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 26th Mar 2020, 9:35 pm   #201
John Earland
Hexode
 
Join Date: May 2013
Location: Middlesbrough, North Yorkshire, UK.
Posts: 411
Default Re: Zx81

Thank you guys for the advice. You’re absolutely right and there’s no way I works even think of forcing the chip out. Like I said I will practice on an old pcb first and only then if I feel confident enough, and that’s a big ‘if’ will I attempt it! Thank you again for all your advice. I do have a plunger style desolder pump-came as part of the kit. I was planning on cutting out the chip as I did in my pet. Food for thought!
John Earland is offline   Reply With Quote
Old 26th Mar 2020, 9:37 pm   #202
John Earland
Hexode
 
Join Date: May 2013
Location: Middlesbrough, North Yorkshire, UK.
Posts: 411
Default Re: Zx81

P.S the PET by the way is working brilliantly even if the cassette player is a little temperamental-though I’ve read that they often are!
John Earland is offline   Reply With Quote
Old 27th Mar 2020, 6:36 am   #203
Slothie
Hexode
 
Join Date: Apr 2018
Location: Newbury, Berkshire, UK.
Posts: 369
Default Re: Zx81

Quote:
Originally Posted by John Earland View Post
P.S the PET by the way is working brilliantly even if the cassette player is a little temperamental-though Iíve read that they often are!
(Off topic I know)
My experience of my PET was you need to demagnetise and clean the heads of the cassette players frequently as some models (particularly the built in one on early PETs) had D.C. bias. Symptoms were unreliable reading and failure to verify. If you look around you can find mains demagnetisers for £15-20, the type that have permanent magnets built into tape cleaning cassettes I never found as effective.
Slothie is offline   Reply With Quote
Old 27th Mar 2020, 8:20 am   #204
John Earland
Hexode
 
Join Date: May 2013
Location: Middlesbrough, North Yorkshire, UK.
Posts: 411
Default Re: Zx81

Quote:
Originally Posted by Slothie View Post
Quote:
Originally Posted by John Earland View Post
P.S the PET by the way is working brilliantly even if the cassette player is a little temperamental-though Iíve read that they often are!
(Off topic I know)
My experience of my PET was you need to demagnetise and clean the heads of the cassette players frequently as some models (particularly the built in one on early PETs) had D.C. bias. Symptoms were unreliable reading and failure to verify. If you look around you can find mains demagnetisers for £15-20, the type that have permanent magnets built into tape cleaning cassettes I never found as effective.
Thank you for that. Yes I have two and both can be temperamental. I have a black one and a cream coloured one. The black one is like the one built into the early PETs and cream one came with my C64 - Iíll check the demagnetiser out
John Earland is offline   Reply With Quote
Old 27th Mar 2020, 12:16 pm   #205
John Earland
Hexode
 
Join Date: May 2013
Location: Middlesbrough, North Yorkshire, UK.
Posts: 411
Default Re: Zx81

Regarding the ULA on the ZX81, this might be a silly question but why does heat cause it to fail? Normally itís fine until it gets warm/hot.
John Earland is offline   Reply With Quote
Old 27th Mar 2020, 1:52 pm   #206
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 4,227
Default Re: Zx81

Components which work when cold but fail when hot are not uncommon. That's why freezer spray is 'a thing'.
SiriusHardware is offline   Reply With Quote
Old 27th Mar 2020, 3:23 pm   #207
Slothie
Hexode
 
Join Date: Apr 2018
Location: Newbury, Berkshire, UK.
Posts: 369
Default Re: Zx81

Quote:
Originally Posted by John Earland View Post
Regarding the ULA on the ZX81, this might be a silly question but why does heat cause it to fail? Normally it’s fine until it gets warm/hot.
If there is a tiny crack in the silicon or between the gold bond wire and die in can contact when cold, and when it heats the silicon and aluminium on the chip expand at slightly different rates bending the chip slightly opening up the gap. The gap only has to be a few molecule wide to stop the electrons!

Last edited by Slothie; 27th Mar 2020 at 3:24 pm. Reason: Autocorrect failure
Slothie is offline   Reply With Quote
Old 28th Mar 2020, 1:20 pm   #208
John Earland
Hexode
 
Join Date: May 2013
Location: Middlesbrough, North Yorkshire, UK.
Posts: 411
Default Re: Zx81

Thank you for this. I understand now.
John Earland is offline   Reply With Quote
Old 28th Mar 2020, 1:25 pm   #209
John Earland
Hexode
 
Join Date: May 2013
Location: Middlesbrough, North Yorkshire, UK.
Posts: 411
Default Re: Zx81

I have bought another ZX81 of eBay! (I have three now; none of which work perfectly!)

