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!

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!

Re: Zx81
 

(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.

Re: Zx81
 

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 - I’ll check the demagnetiser out

Re: Zx81
 

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.

Re: Zx81
 

Components which work when cold but fail when hot are not uncommon. That's why freezer spray is 'a thing'.

Re: Zx81
 

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!

Re: Zx81
 

Thank you for this. I understand now.

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?

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?

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.

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.

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?

Re: Zx81
 

Thank you for this. Would you believe it-it’s 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! I’ve even loaded the 16k Asteroids game. It seems (everybody fingers crossed) to be working well. Yes, I have to say I’m a bit stumped by the light video. In fact when I first tried this on the crt I didn’t 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 it’s 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.

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.

Re: Zx81
 

Yes-that’s what I did! Not bet good at Asteroids-never was but now I have time to practice!

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.

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!

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.

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.

Re: Zx81
 

I think the flight simulator one is in the cassette I have-it’s an old cassette with a tiny insert explaining all the games. I’m amazed how speedy the Asteroids game plays. Was a little confused at first when the starship kept changing from 0 to other numbers as you turn the craft around to fire at the incoming asteroids! Thought something was wrong!

Re: Zx81
 

Can’t seem to find 3D Monster Maze anywhere.

Re: Zx81
 

Not sure whether you mean you can't find an original tape for sale - it's probably the one that people are least likely to sell, but from what you have written earlier you may already have some material found in .TZX or .P file format so I assume you have a way of converting those to audio playback format.

Have a look around on this site, where there is a lot of useful archived material including books, various utilities, and other things.

http://www.zx81stuff.org.uk/

For example, here:-

http://www.zx81stuff.org.uk/zx81/tape/3DMonsterMaze(NGS)

I personally also had great fun with 'Seawolf', which is a kind of early forerunner of 'Silent Service', the submarine simulation.

http://www.zx81stuff.org.uk/zx81/tape/SeaWolf(Newer)

Re: Zx81
 

That’s great-thank you. No I don’t have any conversion software yet-will search for some. What I need is something to give me a simple wav format. I have some software to do that for the PET but can’t get it to work on Windows 7 even though it said it would run. I’ll keep searching!

Re: Zx81
 

There is a utility to convert .P and .TZX files to audio files on the site I linked to in #223.

http://www.zx81stuff.org.uk/zx81/tap.../overview.html

Re: Zx81
 

Thank you for this-Ill check it out.��

Re: Zx81
 

If you see anything you think might be interested on that site, have a go at it by pressing the 'play' button on the page for the software title concerned and you can test drive it using the website's built-in full speed online emulator. All very nicely done.

I'm fairly sure I have that converter software installed on one of my XP machines so if you have trouble getting it to work let me know via PM which ones you want and I'll have a go at converting them for you.

Re: Zx81
 

Thank you! I’ll take a look.

Re: Zx81
 

3D Monster Maze was *the* game back in the day and I understand that in more recent years young retro gamers have come to appreciate it too. Some play it with huge screens and describe the moment the T-rex appears on screen as "pants wetting".

Other games I'd specifically recommend for the ZX81 are the more recent "Mayhem"
https://www.homebrewlegends.com/zx81-mayhem/

I still get kicks out of "Perilous Swamp" from the "Fantasy Games" cassette, and Catacombs another JK Greye title.

Artic's Galaxians was great
Mazogs

Re: Zx81
 

I cant remember if you could buy it as a kit or made up like the ZX80?
Bought mine ready made direct from Sinclair originally , and then bought another from WHSmith a while after .

Re: Zx81
 

Yes, mine (My ZX81) was a kit.

Re: Zx81
 

I made mine from a Kit which guess what - didn't work! So had to go back to Sinclair - their diagnostic helpline asked me to put my finger on the ULA and I said "Ouch its hot" - so he said - hmmm not that then. Turns out the Z80 was faulty so they repaired it FOC but, it was several agonising weeks waiting for it to come back, just reading the manual. I still remember the feeling and love looking at my original manual with its pencil notes.

Re: Zx81
 

And they were great manuals, weren't they? I notice the NEXT manual looks to be of similar quality.

