Troubleshooting a 286.. oscope level!

ethan at ethan at
Sat Oct 3 17:20:40 CDT 2015

> 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?

It does! I just looked in it. So there is a video pin (good news), bad 
news is .. it's Herculeas! The old high resolution monochrome output. 
Dip switch 1 switches between CGA external and internal, 2 turns on 
herculeas mode. When it's in CGA mode it says that a pull up or down 
resistor is applied and the video signal on that pin is disabled.

> 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 tried it with one, nothing. It has a hard drive and hard drive 
controller as well (20MB, custom interface board.)

> 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.

Yea. A while ago I traced back the dip switch to some IC which is likely 
read during startup of the system by the bios, which in turn selects 
internal or external display.

> "SAYS external display"?  how?  by lighting an LED?

Correct, LED shows if external display is selected.

> If you can find a duplicate machine, that can confirm/solve bad BIOS.

I might be able to get hold of the bios, but I'm worried about the 3rd 
EPROM. It sits in the middle of a bunch of Yamaha branded chips. The BIOS 
chips are in the middle of a Chips & Technologies 286 chipset group. The 
people I find might be able to dump the BIOS via dos program but not that 
3rd IC. But at least if I could split the one I could compare it.

Here are mainboard pics I took when doing the cap replacement:
I might recheck that mess near C113

And sharing is caring, here is floppy disk images of the software (more 
rare than the computer rare) that goes with the computer (fairly rare in 
working condition.) I'm going to scan all the documentation I have 
eventually and add it. Looking at the docs the expansion connector might 
be good enough to be ISA. Hmmm PC IDE for it!?!

Ethan O'Toole

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