UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Computers

Notices

Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

Closed Thread
 
Thread Tools
Old 3rd Nov 2017, 12:29 am   #21
Argus25
Octode
 
Argus25's Avatar
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 1,463
Default Re: Nascom repair - Looking for clues please

IF the 6264 is a useful part here, I'm pretty sure that is the same basic 8k. X 8 memory IC used inside the Dallas DS1225 battery backed NVram, it's in there with a controller IC, these are easy to program and remember the data when removed from the socket. Also, in some applications, like Tek scopes, the Fram type FM16w08 can be substituted for the DS1225 and programmers like the GQ-4x will program an FM16w08 if it is set for a DS1225, recently the manufacturers of the GQ-4x updated the supported IC list in this point. So perhaps the FM16w08 could be of help too.
Argus25 is offline  
Old 3rd Nov 2017, 11:15 am   #22
cmjones01
Octode
 
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 1,761
Default Re: Nascom repair - Looking for clues please

Quote:
Originally Posted by julie_m View Post
If "tanning" is a problem for you, it was certainly possible in the days of the BBC Micro to use a 6264 (I think that was the part number) 8k * 8 SRAM IC with an auxiliary lithium battery back-up supply, as a substitute for a 2764 EPROM. The BBC used only memory ICs designed to run from a simple 5V supply (no +12V, -5V or -12V). Most of the pins are just passed straight through; the rest are carefully bent out of the way and the IC stood in a 28-pin DIL socket.
Yes, the 6264 is the one. I used a setup like that for quite an ambitious 6502 development project in the late 1980s. My 6264 had three AA cells in a holder on flying leads connected to its power pin and ground. The power pin was bent upwards so it didn't contact the socket. I had a bit of Veroboard with an address decoding latch on it connected to the BBC Micro's 1MHz bus so that the 6264 RAM appeared in 256-byte pages at &FC00 (I think). Code was assembled (using the Beeb's built in BASIC assembler) 256 bytes at a time to that address, with the origin set to the address it would finally end up at. Run the assembler, pull the chip out of the socket (power still on) and plug it in to the EPROM socket on the device I was developing. Hit reset and the code ran. Rinse and repeat.

It worked fine, with only very occasional data corruption problems. It was very cheap, as well, which was a big advantage at the time!

Conclusion: if erasing EPROMs is a pain, a battery-backed RAM chip will work fine.

Chris
__________________
What's going on in the workshop? http://martin-jones.com/
cmjones01 is offline  
Old 3rd Nov 2017, 6:01 pm   #23
acollins22
Tetrode
 
Join Date: Oct 2010
Location: Melton Mowbray, Leicestershire, UK.
Posts: 78
Default Re: Nascom repair - Looking for clues please

Thanks for the interest folks it's great to get help and advice.

I've been over the board testing the 74 series chips with a tester and they all turned out OK. My tester can't do 74LS163s though so I rotated those around, still no success. I did the same with the 81 series buffers. So I've come to a halt for a while. Everything is fine but it's still not working.

Cheers,

Andy.
acollins22 is offline  
Old 4th Nov 2017, 10:56 am   #24
SiriusHardware
Octode
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 1,785
Default Re: Nascom repair - Looking for clues please

It looks like you're back to trying to write some test code, or 'belling out' every single interconnection on the PCB in case the fault is a physical fault like a cracked track or failed through-hole connection. Not a task I would relish.

Some EPROM programmers of a certain vintage have an 'eprom emulator' mode where they provide a connector for a flying flat ribbon cable with a male DIL connector on the end of it: This plugs into the target system's EPROM socket and the EPROM programmer uses internal RAM to emulate the action of an EPROM, the advantage being that it only takes a few seconds to download altered / amended code to the programmer.

If your own programmer(s) had this feature I'm sure you would already be using it, but it might be worth looking out for an old programmer with this feature. I have one unit which is a serial hex -> eprom emulator only, not a programmer, made by a little outfit called 'JPD' many years ago, and I also have a commercially made EPROM programmer made by ICE TECH which includes an EPROM emulator feature. There surely must still be a few of these around.

You'll still eventually need an eraser of course: I wonder if you can now get UV LEDs which produce light of a suitable wavelength? My understanding is that some LEDs used in LED lighting actually natively emit UV, and they are coated in phosphor which converts the UV light into light of the required visible wavelength. Is anyone already working on a LED eprom eraser?
SiriusHardware is offline  
Old 23rd Nov 2017, 8:27 pm   #25
DigitalNoMore
Pentode
 
Join Date: Jan 2013
Location: Nottingham, Notts. UK.
Posts: 111
Default Re: Nascom repair - Looking for clues please

Hi Andy,

I've only just spotted this thread but in response to the supply voltages you've measured:

What I have so far...
I have the following voltages from the power supply :-
11.93V
4.79V
-5.06V
-12.06V

If the supply rail measuring 4.79V is the 5V supply rail to the processor and other logic circuitry then this could well be the cause of the problems you are seeing. Before going any further I would look to sorting this issue out.

Jerry
DigitalNoMore is offline  
Old 23rd Nov 2017, 9:46 pm   #26
cmjones01
Octode
 
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 1,761
Default Re: Nascom repair - Looking for clues please

Quote:
You'll still eventually need an eraser of course: I wonder if you can now get UV LEDs which produce light of a suitable wavelength? My understanding is that some LEDs used in LED lighting actually natively emit UV, and they are coated in phosphor which converts the UV light into light of the required visible wavelength. Is anyone already working on a LED eprom eraser?
Without wishing to get too far off topic: probably not yet. EPROMs need UV light of a wavelength shorter than 400nm (the 257nm line from mercury vapour discharge does the job) and they need quite a lot of it. LEDs at those sort of wavelengths are still pretty inefficient and expensive.

White LEDs typically use a Gallium Nitride visible blue LED, around 460nm wavelength, with a phosphor coating to create white or other colours like pink. There's no UV involved.

Chris
__________________
What's going on in the workshop? http://martin-jones.com/
cmjones01 is offline  
Old 23rd Nov 2017, 9:56 pm   #27
acollins22
Tetrode
 
Join Date: Oct 2010
Location: Melton Mowbray, Leicestershire, UK.
Posts: 78
Default Re: Nascom repair - Looking for clues please

Hi Jerry,

Thanks for the suggestion. I have just tried running it again but with the 5V rail coming from a bench supply and I still see garbage.
acollins22 is offline  
Old 25th Nov 2017, 12:45 pm   #28
SiriusHardware
Octode
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 1,785
Default Re: Nascom repair - Looking for clues please

Quote:
Originally Posted by cmjones01 View Post
White LEDs typically use a Gallium Nitride visible blue LED, around 460nm wavelength, with a phosphor coating to create white or other colours like pink.
Ah, in that case maybe I was thinking of flourescent lights (produce UV, which is converted to visible light via the phosphor coating?)

Acollins22, it might be worth going back to the vintage computer forums to this thread which you were involved in early on:

http://www.vcfed.org/forum/showthread.php?60295-Nascom

It has since developed into a fairly technical discussion about various aspects of the Nascom (1) and is still ongoing. You might find some useful clues there now.

One thing which comes up over and over again in discussions I've read about the Nascoms (1) and (2) is their apparent sheer fussiness about individual ICs - in some cases replacing one apparently working IC with another working IC, or swapping two of the same ICs over in the machine, will cause a significant change. This is especially the case when it comes to the video circuits.
SiriusHardware is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 10:45 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Copyright ©2002 - 2018, Paul Stenning.