Troubleshooting a 286.. oscope level!

[Fred Cisin]

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