Hi all,
I'm trying to restart an 1812 but I have the MPU LED stuck on. Reading the various posts and suggestions I tried to make various changes but without success. The only doubt I have is the ROM, in reality I don't know if it is corrupt, I tried to download the rom from VPforum but it doesn't work.
Unlike what Clay says, the LED on my board will turn on the same even without GPROM and without battery, I have no other ideas, I need your help.
What have I done:
- potentiometer 500ohm (A2)
- replaced R5-R6 with 680k
- replaced C20-C21 initially with 1uf50v ceramic capacitors and subsequently with two 10uf35v tantalum capacitors (still soldered).
- replaced U11 - U5 - U4
I have 3 roms (I don't know if they work, I'd like to know the correct cecksum):
Original Rom - Rom A - Rom B (vpforum)
I've tried all 3 but the results are still the same except for U7 pin6:
- Original rom = 5v LOW-pulse quickly __________ pressing SW1 = 5v LOW-no pulse
- Rom B ____ = 5v HI-pulse quickly ____________ pressing SW1 = 5v HI-no pulse
- Rom A ____ = 0v LO-pulse slow (6,5sec.) ______ pressing SW1 = 0v LO-no pulse
What I see:
- 5v perfect
- U6 (pinball on) pin1=5v ______pin2=3.20v ___ pin3=5v
- U6 (pinball off) pin1=3.20v ___ pin2=3.20v
- U6 (without battery / GPROM) pin2=4.75v
- U16 pin1=5v(no pulse)____pin2=0v(pulse slow 6,5sec.)____pin3=5v(pulse slow 6,5sec.)
- U14 pin4=5v(no pulse)____pin5=5v(pulse slow 6,5sec.)____pin6=5v(pulse slow 6,5sec.)
- U1 pin40=5v (pulse slow 6,5sec. with 10uf35v tantalum capacitor - instead it pulses fast with 1uf50v ceramic capacitor)
- U1 pin39=2.5v LO-pulse
- U1 pin38=5v HI-no pulse
- U1 pin37=2.5v LO-pulse
- U13 pin9/11 =no activity (HI-no pulse)
- U18 pin11 =no activity (HI-no pulse)
- keeping pressed the reset key SW1 no change:
U1 pin40 lowers from 5v to 0v
U13 pin4 lowers from 5v to 0v
- releasing the reset button SW1:
U1 pin40 immediately returns to 5v
U13 pin4 immediately returns to 5v
- U11 pin/volt
1/0 9/0
2/4.95 10/4.95
3/5 11/5
4/0 12/0
5/5 13/5
6/0 14/0
7/0.7to3.2(continuously) 15/0.7to3.2(continuously)
8/0 16/5
next checks?
Cesare.
(forgive me for translation errors)
mpu LED is stuck on (resized).jpeg