How to install a ZX81 16K Ram expansion - 10 minute mod!

[@pinkmouse4863]

Time for a 3D Monster Maze allnighter stream methinks! :)

[@backofficeshow]

You are the man who made it happen

[@redhawk668]

The RAM chips inside the ZX81 can actually be used as color RAM on C64 longboards and early shortboards. So, it makes sense getting them out intact.

[@j0hnf_uk]

When you switched on after installing the memory, I was expecting the computer to have the slight delay before the command prompt came up, and sure enough, there it was! That would be enough to convince me, but obviously, the peek address program is confirmation of it having worked. 16k was considered a lot of memory, back then, although I do think there were some unofficial 32k, 'RAM packs', available, but they were expensive, (back in the day), and not that common. Weird to think that a tiny bit of electrical work, as shown here, can get rid of the frustration of a crash for having breathed too hard whilst having just spent the best part of a hour typing in machine code. The machine itself can't compare with other machines that came out a year later. No sound and no colour meant it was on a loser from the start. However, as a first computer for someone like myself, aged 11, at the time it was fascinating to be able to learn how to program and write your own within a week of first starting to learn BASIC. All I needed in addition was a portable black and white TV, and somewhere to use it. I remember first starting at the beginning of the Summer holidays, back in 1981, and by the time I started school in September, I knew how to program and was writing all kinds of stuff. Great days. I never liked the ZX Printer. It was too small for any practical use, as the paper, (if you can call it that), was only about 2 inches wide. Printing involved it literally burning the silver off the surface of the paper, which, if you printed anything dark, meant you got to witness a horrible smell of burning. As the paper was in a roll, it would often curl itself up and was prone to creasing, which, if it happened enough, would result in black lines where the silver had deteriorated coming off, leaving the paper almost illegible. Regarding this upgrade; I'd be tempted to want to go the whole hog and install the 32K.

[@backofficeshow]

Thank you for sharing your insights! I think the 32k is not a particularly challenging installation. I suspect after installing the 16k you could upgrade this by removing the hi address line from the pullup and going via the additional logic. Definitely worth a try for the ultimate zx81!

[@Rendell001]

​@@backofficeshow One wonders what the maximum amount of Ram that the machine could handle would be? I know there was a 64K Ram pack from Memotech, they seemed to have produced a range of add ons for the ZX81. I heard very recently that some enterprising soul even managed 128k (!) on a ZX81 and as a test for it, he succeeded in porting Dragon's Lair to it in hi-res black and white! I believe you can find the footage of that here on YT...

[@backofficeshow]

@Rendell001 I am guessing it would be a maximum of 64k directly addressable ram and as much as you would like paged

[@matzebautwas8901]

As far as I know: Mounting the 2k Jumper, as you have done, will connect Pin 21 of the socket to A10. So it is not necessary to bend this pin and solder the wire between Pin21 and A10.

[@NeilGrevitt]

I’ve loved the ZX81 series. I was given one as my first computer when I was about 9 or 10. Still have it, but it’s not been powered up for a very long time. This has inspired me to investigate digging it out and getting it back working!!

[@backofficeshow]

That's wonderful Neil, the best thing is, there is so little on them up go wrong, it will probably work just fine.

[@alanclarke4646]

Yes, the ZX81 checks for RAM counting down from &H7FFF to &H3FFF as part of the start up routine. So with 16 times the amount of RAM, it takes 16 times longer!

[@Paul-bu1te]

A great tutorial. My board is slightly different, as it’s an issue 1 board, but still worked fine. Thanks for sharing this.

[@backofficeshow]

Wonderful!!! Thanks for the feedback, I never know if my videos are useful to anyone 😂

[@redhawk668]

I’ve designed a little PCB for this, so no need to bend pins. Just solder a socket, put the PCB in, set jumper LK2 (A10), solder A11, A12 and A13 and you’re good to go. The ZX81 was my first real computer, I remember the rampack wobble....

[@arcadesunday4592]

Nice mod! I had this exact problem when I was around 10/11 years old! Sat and manually programmed a game from a magazine, and before you could save it to cassette... Small bump... Oooops!! That 16k pack also got super hot! How times have changed!!

[@backofficeshow]

That's interesting about the heat, I wonder what they had going on in there? 🤔

[@JimforbesRitte]

@@backofficeshow I seem to recall there was a coil in mine (haven’t got it any more, unfortunately!), but the Sinclair pack used 4116 ICs like the speccy, which require a 12v supply, and potentially a -12v line too. That might explain the heat, as the zx81 only has +9v and +5v lines, as there will be some iffy voltage conversions happening...

[@spudhead169]

You could use a switch (or an IO port for that matter) to pull the most significant bit of the address lines up and down in order to page in the unused upper 16k of the chip. Not sure what use that would be though.

[@markusjacobi-piepenbrink9795]

Thank you for excellent video! What soldering iron do you use in the video?

[@paulb4uk]

Very helpful i shall have to do mine

[@backofficeshow]

Absolutely, I think it's mandatory 😂

[@dreamvisionary]

I like the look of your soldering iron. What's the make/model?

