Stern MPU100 missing 7th Flash

Reworked an unknown m100 with original battery still attached, surprisingly no corrosion. Set up the jumpers for 2716 at U2 and U6.

Got 6 flashes on the bench (Yay) and off to the game I go.....no 7th flash. OK Cool, f4 on the rectifier must be blown, nope. Still cool, must be the voltage, nope, 45 on the rectifier, 45 on the left side of R113 and 22.5 on the right. R113 & R16 are new. Also verified the 6800 in another game.

Ok, swap in new scoketed U14 and U12....still no 7th flash

Is the 22.5VDC just high enough to stop the 4049?

#2 8 years ago

AMS

barakandl

OP
Quoted from eh97ac:
Reworked an unknown m100 with original battery still attached, surprisingly no corrosion. Set up the jumpers for 2716 at U2 and U6.
Got 6 flashes on the bench (Yay) and off to the game I go.....no 7th flash. OK Cool, f4 on the rectifier must be blown, nope. Still cool, must be the voltage, nope, 45 on the rectifier, 45 on the left side of R113 and 22.5 on the right. R113 & R16 are new. Also verified the 6800 in another game.
Ok, swap in new scoketed U14 and U12....still no 7th flash
Is the 22.5VDC just high enough to stop the 4049?

Check that you have a 120hz pulse leaving u14 and going to the PIA zero cross input pin. U14 can and does fail when the voltage dividing resistors fail and overvolts u14 and/or when the 82ohm resistor right below u14 overheats it.

#3 8 years ago

eh97ac

Pinside member

119.9

#4 8 years ago

AMS

barakandl

Check the IRQ is connected to both PIA and the IRQ pin of the cpu. If good, replace the PIA with the zero cross input (u10 i think it is).

#5 8 years ago

eh97ac

Pinside member

I have continuity between the 6800 IRQ (pin 4) and both 6821's IRQ (pins 37 & 38).
I'm not sure what you mean by "replace the PIA with the zero cross input"

#6 8 years ago

AMS

barakandl

swap in known good PIA

#7 8 years ago

eh97ac

Pinside member

Tried 6 different PIA chips in various combo locations of U10 & U11, all chips are known to be good. I also went ahead and added a new socket to the 6800. 6 flashes only

#8 8 years ago

AMS

barakandl

Ugh... that sucks... i figure it is going to get really hairy from here out.

Maybe the PIA is not getting enabled right. So even though the 120hz zero cross input is proper. It doesn't read that input correctly because the PIA is not in a state to do so.

Check the address, data, and any custom CE signals the PIA may be getting the from the smaller 14-16 pin logic devices on the left. Quick glance at the schematic.... u18 and u19 failed could cause no 7th flash.

#9 8 years ago

eh97ac

Pinside member

I found an old RGP post mentioning the Stern MPU needs a 2K ohm at R113 & R16, I currently have 2.2K in place. Do you think 10% is enough to throw the system off?

I think I have some spare chips, so with nothing left to lose, I'll swap out U18 & U19.

#10 8 years ago

AMS

barakandl

http://www.raltron.com/cust/tools/voltage_divider.asp

No, wont matter... If you have two resistors of exact same value in the divide circuit, it will be 50% division. 2k or 2.2k will both 50% divide.

play with this calculator and you will see =D.

#11 8 years ago

eh97ac

Pinside member

Based on a recommendation from an interwebs pinhead I have the game up and running.

Jumped U14 pin 10 to U10 pin 17

#12 8 years ago

AMS

barakandl

OP
Quoted from eh97ac:
Based on a recommendation from an interwebs pinhead I have the game up and running.
Jumped U14 pin 10 to U10 pin 17

I think you mean U10 Pin 18? That would indicate open circuit.

#13 8 years ago

eh97ac

Pinside member