Vintage RM Nimbus PC Graphics Problem - UK Vintage Radio Repair and Restoration Discussion Forum

valhalla159 · 24th May 2014, 8:10 pm

Hi

Can anyone shed any light on what is causing the graphics problem in my video below.

http://youtu.be/eyoJPCgpIMs

The computer is an RM Nimbus PC-186 manufactured by Research Machines in the UK in 1985. If you're not familiar with them, here is some background info. http://en.wikipedia.org/wiki/RM_Nimbus

I am restoring a few of these and a few seem to have the same problem. There is no obvious problems on the motherboard, all capacitors look visibly OK.

Any suggestions appreciated

paulsherwin · 24th May 2014, 9:47 pm

Just for peoples' info, these were not IBM compatibles though they ran MS-DOS. They used an 80186 processor, the only 80s computer I know about that did.

I can't help with the video fault as I can't remember how the graphics subsystem worked. There was some oddball stuff in these machines, including a proper sound chip on the motherboard a bit like a Macintosh.

Are you sure the monitor is compatible?

WreckTangle · 25th May 2014, 12:14 am

What monitor output are you using? The BBC connector or the "modified" CGA? Does the monitor work properly with a BBC? Have you got another TTL (digital) monitor you could try?

valhalla159 · 25th May 2014, 1:11 am

These machines could be IBM Compatible, sort of, there was a file called runpc186.exe which allowed some IBM software to run. One of my machines has an IBM emulator board installed which allows it to be fully IBM compatible but for the most part they were designed to run native RM applications.

Its the original monitor, I've tried more than one. I have collected a few of these machines and a few have developed this particular fault while others are fine so there must be a weakness in the hardware somewhere.

Its the CGA output. I have 3 of these original RM TTL monitors and its the same on all. I have other nimbus 186 machines that work fine on the same monitors the problem is only affecting a few of them.

cmjones01 · 25th May 2014, 8:27 am

My money would be on faulty graphics RAM chips - the regular striped pattern suggests it to me. Similar problems are very common on vintage video arcade games. I don't know the graphics hardware on RM machines but there should be a reasonably identifiable bank of RAM, either static or dynamic, for the display. Sometimes warming or cooling the chips can affect the fault and help to identify the faulty chip.

Chris

cmjones01 · 25th May 2014, 10:20 am

If you post a picture of the motherboard or graphics card, I can probably identify the RAM chips.

julie_m · 25th May 2014, 10:24 am

Agreed it looks like display memory.

If the memory ICs are socketed, try re-seating them. If that doesn't fix it, swap them around. If you swap over two working ones, the picture won't change; but if you swap a worker with the dud, then the fault will move and so the picture will change. You can work out which is the faulty one by elimination.

If no pair of ICs move the fault when swapped, then suspect the address decoding or bus access logic.

valhalla159 · 25th May 2014, 11:11 am

Thanks for the replies so far. Here is a photo of the mainboard. This machine has 1024kb RAM installed with 64kb Video RAM.

It looks like the chips are soldered to the board.

mike_newcomb · 25th May 2014, 7:22 pm

Hi Valhalla, have you ever seen the failed machines displaying correctly?

Reason for asking is the scrambled display looks similar to that when more advanced software/hardware is being used, but it is not backwards compatible and thus cannot support an old monitor.

Regards - Mike

julie_m · 25th May 2014, 7:40 pm

Looking again, it's almost as though not all the bits of each byte are getting through. Which -- unless there are eight one-bit wide memory ICs; I don't know exactly how the memory is arranged -- points to a faulty or dry-jointed bus transceiver IC.

Are there any "bodge wires" under the board that may have come adrift?

cmjones01 · 25th May 2014, 7:57 pm

I reckon the video memory is the row of 8 identical chips at the top left of the board. They'll be standard 64k x 1 bit DRAMs. If you have, or can get, any spare chips, just piggybacking a known good one (no soldering) on top of each one in turn can often reveal the culprit.

Chris

valhalla159 · 26th May 2014, 2:55 pm

Thanks Chris, I have ordered some chips so will see what happens.

On second glace, the fourth chip from the left isn't looking too healthy!

cmjones01 · 26th May 2014, 8:28 pm

Quote:

Originally Posted by valhalla159

On second glace, the fourth chip from the left isn't looking too healthy!

I noticed that it looked like it had a hole in its top, but put it down to a bit of dust or left over label. If it does have a hole in, it probably needs replacing...

Chris

24th May 2014, 8:10 pm	#1
valhalla159 Retired Dormant Member Join Date: May 2014 Location: Kent, UK Posts: 4	Vintage RM Nimbus PC Graphics Problem Hi Can anyone shed any light on what is causing the graphics problem in my video below. http://youtu.be/eyoJPCgpIMs The computer is an RM Nimbus PC-186 manufactured by Research Machines in the UK in 1985. If you're not familiar with them, here is some background info. http://en.wikipedia.org/wiki/RM_Nimbus I am restoring a few of these and a few seem to have the same problem. There is no obvious problems on the motherboard, all capacitors look visibly OK. Any suggestions appreciated

24th May 2014, 9:47 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,966	Re: Vintage PC Graphics Problem Just for peoples' info, these were not IBM compatibles though they ran MS-DOS. They used an 80186 processor, the only 80s computer I know about that did. I can't help with the video fault as I can't remember how the graphics subsystem worked. There was some oddball stuff in these machines, including a proper sound chip on the motherboard a bit like a Macintosh. Are you sure the monitor is compatible?

