TECH: Diagnostic LEDs on Williams System 7 MPU boards
firepower
Bear with me as might be important to you when troubleshooting System
7 MPU boards.
I bought a faulty System 7 board and was trying to fix it, at one
point the 7-segment display stopped working, so I added the diagnostic
LEDs for the Leon test ROM, as you might do...
But it's not that simple in system 7 land.
On system 4-6 MPU boards, the LED cathodes are grounded and the IC2
(8T28 Tranceiver) raises a pin high to light the LED. OK so far.
Then Williams changed the way the LEDs are implemented on System 7,
due to using the 7447 display driver. On System 7 the LEDs are kind
of "reversed". The LED anodes are always at +5v through the
resistors, and the LEDs will light when an input line to the 7447 is
held low. We are in backwards world.
I read a thread from 2000, where Duncan Brown commented that he
noticed the System 7 LED lights were reversed. If Williams had put
the signals in the right place, the Top LED could still light up for
"1" and the Bottom LED for "2". But they didn't. This gets even
more confusing as a "3" is shown by NO LEDs lit.
In my tests, the logic turns OFF the LED for the number the display is
indicating, and all the other LEDs will stay lit. Not how I would
design a binary indication. It's a good thing the 7-Segment display
usually works!
If you are running some tests like the Diagnostics from the WIlliams
ROMs, or Leon's Test ROM and you want to make sense of the LEDs read
on. It's easier just to try and fix the 7-segment display - maybe
replace a 7400 at IC33 or look closely at the signals at PIA IC18, as
the 7447s don't go wrong that often. Mine was OK.
For me this was a step on the journey:
I added 4 x LEDs and 4 x 150 ohm resistors below on the empty pads to
check this. Top to bottom, the LEDs display A-B-C-D (the resulting
display number shown in BCD). But an "Unlit LED" means binary "1" for
that position. If the board is locked up, you get all the LEDs lit
and a "0" in the display, which I'm used to.
Numbers on the display are shown as "inverse binary" by the 4 LEDs.
In order from Top to Bottom A-B-C-D, with "0" for LED OFF and "1" for
LED ON:
1 = LED "0", LED "1"
2 = LED "1", LED "0"
3 = LED "0", LED "0"
4 = LED "1", LED "1", 3rd LED "0", 4th LED "1". (if 3rd and 4th
added)
5 = LED "0", LED "1", 3rd LED "0", 4th LED "1". (1+4)
6 = LED "1", LED "0", 3rd LED "0", 4th LED "1". (2+4)
7 = LED "0", LED "0", 3rd LED "0", 4th LED "1". (1+2+4)
8 = LED "1", LED "1", 3rd LED "1", 4th LED "0".
9 = LED "0", LED "1", 3rd LED "1", 4th LED "0". (1+8)
0 = LED "1", LED "1", 3rd LED "1", 4th LED "1" (board is locked up,
IC18 PA4 - PA7 will be high)
Here are some tips with the later stages of Game Rom Diags on system
7, in order of probability:
8 - MPU board may be good- it's looking for a Driver board?
Then suspect that IC19 RAM is faulty or finally a memory protect
fault.
9 - First check for coin door closed (or pin 1J4-1 or 1J3-1 is being
grounded),
Then is IC7 faulty or another memory protect component fault, finally
IC19 RAM is faulty.
Another tip is with Leon's test ROM v3,
2 - (That's top LED On, bottom LED Off) can also mean "coin door
closed", or memory protect circuit bad. Not just a bad 5101 RAM at
IC19.
You can prove this by adding a jumper from 1J4-1 to Ground (1J4-2 is a
good place). The test will stop at "2" and not continue until you
remove the jumper, then it will go past "3" and back to flashing if
IC19 and IC16 are good.
Maybe a good way to check your memory protect before putting it back
in a game.
Adding those LEDs may be more confusing than you thought.
-Richard
dwa...@aol.com
thanks.
OHMI_ARCADE
tim
firepower
> Thanks for taking the time to post this info!
> tim
My pleasure - it's something I learned and wanted to pass along.
Many sites and comments I have read assume you can just connect up 2 x
LEDs to a non-working system 7 MPU, and read the LED lights like they
were error codes given by a System 3-6 board. This is NOT TRUE, and
will send you off in the wrong direction.
About all they can tell you is that if they both flash once and go
out, that maybe the board has booted. Like seeing the display show a
"0" briefly. But you know that as the board is probably working
anyway. But without a display, both LEDs OFF could equally mean a "4"
or an "8" . Which are different error codes.
You are really better off using Leon's test ROMs to make sense of the
System 7 MPU errors, especially faulty RAM / PIAs and initial boot.
Once there, you would move on to the real game ROMs and the table
above to make sense of the factory dignostics. if you can't get sense
from your display,
I would definitely add all 4 LEDs as above or use a logic probe to
check the state of all four pads. That way you might get closer to
the real problem.
I plan to get this up on my web site www blackknightpinball dot com as
part of a troubleshooting reference for Sys 7, once I have it fully
documented. In the meantime, I am hopeful some use can be made of the
information here. Best wishes for a successful pinball game repair.
-Richard