Re: Apple IIc woes -- HELP!

[dempson@actrix.gen.nz (David Empson)]

In article <2E4A6424@mkt.eicon.com>, Jim Royal <jimr@eicon.com> wrote:
> 
> My Apple IIc was damaged during an electrical storm.  It seems a spike on 
> the telephone line fried both my external modem and the IIc modem port.  I 
> discovered the damage to the port only after purchasing a new modem.  The 
> DTR line appears to work normally, but the machine locks up when I try to 
> send a single character.

A transmit lockup could be caused by lack of clock signal going to the
6551 (probably a damaged motherboard, crystal/clock source or 6551), or
by any of the RTS, DTR or CTS control signals being false.  CTS is
tied to ground in the IIc, and RTS and DTR are under software control,
which makes this unlikely.

The software may also be checking the incoming flow control line,
which goes to the DCD pin of the 6551, and waiting for it to go active
before transmitting.  Are external changes on the incoming flow
control line reflected in the DCD bit of the 6551's status register?
(Bit 5 of location $C0A9 should be 0 if the flow control signal is
true, or 1 if it is false.)

Another possibility is a damaged control signal going to the chip
(e.g. if it isn't being selected, or has the wrong address, read/write
or data).

> On the advice of a friend, I replaced one of the two 6551 chips on the IIc's 
> motherboard. These chips are responsible for Port 1 and Port 2 serial I/O.

They aren't connected directly to the serial ports.  The signals go
through the 1488 and 1489 driver and receiver chips, which should
provide some isolation from the line.  They are unlikely to protect
against lightning strikes, but I would expect random damage to the
whole IIc in this case, not just the serial chips.  You may have a
serious fault.

> The transplant was successful, but the problem didn't go away.  According to 
> the Apple IIc Technical Reference Manual, there are two other chips in the 
> IIc responsible for serial I/O -- one for incoming signals, the other for 
> outgoing signals.  Should I replace both?  What else could I do?  I don't 
> want to replace parts at random.

If you are going to do this, try replacing the 1489 first.  I would
suggest replacing both of them anyway.

> Needless to say, I dismissed a motherboard swap from Apple Computer as a 
> hideously expensive proposition.

Alltech Electronics may be a cheaper alternative.  I don't know
offhand if they offer IIc motherboard swaps.
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand