Re: Zx81
 

I found them! Please don't go to any trouble. I have a picture and it is as was before. Good resolution but wobbly image. I shall load in a game and let it run.

Re: Zx81
 

OK, it will be interesting to see if your ULA still breaks down when it is no longer driving the buffer transistor. If it keeps on going indefinitely with no change - don't forget, you may need to tweak the tuning on the TV every now and then as the modulator frequency drifts - then I would say have a go at fitting the CCU PCB.

Re: Zx81
 

A little update:
1. I found a spare rf lead and thought I would try that - the result was excellent! I think the cable was faulty to start with! (Picture attached)
2. I kept the computer running for about 2 hours and there were no problems!
3. I will try the PCB as planned.

I will report back.

Re: Zx81
 

To be honest I thought you had already tried a second RF lead (thought you had two of everything). I guess you'd still rather have a composite output to make it more interchangeable with the other machines.

Re: Zx81
 

No-not two of everything-only psu and ‘81s. Yes, still would like to use it on composite.

Quick question-once in composite mode could this be plugged into a computer monitor via an adapter? I don’t plan on doing it but thought it might be possible.

Re: Zx81
 

I assume you are talking about a PC monitor, not an RGB / Video monitor?

If a PC monitor with VGA, DVI or HDMI input you need an intelligent electronic converter which takes in the composite signal and converts it in real time to the desired format, it can't simply be done with a passive adaptor (ie, a piece of cable with the right plugs on each end)

There are any number of generic intelligent hardware video format converters available now, probably including composite to VGA and composite to HDMI, but I can't personally vouch for any of them.

If you mean a typical standard definition video monitor of the sort which would be used with the BBC B / Atari ST / Commodore Amiga, those monitors usually have a composite input.

Re: Zx81
 

You can get composite to VGA converters for £10-15 but you might end up with sensitivity to video standard compliance as you do with LCD screens. I've also read that some composite to HDMI converters can suffer from noticeable lag which makes them less suitable for gaming. I had a comp-VGA converter to use on my Atari 800xl and had no problems.

Re: Zx81
 

Thank you for that. I am happy with the LCD to be honest but thought perhaps in the future sometime.

Here’s an interesting thing. I just put the ‘81 together and when I went to put the kid back on the modulator the picture went completely. It seems when you put pressure inwards on the casing it causes picture issues! I’ve managed it but took a little while. I checked everything and can’t see any sort of short etc.

Re: Zx81
 

I think all that happened was that you shifted the output frequency when you put the lid on - as mentioned earlier, the 'transmitter' in the modulator is just a simple free-running oscillator and putting the metallic lid on it probably just made the transmitter frequency detune to one side a bit.

All you likely needed to do after putting the lid on (at which point the signal disappeared) was to retune the TV a little.

You can test this theory if you like: Tune it in as best you can with the lid on, and then flip the lid off - does the signal disappear? If so, tune around and find it again. Then, put the lid back on - does the signal disappear again? If so, tune around again until you find it again.

If everything in the previous paragraph is true, then all that's happening is that the modulator output frequency is changing slightly when the lid is present / is not present.

Re: Zx81
 

Yes, that makes sense. I will try the PCB today. I feel more confident about the soldering now. By the way, I bought a new soldering kit with temperature control so I hope my skills improve further. It also came with a desoldering unit.

Re: Zx81
 

I tried the '81 this morning and left it running without modifications for just over two hours - unfortunately the same thing happened. The unit rebooted and then the picture was lost. I tried retuning the TV but it was clear that it was the computer. I squirted the freeze spray onto the underside of the PCB where the ULS is situated and immediately the picture returned. I guess this means a new ULA? :-(

Re: Zx81
 

It does look that way, sorry. You could check other things, for example make sure the +5V supply is 5V when the machine is cold and is still 5V when the machine is running warm and the fault has appeared.

Cooling the ULA may just allow it to run on a bad supply voltage for a little bit longer (stranger things have happened). But at the moment it does look as though you have a thermal fault on your ULA.

Re: Zx81
 

Build the world's first water cooled ZX81!

Re: Zx81
 

Some of the retro parts sellers sell a full length low profile heatsink specifically to go on the ULA which, even when working 100%, normally runs quite hot. But I fear that this ULA may already be beyond redemption.

Re: Zx81
 

I did a look around for replacement ULAs and they're all out of stock. - replica cpld based replacements furnish lots of dead links. I guess it would require more dedicated research than the 10 mins I spent.

Re: Zx81
 

This source

https://retrorevivalshop.co.uk/index...roducts_id=560

...claims to have five of the ones made by the New Zealand guy (who also makes the Vdrives for the Spectrum). Have not used this source myself.

Re: Zx81
 

Oh-thanks for that! I’ll take a look-would love to have this little fella up and running again!

