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.

Reply
 
Thread Tools
Old 7th Aug 2017, 1:28 pm   #61
Gulliver
Hexode
 
Join Date: Aug 2011
Location: Luton, Bedfordshire, UK.
Posts: 325
Default Re: Sinclair ZX81

For those interested in getting original hardware up and running....

ZX81's are a bit fickle about cassette players. Definitely use a mono one, any old "shoebox" recorder or walkman in working order should work however. You just need an EAR socket and a volume control. tone control can help, set to boost the treble.

As for RAM expansions, the original was prone to wobble of course. I tended to put Blu-Tak between my ZX81 and the RAM pack. Today there is a device on sale called "ZXPand" which has 32K RAM, SD card adaptor (for instant read/write of programs and data). I found that an old 16Mb SD card pretty much holds the entire ZX81 software library! ZXPand has an optional joystick interface and AY sound chip too.

While I love original hardware, LOADing 3D Monster Maze in 10 minutes from cassette is a bit of a chore when I can have it in under one second from an SD card, running on the original hardware.

Quite amazing that there is still new ZX81 software and hardware being produced, albeit very much cottage industry. Nothing like this exists for Commodore computers as far as I am aware.
Gulliver is offline   Reply With Quote
Old 7th Aug 2017, 1:59 pm   #62
SiriusHardware
Octode
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 1,367
Default Re: Sinclair ZX81

The only drawback with ZXpand is that it (as far as I am aware) connects through the self same edge connector as the original RAM pack did, so is potentially prone to the same 'bump' problems. Nothing beats an internally fitted RAM upgrade for stability.

Audio loading of software is all part of the, er, charm, especially if you are loading from a digital source which outputs the same signal at the same consistent level every single time so it is virtually guaranteed to always work first time.

For those who do go for a ZXpand there is also a version of the remanufactured replacement keyboard which has the alternative key legends printed on it for ZXpand specific commands. They've even got that covered.

The ZX series always did have a good selection of third party hardware to go at, see also those items produced by a gentleman called Paul Farrow who produces beautifully designed and documented add-on hardware for ZX computers - mainly for his speciality, the Spectrum, but also for the '81 as well.

For Ken and his ULA, I bought a late version ('2C210') ULA - the one in which the video 'back porch' problem is fixed - quite recently from an online supplier who had found a large quantity of them somewhere. He is currently offline while recovering from a serious illness, but hopes to be back up and running some time around mid or late August (2017, for anyone reading this in two years' time). So if you still think you need a ULA in a few weeks' time, have another look around then.

I do think you should do a few more checks on clocks, supply voltages, and other signals such as _RESET before buying a ULA - pity you don't have anyone nearby who can loan you a working ULA to try.

This nicely redrawn ZX81 circuit diagram may assist with fault finding.

http://www.mainbyte.com/ts1000/good_schematic_hi.jpg
SiriusHardware is offline   Reply With Quote
Old 7th Aug 2017, 4:27 pm   #63
Ken
Pentode
 
Ken's Avatar
 
Join Date: Oct 2003
Location: Near Glasgow, UK.
Posts: 218
Default Re: Sinclair ZX81

Hello SirusHardware,
Thanks for the heads up on the ula. So far I have not found anybody on the net who still has any to sell. I am hoping it's not the ula. I have spent a good part of today trying to get another old oscilloscope working. It's only 12 MHz but I assume it will do for this. Rotary switches and pots are noisy. It is now working well enough to use so this will help. I am not going to be able to do much for the rest of the week as I have my granddaughters coming for a short visit but I will get back to it. Also the circuit diagram you posted a link to is far superior to the one I've got. I'll print out a copy.

Regards,

Ken.
Ken is offline   Reply With Quote
Old 7th Aug 2017, 6:07 pm   #64
Gulliver
Hexode
 
Join Date: Aug 2011
Location: Luton, Bedfordshire, UK.
Posts: 325
Default Re: Sinclair ZX81

ZXpand does indeed use the regular edge connector but seems to be designed in such a way as to not wobble. I've had mine a couple of years and the machine has only crashed a couple of times.
Gulliver is offline   Reply With Quote
Old 7th Aug 2017, 8:54 pm   #65
IanBland
Pentode
 
Join Date: May 2015
Location: Northampton, Northants, UK.
Posts: 113
Default Re: Sinclair ZX81

Well, you could always buy a ZX81 for parts, there are a number currently on Ebay.
IanBland is offline   Reply With Quote
Old 8th Aug 2017, 9:09 am   #66
Ken
Pentode
 
Ken's Avatar
 
Join Date: Oct 2003
Location: Near Glasgow, UK.
Posts: 218
Default Re: Sinclair ZX81

Hello Ian,
I had thought of doing this but from what I've read on the net the ula seems to run hotter than is good for it and I wondered what the chances were of finding a scrap ZX81 with a working ula.

Regards,

Ken.
Ken is offline   Reply With Quote
Old 8th Aug 2017, 10:36 am   #67
Dave Moll
Dekatron
 
Dave Moll's Avatar
 
Join Date: Feb 2005
Location: West Cumbria (CA13), UK
Posts: 3,118
Default Re: Sinclair ZX81

I must admit that my thought was the same - that a scrap ZX81 is most probably so because of a dead ULA.
__________________
Mending is better than Ending (cf Brave New World by Aldous Huxley)
Dave Moll is offline   Reply With Quote
Old 8th Aug 2017, 2:52 pm   #68
Ken
Pentode
 
Ken's Avatar
 
Join Date: Oct 2003
Location: Near Glasgow, UK.
Posts: 218
Default Re: Sinclair ZX81

Hello All,
I have used my old scope to make the tests below. Scope set to DC.

1. Pin 16 ULA
(a) volts/cm=0.5
(b)Time/cm=2ms

2. Output of one transistor comp video adaption(no signal diodes)
Same settings as above.

Used merlinmaxwell's method to check clock on pin 6 of cpu. Reading on AVO8 was 2.5V so clock might be working.
I have attached the traces for 1. and 2. above.
You will realise how little I know about this stuff based on my question. Do the traces in 1. and 2. above look as you would expect if the ula was working?

Regards,

Ken.
Attached Thumbnails
Click image for larger version

Name:	Pin16ULA.JPG
Views:	54
Size:	89.1 KB
ID:	147534   Click image for larger version

Name:	CompVideoAdaptionOut.JPG
Views:	57
Size:	91.3 KB
ID:	147535  
Ken is offline   Reply With Quote
Old 9th Aug 2017, 11:50 pm   #69
SiriusHardware
Octode
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 1,367
Default Re: Sinclair ZX81

Quote:
Originally Posted by Dave Moll View Post
I must admit that my thought was the same - that a scrap ZX81 is most probably so because of a dead ULA.
Maybe not, actually - the keypads are common failures on them (which is why it was worth someone's while to get some remanufactured). It usually manifests itself as dead indivdual keys or a whole key row out.

That would have been enough reason for them to be discarded or 'lofted' at one time, especially in the years between when the machines were originally made and when the new keypads started to be made. The keypad/membrane was a unique part, nearly impossible to repair once it started to fail and for a long time, impossible to replace with like for like.

Finding an '81 which does or does not have a working ULA is not the only issue, because ideally you want one with a ULA2C210, the version on which the video back porch issue was finally fixed.
SiriusHardware is offline   Reply With Quote
Reply

Thread Tools



All times are GMT. The time now is 3:53 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 - 2017, vBulletin Solutions, Inc.
Copyright ©2002 - 2017, Paul Stenning.