However, with this latest one-this is really interesting. It is an issue one model dated 1980. I have modified the output so that it works perfectly on LCD. The ribbon is badly damaged though. When I opened it up someone must have tried to do something because it’s ripped. I’ll have to source a replacement unless I can come up with something to link the keyboard and input.

However, I have tried it with another keyboard attached and it works really well except, that it won’t load from a cassette. Every time I try, the unit freezes and I have to unplug it. Could this be a bad ULA again? Is this worth starting a new thread for?
John Earland is offline   Reply With Quote
Old 28th Mar 2020, 1:29 pm   #210
John Earland
Hexode
 
Join Date: May 2013
Location: Middlesbrough, North Yorkshire, UK.
Posts: 411
Default Re: Zx81

I’ve just spotted a replacement keypad on Sell My Retro which I’ll get - easiest option. Any ideas though about the data reading issue?
John Earland is offline   Reply With Quote
Old 28th Mar 2020, 8:11 pm   #211
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 4,227
Default Re: Zx81

You are quite fortunate that such an early machine (presumably with an early ULA) works OK with the LCD display after conversion to composite. In theory it should have the same 'dark screen' problem as ZX81 #2 which you gave away.

That location of yours seems to be some kind of Bermuda Triangle for cassette mechanisms (loading problems on PET, Spectrum and ZX81).

The first thing I am wondering is whether you are still trying to use a stereo lead, where the contacts on the ear / mic sockets on the ZX81 and Spectrum (which are the same) are designed / spaced for a mono plug. Are you now using a mono lead? If not, you are likely to have problems because the contact spacing on the sockets is such that one of them tends to short two sections together on a stereo plug.

When you say the machine freezes and you have to unplug, when exactly is that, you go load "", the screen goes blank and you don't see any squiggly lines when the program starts playing from tape? Can you break out of that state with 'Break'? If you can, the machine has not frozen or crashed, it is just failing to see any audio coming into the EAR socket for some reason.
SiriusHardware is offline   Reply With Quote
Old 28th Mar 2020, 8:40 pm   #212
duncanlowe
Heptode
 
Join Date: Jan 2016
Location: Stafford, Staffs. UK.
Posts: 780
Default Re: Zx81

The POKE command reminds me of my schooldays. The school bought a TRS80. Very few were allowed near it, but of the pupils only two of us were. We were trying to develop a game. To put something on screen we would POKE a value to the graphics RAM. So during our experiments to do this, we would POKE a character to the first location, then POKE a blank, then add one and POKE a character again. A loop would then get the character to move to each location on screen. Problem is we miscalculated the number of character positions, and we think we ran over the end of the video RAM and into ROM. Whatever the actual explanation, it broke the computer. I'm quite happy for someone to prove me wrong and tell me that ROM wasn't immediately after video RAM on a TRS80 or any other explanation, but I'm still fairly convinced it was a design flaw. So POKE with care.
duncanlowe is offline   Reply With Quote
Old 28th Mar 2020, 8:57 pm   #213
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 4,227
Default Re: Zx81

Offcially this can never happen, ie, the user should never be able to do anything from the keyboard which ends up damaging the hardware.

In practice, maybe it is possible with badly designed address decoding, such as would allow the user to try to write to ROM, which is read-only. If the ROM activates (in read / output mode) at the same time as the system is trying to write to it you end up with a bus battle between devices. One of them could potentially cause damage to the other.

With regard to the TRS80, I assume this is based on the Z80 which usually begins executing from address 0000 after reset, so the ROM is usually the first thing in the memory map and everything else comes afterwards. It is unlikely therefore that the ROM was mapped at a higher address than the screen RAM but I don't know the TRS80 at all, so who knows?
SiriusHardware is offline   Reply With Quote
Old 28th Mar 2020, 9:13 pm   #214
John Earland
Hexode
 
Join Date: May 2013
Location: Middlesbrough, North Yorkshire, UK.
Posts: 411
Default Re: Zx81

Thank you for this. Would you believe it-itís now working! I found something you wrote Sirius way back (2016 I think) in a forum for someone who was having exactly the same issue as me! I played around with the cassette player and have obviously got the correct volume level. I did try this first actually before I wrote on here but now it seems happy! Iíve even loaded the 16k Asteroids game. It seems (everybody fingers crossed) to be working well. Yes, I have to say Iím a bit stumped by the light video. In fact when I first tried this on the crt I didnít get a great picture-very, very unstable. It looked dark to me just as I has expected.

