Stern MPU100 Flash sequence repeating HELP!!! SOLVED

[Boots]

Hey guys
I am fixing up a Stern MPU100 for a mate and I can't work out what is going on with it.
When you turn the machine on it goes through all of the seven flashes and the starts all over again.
I found some corrosion on U11 socket pins so I cleaned up the board and replaced the socket.
I also replaced the U7 and U8 sockets while I was at it.
I set it up in one of my machines and it is still repeating the flash sequence endlessly.
I have tried swapping the chips from U7 U8 and U11 into one of my working boards and they are fine in the other board.
This MPU was working fine for the last 12 months and just stopped all of a sudden.

I have run out of ideas I need help please!!!

[The pinballist formally known as Dean Morgan]

Try replacing the transistors, capacitors and diodes etc in the reset circuit

[Strangeways]

Corrosion around U11 could have spread and not be visible. Check pin 40 on the CPU. Is should go from low to high after the 5V has stabilized. This is a normal boot. If Pin 40 stays low, then the LED will stay on. It sounds like it is booting and then rebooting.

U11 PIN 34 is also the RESET signal. Check your work around this pin.

Sounds to me like the RESET section is not holding 5V. Check Q1 and Q5 - these are easy fixes and cheap to replace.

Before looking at the board - Could you eliminate something - Connect up only J4 to the MPU and see if it still reboots without the other connectors.

[beaky]

is it actually going through the whole boot sequence or is it just continuously flashing the led and just looking like it's going all the way through?
I was sent one that continuously flashed the led and it turned out to be the solder throughs above U8 ram.

try what Neno has suggested and see what happens.

Does it still have the old style I.C, sockets?
if it does the pins on these sockets tend to disintegrate when there has been a battery leak.

[Boots]

Corrosion around U11 could have spread and not be visible. Check pin 40 on the CPU. Is should go from low to high after the 5V has stabilized. This is a normal boot. If Pin 40 stays low, then the LED will stay on. It sounds like it is booting and then rebooting.
U11 PIN 34 is also the RESET signal. Check your work around this pin.
Sounds to me like the RESET section is not holding 5V. Check Q1 and Q5 - these are easy fixes and cheap to replace.
Before looking at the board - Could you eliminate something - Connect up only J4 to the MPU and see if it still reboots without the other connectors.

Thanks, I will check the 5v around those pins.
I have put the board in one of my machines (a different game) to eliminate machine problems, so I only connected J4, still no good.
I have replaced U7, U8 and U11 sockets and the problem is still there, I have also checked the through traces and they seem ok, but if I can't find any other issues I will drill them and solder a link through the board.

[benpin]

Look like the reset section problem on MPU

Overhaul the reset section with new parts and clean up the corrosion as well.

Should good as gold and no problem

Hope this help?

[Boots]

Thanks everyone for their help, I finally worked this one out.
After checking everything in the reset section and where the very small amount of corrosion was I decided to have a look at the chips that I hadn't touched.
I reseated U2 and swapped the PIA chips.
U10 had a 6820 in it and U11 had a 6821 in it.
From memory I reckon I changed U11 for him when he first got the machine so I was confident it was ok (it was new when I installed it).
When I started the machine, it started perfectly so then I wasn't sure if it was the reseating or the swapping of the PIA's, so I swapped them back, sure enough the endless re-booting.
I also tried another PIA in U10 and the machine started fine.
So it looks like the MPU was a bit fussy about the 6820 in U10, funny though it had been running fine for at least the last year.

[Homepin]

6820 and 6821 are basically the same - I would be looking more at the sockets for these as you may have disturbed the dodgy connection???

[Strangeways]

U10 socket would be suspect. Reseating U10 and changing U11's socket has "disturbed" the area. I'd say changing U10's socket will give you a stable board. Reseating will fix the problem temporarily. at least you know roughly what would need to be done if the problem persists.

[Marty Machine]

Perhaps the PIA pin(s) itself were corroded not the socket, hence the problem going away when the chip was swapped...

[Boots]

Perhaps the PIA pin(s) itself were corroded not the socket, hence the problem going away when the chip was swapped...

You could be right Marty, when I swapped the chips the board booted, I then swapped them back to their original position and the fault re-occurred.
If the U10 socket was bad then it would have had trouble with the chip from U11, unless it is some weird combo of partly bad socket and partly bad chip???
The chip from U11 was newer so maybe the legs were cleaner than the older chip from U10.

Anyway I will definitely be writing this one down for future reference.