25th May 2014, 12:14 am	#3
WreckTangle Tetrode Join Date: May 2014 Location: Gloucester, Glos. UK. Posts: 71	Re: Vintage RM Nimbus PC Graphics Problem What monitor output are you using? The BBC connector or the "modified" CGA? Does the monitor work properly with a BBC? Have you got another TTL (digital) monitor you could try?

25th May 2014, 1:11 am	#4
valhalla159 Retired Dormant Member Join Date: May 2014 Location: Kent, UK Posts: 4	Re: Vintage RM Nimbus PC Graphics Problem These machines could be IBM Compatible, sort of, there was a file called runpc186.exe which allowed some IBM software to run. One of my machines has an IBM emulator board installed which allows it to be fully IBM compatible but for the most part they were designed to run native RM applications. Its the original monitor, I've tried more than one. I have collected a few of these machines and a few have developed this particular fault while others are fine so there must be a weakness in the hardware somewhere. Its the CGA output. I have 3 of these original RM TTL monitors and its the same on all. I have other nimbus 186 machines that work fine on the same monitors the problem is only affecting a few of them.

25th May 2014, 8:27 am	#5
cmjones01 Nonode Join Date: Oct 2008 Location: Warsaw, Poland and Cambridge, UK Posts: 2,679	Re: Vintage RM Nimbus PC Graphics Problem My money would be on faulty graphics RAM chips - the regular striped pattern suggests it to me. Similar problems are very common on vintage video arcade games. I don't know the graphics hardware on RM machines but there should be a reasonably identifiable bank of RAM, either static or dynamic, for the display. Sometimes warming or cooling the chips can affect the fault and help to identify the faulty chip. Chris __________________ What's going on in the workshop? http://martin-jones.com/

25th May 2014, 10:20 am	#6
cmjones01 Nonode Join Date: Oct 2008 Location: Warsaw, Poland and Cambridge, UK Posts: 2,679	Re: Vintage RM Nimbus PC Graphics Problem If you post a picture of the motherboard or graphics card, I can probably identify the RAM chips. __________________ What's going on in the workshop? http://martin-jones.com/

25th May 2014, 10:24 am	#7
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Vintage RM Nimbus PC Graphics Problem Agreed it looks like display memory. If the memory ICs are socketed, try re-seating them. If that doesn't fix it, swap them around. If you swap over two working ones, the picture won't change; but if you swap a worker with the dud, then the fault will move and so the picture will change. You can work out which is the faulty one by elimination. If no pair of ICs move the fault when swapped, then suspect the address decoding or bus access logic. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

25th May 2014, 11:11 am	#8
valhalla159 Retired Dormant Member Join Date: May 2014 Location: Kent, UK Posts: 4	Re: Vintage RM Nimbus PC Graphics Problem Thanks for the replies so far. Here is a photo of the mainboard. This machine has 1024kb RAM installed with 64kb Video RAM. It looks like the chips are soldered to the board. Attached Thumbnails

25th May 2014, 7:22 pm	#9
mike_newcomb Rest in Peace Join Date: Apr 2012 Location: West London, UK. Posts: 665	Re: Vintage RM Nimbus PC Graphics Problem Hi Valhalla, have you ever seen the failed machines displaying correctly? Reason for asking is the scrambled display looks similar to that when more advanced software/hardware is being used, but it is not backwards compatible and thus cannot support an old monitor. Regards - Mike

25th May 2014, 7:40 pm	#10
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Vintage RM Nimbus PC Graphics Problem Looking again, it's almost as though not all the bits of each byte are getting through. Which -- unless there are eight one-bit wide memory ICs; I don't know exactly how the memory is arranged -- points to a faulty or dry-jointed bus transceiver IC. Are there any "bodge wires" under the board that may have come adrift? __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

25th May 2014, 7:57 pm	#11
cmjones01 Nonode Join Date: Oct 2008 Location: Warsaw, Poland and Cambridge, UK Posts: 2,679	Re: Vintage RM Nimbus PC Graphics Problem I reckon the video memory is the row of 8 identical chips at the top left of the board. They'll be standard 64k x 1 bit DRAMs. If you have, or can get, any spare chips, just piggybacking a known good one (no soldering) on top of each one in turn can often reveal the culprit. Chris __________________ What's going on in the workshop? http://martin-jones.com/

26th May 2014, 2:55 pm	#12
valhalla159 Retired Dormant Member Join Date: May 2014 Location: Kent, UK Posts: 4	Re: Vintage RM Nimbus PC Graphics Problem Thanks Chris, I have ordered some chips so will see what happens. On second glace, the fourth chip from the left isn't looking too healthy!