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!

Re: Zx81
 

I've just tried cleaning it and on the one pin hole it appears to be burned. Luckily the connection is not the other side which is intact. Don't know if that will make a difference?

Re: Zx81
 

The problem with nail varnish remover is that it is acetone based which attacks plastics and unless you were very quick to remove it i imagine it will dissolve screen printing and solder resist. It also usually contains. Oils to mitigate drying of the skin which might be annoying to get off. You could try meths or surgical spirit which are similar to IPA and more likely to be lurking in a cupboard or the local chemist (although ive not tried buying meths since the 70s when I had a mamod steam engine!).
Looking at the photos theres not too much flux residue and i'd probably leave it if I didnt have anything to hand. The main risk from rosin flux residue is that in a damp environment it can absorb moisture and become corrosive and possibly conductive - but hopefully this board wont be kept in a damp place! I have circuits i built in the 80s with flux residue without any detectable corrosion because they have been kept dry.

Re: Zx81
 

I take it the burned pad is one which has no tracks connected to it on the lower side of the PCB? Those are the ones most prone to being burned off, because there is no track to conduct some of the heat away.

There are three aspects to each solder hole on a double sided PCB - a pad on the top side of the PCB (which is OK here) a pad on the underside which is (presumably the burnt one) and a hollow tubular conductive sleeve running through the hole, connecting the bottom side pad to the top side pad.

As long as the top side pad and the tubular sleeve are intact you should be able to run solder onto the pin and it will run down inside the tubular sleeve and connect the pin to the top side pad and anything which is connected to that.

Re: Zx81
 

Ive repaired connections to ICs where the damaged pad is on the top by wrapping a very fine wire (one strand from a multistrand wire) around the pin on the socket, carefully soldering the wire to the pin, then fitting the socket and soldering the 'tail' of the fine wire to the track. This works even if the through-hole plating has come away. You can even connect to both sides of the board with a little care if required.

Re: Zx81
 

Thanks for the suggestions. There is a hollow tube. The conection on top runs into this hole and through to the bottom and it is the bottom which is burnt - there isn't any obvious underside connections to that hole.

Re: Zx81
 

In that case I think you will be OK just running solder onto the pin and down inside the hollow sleeve.

Re: Zx81
 

Ok guys-I’ve soldered in the socket! I would appreciate your comments regarding the soldering?

Re: Zx81
 

Is that an unsoldered pin, seventh from left on the bottom row in your photo? Also clean / flick off the splash leading away from the leftmost pin, upper row.

Otherwise, not bad if that is the first time you have ever soldered a socket in.

Re: Zx81
 

Thank you but I’m not out of the woods!
Okay so I put in the replacement ULA and I got a picture! It was very fuzzy and grainy with very thin strips/horizontal lines-and was flashing colour so I fiddled with the switches on the ULA which stopped the flashing and in some configuration stopped the picture i.e. the cursor disappeared. So I guessed it might be something to do with the RF mod I did-so I’ve taken it out and restored it (all in the picture). Sadly, now I can’t get anything at all. I’ve checked the 5v and video connections into the module and there is a good connection between going in and through the components. So I’m thinking of putting the mod back in but not sure where to go from there. It’s highly likely of course that my first time socket soldering has not been a success!

Re: Zx81
 

Can you post a link to the instructions which came with your replacement ULA?

One of the things I remember about it is that the output is switchable between composite suitable for input to the modulator / composite suitable for direct output to the composite input on a monitor or TV.

In other words, this ULA does not require the transistor buffer, you can do the simpler straight-through mod as on your Spectrum if you have the ULA output configured the right way.

Did we not come to the conclusion that the RF modulator was also faulty in that machine? I would suggest the simplest configuration with this new ULA is to set the output for direct composite out and do a straight-through mod from modulator input to output.