Solid State Pinball Repair Tips®
Bally MPU LED Flash Diagnostics

The following diagnostics are valid for Bally -17 and -35 MPU's, as well as Stern MPU-100/200 boards.
Upon power up, the on board LED should flicker briefly. Following this, the LED will flash, indicating completion of varying stages of the internal diagnostic program in the game ROM's. If the LED does not flash at all, check for proper checksums of the game ROM's, the ROM sockets, and the power supply voltages.

First Flash:
Signals the end of the ROM test.

Second Flash:
Test the NMOS RAM (U7). There will be a bit of a delay between the first and second flash, be patient. If no flash, replace U7.

Third Flash:
Tests the CMOS RAM (U8). If no flash, replace U8 - use caution the 5101 chip is NOT diode protected and is very static sensitive!

Fourth Flash:
Tests the first PIA chip (U10). If no flash, swap U11 for U10. Note that although you may get the fourth flash, U10 may still be defective.

Fifth Flash:
Tests the second PIA chip (U11). As above. If you have already swapped U10 and U11 and don't have the fifth flash, try a new 6821 in U11. If still no success, swap once more. Worst case-replace both PIA's.

Sixth Flash:
Tests the Display Interrupt Generator. This actually tests both U11 and U12. If U11 has been replaced, and no sixth flash, replace U12.

Seventh Flash:
Tests the "Zero Crossing Detector". This tests U10 and U14 as well as the presense of +43 volts from the rectifier board. If no seventh flash, check for +43 volts before changing anything else. Fuse F4 (+43 volt) is a common failure. Unless U10 has already been swapped for a new or known good 6821, change it before U14.

© 1997 by Allan Perrins
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