this with my Braze Multigame kit, and it also indicates a problem with RAM #7. Probing the address lines of the 8080 with the Braze installed now shows activity on A11, A14 and A15 (A15 only pulses for a short time after reset)
 

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Check to make sure you haven't got any shorted or broken traces along with the chip select/enable i had a same type of issue before turned out to be a broken track in the ram area flagging the same ram error all the time but the ram fitted tested OK

What happens if you remove the first ram in the bank does it display 1/A error or still 7
 
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Thanks for the reply.
I have carefully looked over the board, but will certainly do it again.

If I remove the RAM from the first bank, it displays '1' or 'A' respectively.

Cheers
 
I'm running off a switcher, but I'll check just incase something is being dragged down.

I've been going over the board to look for shorts or broken traces...can't find anything and after an hour and a half I'm going crosseyed :-/

Thanks again
 
I'm running off a switcher, but I'll check just incase something is being dragged down.

I've been going over the board to look for shorts or broken traces...can't find anything and after an hour and a half I'm going crosseyed :-/

Thanks again
Where are you getting the +18v?
 
From memory I've wired it up using the 12V line.
This setup has worked perfectly on 3 other Midway SI and SI Deluxe boards, so I have to assume it should work for this board.

Phil
 
Ok so removing it shows 1/A thats fine ok so if you put 7 into 1 or A does it still get to 7 and stop? I would be checking the chip enable and chip select along with traces also good idea ot use a ohms meter to check these
 
From memory I've wired it up using the 12V line.
This setup has worked perfectly on 3 other Midway SI and SI Deluxe boards, so I have to assume it should work for this board.

Phil

Yes 12 is fine as thats for the sound section even with it disconnected it wont stop the game from running
 
No the from Ram 7 back you will find it goes across the PCB i don't have the schematics in front of me at the moment just check the chip select and enable signals of Ram 7 and meter them back
 
DIP22
+--\/--+
Vbb |1 22| Vss
A9 |2 21| A8
A10 |3 20| A7
A11 |4 19| A6
*CS |5 18| Vdd
Din |6 17| CE
*Dout |7 16| NC
A0 |8 15| A5
A1 |9 14| A4
A2 |10 13| A3
Vcc |11 12| R/*W
 
Ok so removing it shows 1/A thats fine ok so if you put 7 into 1 or A does it still get to 7 and stop? I would be checking the chip enable and chip select along with traces also good idea ot use a ohms meter to check these

Unfortunately moving the chips around does not help, I still get error at location 7. I'll look into the Chip Enable lines as you suggest.

***Update*** So I was just probing pins 2 and 7 on the 3245 ripple counter that provides the CE signal for the RAM, and then the screen suddenly went ballistic, like a strange scrolling effect like the board is cycling through something. Think I may have killed the 3245. I do have a 74HC4060 that I believe will work as a replacement. If so I'll remove the 3245 and put it in.
 
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I think you got your chip numbers messed up ive never heard of a 74HC4060 dont you mean an 4060 cmos and the 3245 thats the quad clock chip you cant replace with anything else than the 3245

Why do you think you have destroyed that unless you shorted pins together but anyway thete a tri voltage chip

I think at this point your best of using a scope if not finding someone with one to help
 
They go into a 8216 ive had these fail before

Why are you probing around the quad click chip?

Doesn't the 3245 pins 2 and 7 connect to the Chip Enable on the RAM?

Yep, I guess I might have shorted something. Certainly i didn't touch any other IC, and then things went wonky. I hate it when I make things worse.

If I can't get a 3245 then I'll look into the 75365 hack...thank for the link.

Phil
 
Doesn't the 3245 pins 2 and 7 connect to the Chip Enable on the RAM?

Yep, I guess I might have shorted something. Certainly i didn't touch any other IC, and then things went wonky. I hate it when I make things worse.

If I can't get a 3245 then I'll look into the 75365 hack...thank for the link.

Phil

If that chip was stuffed it wouldnt work at all you wouldnt even get that ram 7 error or anything for that matter thats not the problem i would be looking at now the 8216s ect but i really think you need a scope now
 

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Well I do think I stuffed it. It was working (showing the error) and then...it wasn't.

I'll pull out the scope from storage...Just a word of warning though, It's horrible and old.

Phil
 
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