Altos 5-5AD board repair
With tutoring from Bob Grieb, I learned how to better troubleshoot
the motherboard. I can again see the self test boot screen, referred to
earlier in this thread.
Here is what we did (Bob feel free to add/correct):
Through careful observation we determined that something was wrong with
the communications between the terminal and the RS232 port. With the
terminal plugged in, powered on the computer appeared to stall early in
the boot process. With the terminal unplugged the computer appeared to go
farther (drive would spin, hard drive would light). No characters to the
1. clean all sockets and re-seat all chips
2. check chip connections (with power off) using Ohm setting in Fluke
meter. By probing a pin connection under the board and at the pin itself
one can verify that there are no bad solder joints. My fluke has a beeper
to signify a connection, but it's so sensitive that you really need to
let the ohm readout to make sure proper voltages were coming through. We
also experimented with a sheet of tin foil pressed to the bottom of the
board, and used it as a proxy for ohm probes, with limited
3. Locate the RS232 driver chips. These chips are near the RS232 ports,
and will end in 1488 (send) 1489 (receive). To verify that they're
getting the proper voltage, probe pin 7 and pin 1 = -12v and pins 7 and
14 should = +12v
4. Took a look at the cable that connects the terminal with the computer.
It's a straight-through. The cable should plug into the "modem"
port of the Wyse terminal (not Aux)
5. Using Oscilloscope to test RS232 output (1488)
-ground clip to computer
-probe to pin 3 of rs232
-oscilloscope reports = -5V (OK)
Check oscilloscope on pin 2 of the RS2232 receiver chip (MC1489P) - found
it to be bad, not receiving the signal. Replaced the chip.
6. For the TTL chips, you're looking for V=~5 (4.95/5.05 OK) *in general*
there are exceptions and you should check your schematics. We did not
have schematics so we had to guess. Probe the corner pins to check the
voltage while the system is on.
7. Disk drive not responding: Found that the TTL chip in position D10 (a
74LS32) was bad by measuring pin 12. Going 14 ohms to ground. Replaced
8. Discovered that the C2 jumper in slot 3, if pulled, will turn off
autoboot to hard drive. By inserting a disk that does not
boot (but is formatted for an Altos system) causes system to prompt hard
drive or disk boot.
MP/M v2.14 Boot Disk
I was able to make an MP/M boot disk v 2.14. Thanks Dave Dunfield and
I was able to boot by floppy to both MP/M and CP/M, and through these
regain access to the hard drive without replacing it (drive C) as I
originally thought would be necessary. My guess is that when the system
originally farted out, it chewed up a little of the file table making it
loose it's ability to read track 0. I should be able to clean it up and
boot without disk soon.
NOTE: If anyone in this group would like to join a restoration
project please contact me, there are plenty of interesting systems that
need analysis and restoration at InfoAge.