Had to laugh out loud at the anecdote regarding the warm ULA, operating temperature never has been any guide or clue to whether a Sinclair ULA is OK or not. If the ULA is red hot and the computer is working, the ULA is OK. If it's red hot and the computer is not working, the ULA might still be OK. No wonder they caved in so often.

Re: Zx81
 

For those interested , I've found an emulator based around the zx spectrum 48K & 128k models. It's called BASin, and the package contains a lot of useful information. Nice to play with to get back to the old days .

Re: Zx81
 

I still can't believe all the time I spent writing machine code, typing it in as hex, saving it, then running it and it crashing and having to load it back in from tape again, modify, try again. A 16K program took about 20 minutes to load! The worst thing was there was no way to verify, and often it would wait until the end of the recording before deciding there had been a read error somewhere and crashing. But I persevered. I managed to write the first level of the arcade game Scramble. A lot of work.

For some while now I've had an ongoing off/on project of writing a full screen, full feature version of Space Invaders for the 1K ZX81 (I use an emulator these days for reasons stated above). Haven't managed it yet. It's nice to have an unattained goal. I was very fond of my ZX81.

Re: Zx81
 

I remember someone created a VERIFY routine for the ZX81, but I'm afraid any copy I had is long since gone by now. The code simply sat in the usual REM statement in the first line of the program, and a successful SAVE could be VERIFYed by evaluating USR 16514.

It shouldn't actually be very hard to recreate, with access to the ZX81 ROM disassembly. You would have to base it on the LOAD routine in the ZX81 ROM; but instead of storing the byte read from tape into memory, just CP it against the byte at that location. Note the first few bytes of the file are system state, and could have changed since the program was SAVEd! It should be obvious from the code what is what. You can bomb out with an error code straight away in the event of a mismatch, or signal no error if you made it all the way to the end of the file.

If a 1K invaders game is going to be possible, you'll have to use nearly all the available RAM for the screen display and so pull optimisations like increasing the score in place in display memory and stomping on anything you aren't going to drop back to BASIC and need again.

Re: Zx81
 

Well, someone famously did manage to write a pretty respectable 1K chess game although the game board itself was shunted into one corner of the screen.

Re: Zx81
 

Well I don't think there's much call for a verify routine in this day and age, even a ZX81 enthusiast wouldn't actually develop on the actual hardware any more. I wish I'd had one in 1981 though!

The 1K Invaders thing has been through several ideas involving custom screen buffer routines (in fact this thread just started me thinking of another approach) but please don't help me, it's something I like to contemplate on and off and maybe one day actually achieve.

And nowadays at least I have an assembler on PC to write it with!

Re: Zx81
 

Okay, without encroaching on your ideas for how to tackle the programming project, how will you load the finished code onto a real ZX81? Just interested.

I don't even know how developers did that back in the day, I assume it was rather unlikely that professional devs wrote on the actual machine (although I'm sure the one-man back bedroom programmers did, in the early days).

Re: Zx81
 

Well the easiest way without any hardware would be to play it into the cassette port from a WAV file :)

I don't know myself how people did it in the day. IIRC I think I read that Nine Tiles, who did Sinclair's software development, had some sort of emulator running on a minicomputer. There wasn't much of a commercial scene for the '81 and other than Sinclair themselves it was pretty much a cottage industry so they would've used their own proprietary solutions I would guess. For instance you could burn code onto EPROMS, or map some kind of interface into the spare memory map, that sort of thing.

Wouldn't be surprised if some of the few were doing the same as me, saving onto cassette and hoping for the best. With the exception of the legendary 3D Monster Maze, even the few 3rd Party commercial programs were pretty, um, basic.

Re: Zx81
 

Well, think that through. Unlike the later Spectrum the ZX81 did not have a specific tape format for loading and saving binary files, so the only thing it could load from tape, technically, was BASIC. Somehow, you have to get the code which your assembler has produced into a format which looks to the ZX81 as though it is a BASIC program.