However, I thought I would have a go at the composite mod to see what would happen. To my amazement itís a perfect picture! There is an occasional bounce ever so slightly now and again but nothing that bothers me.

Interestingly, the cpu, the ULA and some other ICs are not soldered in making replacing them easy!

Picture attached.
Attached Thumbnails
Click image for larger version

Name:	583480C4-ABB9-4F7B-85ED-8D11247951DB.jpg
Views:	30
Size:	28.9 KB
ID:	201732  
John Earland is offline   Reply With Quote
Old 28th Mar 2020, 9:22 pm   #215
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 4,227
Default Re: Zx81

Well, now that you've got this far you could (temporarily) repair this machine (Henceforth known as ZX81 #3) by fitting the lid / keypad from ZX81 #1, the one which has the ULA problem. (Maybe you've already done that, otherwise how were you playng Asteroids)?

That will give you all the time in the world to attempt the ULA change, as you can be using ZX81 #3 while you fix ZX81 #1 at a relaxed pace.
SiriusHardware is offline   Reply With Quote
Old 28th Mar 2020, 9:58 pm   #216
John Earland
Hexode
 
Join Date: May 2013
Location: Middlesbrough, North Yorkshire, UK.
Posts: 411
Default Re: Zx81

Yes-that’s what I did! Not bet good at Asteroids-never was but now I have time to practice!
John Earland is offline   Reply With Quote
Old 28th Mar 2020, 10:25 pm   #217
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 4,227
Default Re: Zx81

As previously mentioned, you should also try '3D Monster Maze'. Genuinely nail-biting, but it will make you curse that rather insensitive keyboard whenever it fails to translate your lightning reactions into actual actions.

As far as the cassette loading is concerned, basically the way to tell if you have the volume set correctly is: The black and white lines you see on the screen during loading should be approximately the same width as each other. If the black lines are much thinner and spindlier than the white ones, the volume is not high enough.
SiriusHardware is offline   Reply With Quote
Old 28th Mar 2020, 10:47 pm   #218
John Earland
Hexode
 
Join Date: May 2013
Location: Middlesbrough, North Yorkshire, UK.
Posts: 411
Default Re: Zx81

I’ll remember that. It was always easier with the Spectrum because you could hear it at least! I have a cassette with a range of 1K games! Unbelievable that now isn’t it? Amazing really. Asteroids is the only 16k game I have-I have though downloaded a whole lot of software from a website so I shall transfer them to a blank cassette and give it a try!
John Earland is offline   Reply With Quote
Old 29th Mar 2020, 12:09 am   #219
emeritus
Dekatron
 
Join Date: Jan 2012
Location: Brentwood, Essex, UK.
Posts: 3,700
Default Re: Zx81

AFAIR, Asteroids was a game that was included in the deal, along with a 16k Rampak, when I bought my ZX81 from W. H. Smith. Still got it somewhere, along with the Rampak.
emeritus is offline   Reply With Quote
Old 29th Mar 2020, 12:48 am   #220
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 4,227
Default Re: Zx81

All of the 'launch titles' for the ZX81 were by Psion, presumably the same company who later gave us the 'Organiser'. Ones I remember are:

Space Raiders (Space Invaders? Included 'Bomber' on the 'B'; side)
Fantasy Games (Two D&D / RPG type games)
"Thro' The Wall" (Breakout? Included 'Scramble' on the 'B' side)
Flight Simulation *
Chess (16K)
Backgammon
Vu-Calc (Spreadsheet)
Vu-File (Database)

Can anyone remember any more ZX81 titles by Psion? (Not to be confused with Psion's Spectrum titles, of which there were quite a few).

The Psion box cover art was to a very professional standard, I suspect by the same hand as whoever did the illustrations on the ZX81 and Spectrum user manual covers.

* The graphics, especially out of the cockpit, were nothing to look at but it was a great trainer for instrument flying and navigation since there was almost zero external detail. I played this one to death and all the basic stuff learned on that one came in very useful when I carried on 'flying' much more realistic simulators on later generations of computer.
SiriusHardware is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 6:20 am.


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 - 2020, vBulletin Solutions, Inc.
Copyright ©2002 - 2020, Paul Stenning.