Troubleshooting a 286.. oscope level!
cisin at xenosoft.com
Sat Oct 3 16:05:34 CDT 2015
On Sat, 3 Oct 2015, ethan at 757.org wrote:
> Yamaha C1, one of these:
> It doesn't behave at all. The LCD backlight never comes on, and it always
> thinks that it's in external video mode. There is a dip switch on the back to
> set this between internal and external display, and it never switches to
> internal. I don't have a CGA monitor, but I tried to cross wire a Commodore
> 1084S over to it and nothing.
Using your oscilloscope, does the external video connector show anythings
resembling a video signal?
BTW, SOME after-market "CGA" connectors added a signal for composite
video, which would be relatively easy to interface to. Does your manual
have connector pinout?
> Yea the CS is staying active. Since it never really POSTS or brings up the
> LCD backlight, it never does a floppy seek.
Do you have a floppy in the drive? Some "smart" systems won't try to seek
the floppy if they already figured out that there's no disk in it.
> I'm guessing the CPU is stuck in
> an instruction loop or something, possibly because BIOS is damaged?
That, unfortunately seems likely. On a 286 machine with internal/external
video, the BIOS is likely to be farily unique, unlike most desktops, where
many other BIOSs would work.
> No display, the LCD actually never lights up. There is an option to switch
> between external CGA or internal LCD, no matter what setting the dip switch
> is set to it always says external display -- another hint that something is
> very wrong.
"SAYS external display"? how? by lighting an LED?
> I'm trying to find someone with a working machine and going to ask them to
> run a bios dump utility. Not sure if it will work but short of finding a
> working specimin it's my next hope.
If you can find a duplicate machine, that can confirm/solve bad BIOS.
Grumpy Ol' Fred cisin at xenosoft.com
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