Re: IIgs MB repair info

["Michael Pender" <mpender@hotmail.com>]

Bill Garber <willy46pa@comcast.net> wrote in message
bpadnXU6LuAYw5eiXTWJiQ@comcast.com">news:bpadnXU6LuAYw5eiXTWJiQ@comcast.com...
>
> I have a IIgs MB I am in process of trouble-shooting.
> Any FAQs, docs, or other info deemed worthy of use
> will be greatly appreciated. Now, to avoid responses
> concerning already tried procedures, all the socketed
> ICs "do" work. There is no start, I repeat, will not pass
> video pretest. I have an 800k drive attached. There is a
> small grunt from the drive with no led activity, and
> display shows black screen with lower half white and
> 6-7 pixilated rectangles up the left half of the black,
> and black ones down the right side of the white area.
> If anyone knows what is causing this, please inform.
> If I hear from several sources that the board is dead,
> I will proceed to remove all component parts that I
> can and scrap it.
>
> Thanks,
>
> Bill @ GarberStreet Enterprises };-)
> Web Site - http://garberstreet.netfirms.com
> Email - willy46pa@comcast.net

If I remember correctly (and that's a *big* if), the drives are supposed to
start the spin-up cycle as soon as power is applied, whether or not the
motherboard starts talking.  The 'grunt' from the drive sounds like one of
the voltage lines from the power supply is no good.

That could either be due to a bad power supply or a short on the board.  I
would suggest:
1) bench the power supply and check the voltage of each power line while
driving a test load
2) remove any peripheral cards in case one is misaligned
3) also disconnect any 'unnecessary' peripherals in case, for example,
somebody mis-indexed a pin when the drives were plugged

and if that doesn't turn anything up:
4) try a different power supply.

The most likely failure modes:
#1:  90% - something is misaligned such as a Disk II cable connection or a
socketed chip
#2:  9% - bad power supply
#3:  1% - failure of one of the unsocketed chips (expressly assuming that
all socketed chips are 100%)

Problem #3 is far more likely to occur in combination with #1 than by
itself.

Post-infancy chip failure usually does not occur unless induced by an
electrical short circuit.  I've seen #3 occur only twice, once when somebody
was hot-plugging peripheral cards on the bus and once when a defective
peripheral cable shorted the power supply.

Just my $0.02.

- Mike