Re: Zx81
 

In the meantime maybe you can get your ZX81 fix by fitting the CCU PCB to the other ZX81, the one with 'dark' video output.

Re: Zx81
 

All this chat about ZX81s has made me get mine out of hibernation as im stuck at home now for the duration .
Might get the old Speccy out too and have a bash at Horace goes skiing !

Re: Zx81
 

Mine has been out a few times as well over the last few weeks - must try again now I have a tzxduino loading some .p files into it!

My Spectrum and the new Next are always on my desk...

Re: Zx81
 

Have any of you seen the new '81 software titles produced by Paul Farrow? His whole website is a treasure trove of interesting ZX things but here he's done a 3D isometric maze puzzle game in the style of Knight Lore / Alien 8 for the ZX81.

http://www.fruitcake.plus.com/Sincla...heElements.htm

Free to download, I believe.

Re: Zx81
 

There was a game called 3D Monster maze for the ZX81 was pretty good considering the constraints of memory.

Re: Zx81
 

Oh, that was virtually the 'Killer App' for the ZX81. I don't think anyone didn't have that game.

Re: Zx81
 

I remember playing that on my friends ZX81. A year or so previously I'd written a 3d maze program for my Commodore PET and I remember being annoyed I hadn't bothered to sell my game as everyone I knew with a Zx81 had 3D Monster Maze :)

Re: Zx81
 

I never had a zx81-Spectrum was my first and I loved Horace and the Spiders!

Re: Zx81
 

I never had a Spectrum but I was very impressed with it at the time. It's no wonder it spawned the careers of many a computer programmer, you got a lot of computer for not much money. It's a shame Sinclair perused the business market and didn't realise what an opportunity he had in the home computer/games market.

Re: Zx81
 

My recollection is that the ZX81 manual encouraged you to PEEK into the memory and see what was in the various locations. A good way of getting an insight into to what was going on. Data for BASIC programs usually had to be stored in REM statements.

Re: Zx81
 

I never quite got PEEK (still don't to be honest) but as I'm going to be working from home for some time, I might get out my old manuals and start!

Re: Zx81
 

Did anyone have any experience of the Sinclair ROM carts?
They are very collectable nowadays , but they have always eluded me.

Re: Zx81
 

For the ZX81? They were available for Spectrum (for the interface II), but I have never been aware of any made for the ZX81.

For some time now there has been a ZX81 hardware add-on called the ZXpand (see what they did there) which, among other functions, allows ZX81 software in .P format etc to be loaded from flash media. There was, possibly still is, a custom version of the ZX81 keyboard membrane available which has the ZXpand specific keywords / commands on it.

If we want to wax nostalgic about the Spectrum, maybe it's time for a specific thread about that too?

Re: Zx81
 

Yes sorry meant the spectrum. 8-o

Re: Zx81
 

My diary from Thursday 11th June 1981 says:

"I also found out why the system crashes it has a 33rd column containing CHR$ 118 (NEWLINE) and POKEing this puts it off balance and crash"

Agreed on the new thread for Spectrum but, I will leave you with the note that the Spectrum Next contains the Farrow ZX81 emulator so you can play the included .p file of 3D Monster Maze - it works under the increased speeds as well...

Re: Zx81
 

In common with all computers of that era you got a very decent manual with the ZX81 which encouraged you to try to program the machine, at least in BASIC. Once the 16/32 bit home computers faded away and the PC became dominant that whole culture very nearly died altogether and has only been kept alive by a few hardcore enthusiasts who continue to write demos and come up with new hardware for their beloved machines. The resurgent 'Maker' scene owes a lot to Eben Upton and his Raspberry Pi, and other programmable platforms like Arduino and the Micro:Bit.

I think if I was trying to write assembly language code on the ZX81 now I would come up with some system of injecting the assembled code directly into the RAM through the rear edge connector - getting the Z80 to then run it automatically would need some extra trickery. No doubt someone has already done it.

Re: Zx81
 

Use your MK14 loader to type in POKE commands ....

Re: Zx81
 

Yes, in theory that approach can be used in conjunction with any computer with a row / column key matrix. I don't know if there isn't an upper limit on the number of characters one REM line can contain though.

Then again, if you first poke RAMTOP to be just above what is needed for the OS to minimally run, then you can poke the RAM beyond that address with whatever you like and jump to the start of it with a BASIC RANDOMIZE USR address... statement.

It might be quite fun to watch a ZX81 'type in' and then run ~16K worth of BASIC program. From memory though the longer the program is, the slower the screen update after each line entry, so the entry speed might have to scale slowly downwards. (You can tell I am actually thinking about doing this).

Re: Zx81
 

I was just thinking POKE commands if they can be used on the command line but entering them as program lines would mean you could save it. But taking over the bus and writing direct to memory would be quicker if the problem of running it could be solved.