[@backofficeshow]

kzbin.info/www/bejne/i3Kmi4CrbNRjbM0 It's a sugon t26

[@mibnsharpals]

this type is the cheapest and most convenient way to expand the zx and 99% of all programs get by with 16k. In addition, the zx81 takes a relatively long time to run the ram test. The area on top ​​32k is also required for the screen structure. The z80 is used as a line counter for the composition of the image. In the lower area you can change the decoder so that the ROM is not mirrored, so you can still add 8k , but these are not seen by the basic. But it will not deleted when you type in NEW.

[@PedroJafet]

Hello, I did the installation, step by step, on my Timex 1000. It's showing the white screen, but without the cursor. I know it would take a few seconds, but it doesn't show up. I'm guessing that when using the socket for the memory, soldering only occurred on the lower tracks. It makes sense? I'm thinking of extracting the socket and soldering the memory directly to the board. What do you think?

[@daishi5571]

I got a laugh out of the line "that looks better than factory" I bought the Kit version and soldered it together using my dads frankly horrible soldering iron on the dining room table. Worked 1st time, I was so proud but couldn't tell my parents as I had been forbidden to buy the computer (they found out later, but by then they had discovered this was something I was going to do no matter the punishment). And my 16K RAM pack cost more than the computer lol

[@backofficeshow]

Brilliant, must have felt amazing to see it boot up!

[@EmmittBrownBTTF1]

Replace the 7805 power regulator with a new switch-mode regulator - reduces heat.

[@9thbitvideos730]

it wouldnt be a zx81 without the heat :)

[@claudioquintanilla1471]

that's good!! to use a switching voltage regulator... less heat inside means extend chip life...

[@EmmittBrownBTTF1]

@@claudioquintanilla1471 exactly, especially caps.

[@StephenBlower]

Hardly a 10 minute mod when the video is 15 minutes and stuff was cut out. LOL Still a good tutorial.

[@LeftyLabs]

There are dozens of 32K DIP-28 SRAMs available from the usual suppliers (in the US, Digi-Key, Mouser, etc). Are there recommendations for any particular brand or version over another? I have an Issue 3 I’m repairing/restoring now and I’d like to upgrade the internal RAM while I’m doing all of this work anyway.

[@GrahamTinkers]

There was no need to bend or cut the legs on the IC, simply remove the pins from the IC socket before installing it.

[@6581punk]

Remove the entire wiper from the socket? that works. When I knackered a PCB trade I removed a bit of plastic and soldered to the wiper on the socket, so I could still remove the chip easily.

[@backofficeshow]

They still need soldering onto though. I think a little daughter board might be a nice move, perhaps with a few jumpers and the ability to use the full chip. If make one of I didn't think the market for them was 20 units globally 😂

[@6581punk]

At school we used to joke about the ZX81. One of the teachers said it wasn't that bad a computer and it was used as a car engine management system. I hope that was just a myth :)

[@backofficeshow]

I wonder what car 🤔

[@bazza5699]

@@backofficeshow C5

[@KevinRiley]

@@backofficeshow the sinclair C5 of course!

[@backofficeshow]

I thought it a hoooooman powered

[@RetroBoxRoom]

If a ZX80 could be used to control a power station, I'm guessing you could do a whole lot more with a ZX8116K. LOL

[@JCMayPE]

Solder wick: use more flux!

[@bazza5699]

can you see any noticeable difference in programs since the ram increase?

[@backofficeshow]

Not really, but mainly owing to the fact I don't have any

[@RetroBoxRoom]

There's a huge increase in ZX81 games that use 16K. For example, you can play games that have "Spectrum like graphics", from the likes of "Software Farm". There's been a few more hi-res games made since. A ZX81 needs 16K, to be some what useful at all.

[@backofficeshow]

Any that need 32k? It's it work continuing this mod to use the whole chip?

[@RetroBoxRoom]

@@backofficeshow Some games only need 16K, there are others that have been made for 32K. I know there's stuff out there for 64K, but I can't remember if it was internal or external. (It was of course external back in the day)

[@joefish6091]

@@RetroBoxRoom 8K to 16K is vacant, so a max of 56K if no trick character unit Installed. To be honest, the 16K install is good enough for casual retro use or demonstration purposes. It makes the computer typical. A composite output mod would be useful, a dedicated LCD display would sweet but prob tricksy.

[@gdutfulkbhh7537]

It looks so weird to see a PCB without a solder resist layer on the top...

[@backofficeshow]

Yeah. It used to be out much the only style of PCB you would see in cheap things

[@davidw.2467]

If clearance is not an issue, bending and cutting the legs of the CY62256 sram could be avoided by using 2 stacked turn-pin sockets, cutting the legs and soldering the required jumper wires to the upper one.

[@vanhetgoor]

And what is wrong with a 32K RAM expansion?

[@backofficeshow]

Nothing, but if I recall when looking into it you have to deal with the extra address line so it's not a drop in replacement

[@joefish6091]

Just a little bit pointless in 2022, a full 48K or even 56K could be done if really keen. Building a ZX Specrum clone ie Harlequin, Pentagon, or Nucleon would be a bether move. Then theres the ZX-UNO which uses personality code modules to emulate many different types of 8bit computers.

[@ScottsGameAsylum]

you could make so many out of context clips on this video.

[@backofficeshow]

Really? I thought I was exemplary in my professionalism

[@SolarLantern424]

Theres some games and other software that work on a 32k zx81: kzbin.info?search_query=32k+zx81 To not use the whole 32k of space seems a waste to me.