Can you post a photo showing the area around U8?
Let’s see if we can get the MPU to boot before doing anything else. The MPU only needs the power connector so remove all connectors from the MPU board except for J4. Power up the machine and watch the LED on the MPU board closely. Refer to post #23 by mrm_4 detailing what the flashes mean. The first flicker is very quick and faint and can be missed. This first flicker is not counted as a flash.
When you see a flash that means the test passed.
Count the flashes and report back. If the same as before, only 2, then there is a problem with U8, cmos ram.
Check voltages on the MPU board as follows:
Set your dm to dc volts and connect thEblack ground lead to TP4
Red meter lead to TP5, should be close to +5 vdc
Red meter lead to TP2, should be close to +12 vdc
Red meter lead to TP3, should be close to +19 vdc but will bounce around since this is unregulated voltage. This voltage comes through a resistor from pin 15 of J4 which should be +43 vdc. This is only neede to get the final 7th flash.
So the voltage to the board looks good. So likely U8 is bad. You checked the pins and none are bent so that is good. With U8 back in check the voltage on pin 22 of U8, should be +5 vdc.
Just need to replace individual chips as they are found bad. If the 5101 is bad I can send you one, I have a number of good ones from installing NVRAM on boards.
The board booted past the 2nd flash before you started reseating chips, correct? The U8 is cmos and can be damaged by static although I have not seen this myself but I am also very careful handling chips. Also, chips have to be oriented correctly, if you put it in wrong after reseating it would have be damaged.
So next step, short of getting out the logic probe or oscilloscope, would be to replace U8.
Lack of 7th flash in your original problem is classic lack of 43 volts which typically is caused by a fuse blowing caused by selenoid problems.
If you want me to mail you a couple of 5101s PM me you address.
Great to see you're enjoying Pinside! Did you know Pinside is able to run thanks to donations from our visitors? Please donate to Pinside, support the site and get anext to your username to show for it! Donate to Pinside