I don't know if the low level tape routines were able to be called by the user, but if they were then perhaps a short piece of machine code contained in a REM statement could launch a loader utilising the tape routines in ROM to load in raw data. Alternatively a larger REM statement could contain a self contained 'loader' which would load the 'raw' code in from the tape.

In more recent years we have seen the emergence of tapes represented by files and sometimes converted back into tapes, so examination of how the .TZX and .P file formats are constructed may yield clues.

I think you are probably right that the larger and more prolific software houses started to use hardware emulator / development systems, although probably more towards the peak of the 8-bit era, your Spectrums, C64s, Amstrads, BBC Bs, etc.

Re: Zx81
 

Well, this being the 21st century, one uses a modern IDE-

https://www.sinclairzxworld.com/view...php?f=6&t=1064

It's amazing what we have these days, thanks often to the unpaid toil of enthusiasts. Just forums like this are amazing compared to the days of thumbing through magazine articles for hints and tips on technical hobbies. Datasheets a Google away, circuit simulators and, God bless us, even an advanced programming environment for the ZX81!

But we still pack our assembler into REM statements in the end...

Re: Zx81
 

I wouldn't be surprised if Nine Tiles had some sort of minicomputer. The order of the cursor keys on the Sinclair machines is 5=left, 6=down, 7=up, 8=right, which is a lot like H=left, J=down, K=up, L=right. Almost as though they were used to vi!

Re: Zx81
 

I bit the bullet and removed the faulty ULA and used the desoldering device! Not too bad for a first attempt-though one of the socket holes looks tinged! I’ve tested the connection and it’s still intact it seems - honest comments welcomed. Not sure I am confident enough to solder in the new replacement though.

Re: Zx81
 

Well, the main hazards were:-

-Heating the pads too much so that they came away from the PCB
-Trying to remove the IC while it was still attached to some pads, resulting in the pads coming away with the IC.

Neither of those things happened, so well done!

If you have some 'proper' PCB cleaner or something which will do (suggestions anyone?) you mind find those burnt looking bits are just left over burnt flux and will clean off.

I would say you have done by far the hardest bit. Do you have a socket you can fit in the holes where the IC was removed from?

Re: Zx81
 

Yes I have a socket which I can use and I ought to have a go really!

Re: Zx81
 

A few reminders:

-Try to remember to fit the socket with the 'notched' end (the 'Pin 1' end) nearest to the edge of the PCB, looking at your second photo. This serves to remind you (and anyone else) which way the IC should be inserted into the socket.

When soldering a large socket, IC, or other device into a PCB, solder two opposite corner pins first, say pin 40 first, followed by pin 20. Then have a look at it, if it is not sitting right down flat on the PCB, apply gentle pressure on the socket or device and briefly melt the solder on each corner pin in turn until the socket or device settles down perfectly flat on the PCB.

This is easiest to do if you are holding the PCB vertically by the edges between your thumb and little finger and you use one of your middle fingers on the same hand to apply a bit of pressure to the centre spar of the socket. Your other hand, of course, is needed for the iron.

Once you have the socket on perfectly flat and with two opposite corners soldered, you then have all the time in the world to solder the other 38 pins.

Your soldering looked greatly improved the last time we saw some, so just remember, heat both pin and pad with iron for several seconds, dab solder on hot joint, remove solder, remove iron.

Re: Zx81
 

The best thing for cleaning off rosin flux residue is 99% isopropyle alchohol which is cheap on eBay and has the advantage over other solvents in that it doesnt attack most plastics including importantly PCB materials. Be careful about kitchen cleaners claiming to be IPA because they are usually lower in concentration and may contain other chemicals.

Re: Zx81
 

I use IPA here at work for that purpose, but I was assuming John wouldn't have any handy. At the moment it's not just a simple matter of strolling out to the shop to get stuff. (A pharmacist might have some, I guess).

I was thinking maybe something like nail varnish remover or some other common household solvent. It has to be something which won't lift the screen printing or the red solder-resist (come to think of it, nail varnish remover probably will).

Re: Zx81
 

Thanks everyone - I do have some 99.9% Isopropyl alcohol -acquired before the lockdown! I shall give it a go!