Re: Zx81
 

I think that on the ZX81, like its later big brother, you can switch out the onboard ROM and replace it with an external ROM just by pulling one line on the edge connector.

If you knew that the entry / execution address for your direct-injected code was going to be address nnnn, then you could clone the normal ZX81 ROM and just make a few changes to the code it runs at reset, so that it would jump straight to your code instead of initialising the RAM, which of course is the last thing you would want it to do.

This would also leave the majority of the original ROM content as found so that your code could make use of the OS subroutines, character set, etc, if required.

Re: Zx81
 

Actually, I think the ULA needs to be able to see parts of the ROM code at all times, not least the character / graphics set, in order to be able to maintain generation of the display.

Re: Zx81
 

I think I need to do a whole lot of learning!

Re: Zx81
 

Well the good news is that although ZX81s are stupid prices these days, the books are plentiful and cheap on auction sites. Also many are available as free downloads along with loads of other pages and forums full of info!

Re: Zx81
 

...and anyway John, first you need a working ZX81. So how's that CCU PCB installation coming along?

Re: Zx81
 

I haven't fitted it to be honest. I initially bought the ZX81 for my brother who is 50 this year and he wanted a vintage computer to tinker with. So, I thought I would either fit the mod myself or let him have the fun - so I've decided upon the latter!

I have ordered the vLA81 you mentioned though and will replace the faulty ULA. (Draws breath) - I may attempt the installation myself! I will practice on a small amplifier pcb to hone my soldering and desoldering skills and have a go. On the other hand I might wait until I can meet my friend again and ask him!

Re: Zx81
 

Here's a,summary of desoldering techniques.
https://m.youtube.com/watch?v=77JgIqraX_I
I've got a soldering pump but for years I used th flux and desolder braid technique. I desoldered 40 pin chips this way with no problems, as long as you do diagonally opposite pins to stop overheating the board. Once the solder has been sucked out wiggle each pin to break any adhesion - if it doesn't free up use more flux, more heat & braid. Practicing on an old board that doesn't matter is an excellent idea.

Re: Zx81
 

Thank you for this. I like the braid technique. What sort of braid is that? Can any wire be used?

Re: Zx81
 

Quick question - not entirely sure it's for this thread but there is a link:
I had modified the ZX81 using the transistor and two resisitor method-still awaiting the diode. I know this works for the ZX81, would this also work for the ZX Spectrum? I'm assuming it will.

Re: Zx81
 

Not ordinary braid but a product like this https://uk.farnell.com/chemtronics/8...ING-TEST-23MAR

Re: Zx81
 

Thank you!

Re: Zx81
 

The soldering braid is coated with flux so ordinary braid won't work. Use extra flux to maximise the amount of solder removed. In the video they used gel flux but you can use liquid flux it just is a bit messy, you can even "draw" on the braid with a flux pen (that's what I did). Rosin flux is a bit messy for desoldering as it leaves a hard brown deposit that needs scraping or washing off with isopropyl alcohol.

Re: Zx81
 

I don't really like to be the *Voice Of Doom* but I think it might be a bit early for John to try to remove a 40-pin IC intact from an antique double sided PCB, especially when there is absolutely no need to attempt that.

As with the keyboard decoder IC which was replaced in his PET, by far the better approach at this stage would be to cut all of the ULA pins high up beside the body of the chip, remove the body of the chip, desolder each pin individually and then use the desolder tool(s) to clear the holes ready for the replacement ULA (or the socket for it) to drop into.

Needless to say this should only be done when the replacement ULA is already physically in front of you - you don't want to chop up the old chip and then find that oops, there is a problem in the supply chain and you aren't going to get one after all.

John, if you do decide to try to get the original IC out in one piece as some sort of challenge, the one thing you absolutely MUST NOT do is remove 90% of the solder and then try to lever the chip straight up off the PCB with a screwdriver or similar wedge between the body of the chip and the PCB. If you do, some of the top side PCB pads and probably some of the through hole plating will come off with it. The sharp end of the screwdriver / lever may also cause damage to any PCB tracks under the ULA.

If you get to that stage the correct way to release the chip is to use a blunt wooden object like the square end of a small brush handle to push the chip from side to side, back and forth, until the little traces of solder which are keeping the chip in place crack and let go.

Let me put it this way: The PCB is irreplaceable, damage that and the whole machine may be lost. The chip is almost certainly duff, so it's better to destroy the chip and remove it in pieces than it is to destroy the PCB.

Re: Zx81
 

I would cut the old chip out but I wasn't sure if John wanted the option of refitting it. The best option is a desoldering pump, they're much less expensive than they used to be. I have one of the ZD915 ones that are around £100 and it is excellent. The plunger style ones at £5 work well if your not doing too many chips or you have the patience of a statue, just heed Sirius's advice about not forcing anything; if you've removed enough solder and freed the pins as described the chip will almost fall out.