Xerox 820/II 8/16 Prom/Eproms ( Masters ) on e-bay

[Mark G Thomas]

Tom,

On Wed, Aug 11, 2021 at 11:05:59AM -0500, Tom Uban wrote:
> Ok, I was going by the appearance of my case being the older style. I had not considered the number
> of PROMs, but now that you mention it, my old Ferguson Big Board (which was an 820 clone) only had
> two PROMs.

If you still have any 8" floppies from your Big Board, these will probably 
boot and work in the 820 as well as 820-II.

...
> >> At some point, I need to ask someone to make me bootable 8" floppies, but I suppose I need to
> >> determine if it is 820 or 820-II first...
> > I can able to help you with floppies. The floppies are standard 
> > IBM 3740 Single Density and easy to write with Imagedisk software 
> > and a PC-connected 8" drive.
> What do you use to connect an 8" drive to a PC?

This adapter is useful: http://www.dbit.com/fdadap.html

But, if your floppy doesn't require a TG43 signal, you could just
wire a custom cable with no logic needed, or get something like this 
on e-bay: https://www.ebay.com/itm/114924378003

Additionally, some PC floppy controllers do not work for some kinds
of floppy encodings. Here is the article I used as a guide. I've been
using an Adaptec AHA-1522A SCSI+Floppy interace, which has a good 
FDC chip for optimum floppy format support.

      https://retrocmp.de/fdd/general/disk-imaging.pdf

> > Both the swithing supply in my 820-II and it's external 8" drive box 
> > had failed. I replaced the supply in the 820-II case with a modern 
> > switching supply that easily fit. The HV bleeder resister for my CRT 
> > was arcing, so I replaced that. I replaced all the electrolytics on 
> > the monitor board. I also replaced the sockets for my ROMs, as some 
> > of their contacts "sprung" when I replaced the chips, but I do not 
> > recommend doing this unless it is absolutely necessary and you have 
> > good tools and practice.
> I had not turned on my box for a number of years, but when you posted, I decided to try it and it
> worked perfectly, which I suppose is just lucky. I do have the tools and skills to work on it if it
> were to fail, but won't likely make changes unless it does.

I'm all for not fixing it if it's not broken. But, I do suggest careful 
inspection of the electrolytics on the motherboard and monitor, especially 
since the monitor board is right above the logic board, so if a cap 
phsically leaked, the corrosive stuff that comes out could drip onto the 
logic board. I see this mostly in SMD style electrolytics, but have seen 
those like the ones in my 820-II physically leak, and the damage can be 
serious.

> > The 820-II restoration was a fun and rewarding project. It is well 
> > documented, easy to work on. It was also my first ever CP/M computer. 
> > When I was in high school, the 820 motherboards were readily available 
> > for $75.
> >
> > Mark
> It sounds like you had a good time, which in my opinion is the main goal!
> 
> --tom

-- 
Mark G. Thomas <Mark at Misty.com>, KC3DRE