I have a Data East mpu 520-5003-03 board on my workbench.
The board isn't booting like it should be.
No acid damage what so ever.
Both 5V and PIA leds come on.
I've used a old ATX computer power supply, so the voltage isn't the problem.
Even with the Data East test eprom the board isn't booting.

Here is what I've done so far.
I've replaced ALL the pia chips with known working 6821 chips.
Tested the reset generator (IC1) on another mpu board and verified it's working correctly.
Unsoldered all the small chips that have anything to do with chip select ect....
Tested them all with the chip tester. All oke.
Replaced the NE555 timer chip.
Unsoldered both 74LS244 chip and tested them, both oke.
Replaced the 74LS245 chip, which was bad.
Exchanged the cpu with a 6802 chip.
Replaced the sockets for the roms and ram.
Verified the game roms, both oke.
Checked the J5 jumper, set correctly (for a 27C512).

What can pull the reset line low (1.4V)?
I'm at a loss.....
Please help me.

Peter

No Data East specialists here?

Sometimes the reset line goes high for a second or two, but drops to 1.5V

I did check the 47uF cap with an ERS meter and it is correct.
I'll replace it anyway.
I'll also replace R4.
Thanks for pointing me to another direction.
I'll let you know what happens.

Today I replaced the capacitor and R4 resistor.
No progress....

At first the reset signal is high.
The onboard relay starts clicking 3 or 4 times.
But the reset goes gradualy down over a period of 4 or 5 seconds to 1.5V

After that the signal does not go high again after rebooting the board.
It's just like when het board is cold, it works, but something gets heated and draws the voltage down.

The cpu socket is the last socket I did not replace (jet).
I will do that today.

I've replaced the cpu socket, but it did not help.
Being at the end of the rope I decided to remove all the PIA's, CPU, 555 timer and go bare bones.
The signal still stays low at 2.5V
I made some cuts in the reset line to narrow down the suspects.

One below the cpu...

Data_East_MPU_03a (resized).jpg

One at the back (the right one) and later a second one on the back (the left one).
The second one (on the left) I made to cut out any possible problem under the chrystal.

Data_East_MPU_02a (resized).jpg

Still the problem persisted....
It had to be in de reset generator/timer section.
After a very close look I saw something between resistor R64 and R65 legs.
I removed the capacitor and the reset generator to make some room to clean the area.
When cleaning C88 also broke in two and needed to be replaced.
After putting things back together the reset line stayed at a clean 4.6V
Mission accomplished!!!

Data_East_MPU_04a (resized).jpg

I've put all the original part back on the board.
All is working fine and steady.

After all it was just some leftover (conductive) residue from battery leakage.....

It is a ceramic capacitor which is use with all ic's.
It's exact function I don't know.
The main problem was the residue which caused it to break.