On Thu, 5 Mar 2009 05:02:37 -0800 (PST), b
put finger to keyboard and composed:

I am trying to get this analogue tape langauge lab system ASC model 4M
up and running. I believe it dates from the late 80s. It consists of a
master control panel and several 2-track slave decks. These are
interfaced using a bus and ASCII command strings (that's as far as my
knowledge goes on the IT side of things!).

schematic part 1: http://www.fileden.com/files/2007/6/...%201%20new.jpg
part 2. http://www.fileden.com/files/2007/6/...%202%20new.jpg

print side of the pcb: http://www.fileden.com/files/2007/6/...side%20new.jpg

There are some pictures of the equipment he
http://www.fileden.com/files/2007/6/...3/console1.jpg
http://www.fileden.com/files/2007/6/...0underside.jpg
http://www.fileden.com/files/2007/6/...20elements.jpg

The problem was caused on assembling the master panel's processor
board: I accidentally connected the cable 105 which was supposed to go
into connector 105 (from student control unit which supplies the slave
decks) into socket 103 (they are the only two in the whole system
which are the same size!).
You can see on the schematic that pin 25 of 105 carries a +7.5v supply
and 13-14-26 appear to have +5v on them. So these voltages were fed
into the corresponding pins of number 103, and ended up God-knows-
where, the 7.5v went on the AO line.

On powerup, a few lights came on on the console, but it was several
seconds before I realised something was wrong and yanked the plug.
Since then, the main console appears dead, with a very faint display
on the LCD and nothing else. the individual slaves work, but receive
nothing from the master console. The 'good' news is that since 105,
which supplies a 5v and 7.5v supply, was not in its place, there were
no other voltages around on that panel whilst the misconnection
happened.

Could this be some eeprom got scrambled or one of the IC's blew? What
I need to know now is what components are likely to have been affected
by this misconnection, and what to start replacing. I tried replacing
IC115 (8155) by subsitution from a student machine, no change.
removing the ICs one by one (except IC108 to IC111) usually made no
difference, but IC107 74HC138 caused the LCD to come on with garbage
on, and a few random lights to come on. However, exchanging it for one
of the others made the unit dead again.

ICs include.
74HC138AP
SS74HC241E
PC74HC00P
D8259AG2
TMP8251AP
D8505AHC-2
M74HC623B1
74HC573AP
ULN2803A
RAM: 6116
2764
27128

I know this is a long shot, any ideas on what to start replacing? I've
no idea how easy to come by most those chips are.
Thanks for any ideas
B

The first parts I would check would be the EPROMs (2764, 27128). I
presume the contents would include recognisable text strings, so if
some text is garbled, then you can be almost certain that the PROMs
are bad. For example, I would expect that a fault in address bit A0
would cause a text string such as "COPYRIGHT" to appear as "CCPPRRGGT"
or "OOYYIIHH".

If reading the EPROMs several times produces different results, then
once again you can be sure that they are bad. OTOH, if they have a
zero checksum, or some other pattern that looks like a checksum (eg
0x55AA), then you can be confident that the part is OK.

If any of the EPROMS are bad, then you will need to find a working
device to make a copy. In any case it would be a good idea to backup
the contents of good devices to a file.

If you have difficulty obtaining any parts, I believe I may have a few
of the bigger ICs in storage ... somewhere. Otherwise you may find
that many of these ICs were used in old arcade games, IBM XTs, PC/ATs,
etc.

BTW, IMO any design that allows you to do what you did is inexcusable.
I wonder how many boards were destroyed in the factory in this way.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.