ZX Spectrum 48K - How to replace the ROM with an EPROM

[@SpeccyMan]

I modded my ZX Spectrum with a custom EPROM back in the 1990s. I had built a 24 I/O board based on an 8255 PPI and the EPROM programmer was an add-on board I built for it. Great times. :D

[@ahmedalshalchi]

I did exactly but in 1988 and yeah those days were the best in my life in electronics when electronics means electronics !.

[@syrus3k]

@@ahmedalshalchi and you had no web to help you 😱

[@ahmedalshalchi]

John Hunt ... In 1988 and earlier , I was Hi-School student and was not sure if I could study electronics engineering at the university so I self trained myself using every small piece of printed tech. material I could get and self funded myself as well using spare time jobs.... They were the best years of my days in electronics but now I am certified professional electronics engineer with well experience and wide business market relations , I find my self as lost with no ability to accomplish same steps compared as before....

[@ahmedalshalchi]

Tooooo late to explain this my friend Ian , I did it before you in 1988 and I made a great bang in ZX Spectrum market so I could dominate the market because of this modification to EPROM version because the ROM chip was veeery expensive and also programming an EPROM was a miracle method to happen by late 80's. Anyway, good to refresh my old memory for good old gold days !!.

[@IanScottJohnston]

My first Spectrum I got when the issue 2 came out in 1982 or 3. It was about 2 years after that when I swapped out the Rom for a 27128 using a home made eprom programmer. Alas I kept it to myself! But yes, fun times were had ground-breaking with computers at home.

[@ahmedalshalchi]

Ian... I had my ZX Spectrum + in 1987 and I was trying to interface it to another circuitry I used to design in my earlier engineering college days but I damaged it for some stupid short circuit I made by wiring out the board so I freak out to secure the ROM from loss so I investigated to find a modification to EPROM version rather the ROM and I could do that by the end of 1988 with my also home-made EPROM programmer , MSX based which my father bought it for me in 1985.... Those were the days my friend we thought they will never end !... but.... kzbin.info/www/bejne/r2SudpuNidOKrKc

[@IanScottJohnston]

Ahmed AlShalchi My home made programmer was designed around the 6402 UART IC and had a RS232 connection to the ZX Interface 1. I wrote a small BASIC program to send the hex file to the programmer which burned the eprom on the fly. The first thing I ever modified just to test the new eprom was the "Copyright 1982 Sinclair Research" message on boot.......umm, it had my own name now!

[@ahmedalshalchi]

Ian... It seems you were having great knowledge about interfacing at that time more than me. I built my 2nd EPROM programmer by 1992 with RS232 interface to have the ability to transfer hex codes from IBM PC to my MSX based EPROM Programmer2 as an undergraduate engineering project with full BASIC programmer utility demo. That was an amazing to my project supervisor of how could I do it by H/W & S/W, you know, early 90's were completely new to PC world. Again, those were the days my friend we thought they will never end !... but... Check your Email please... thank you.

[@basicmicrouk486]

Great to see inside the ZX Spectrum after many years, When i was young i used to have a computer shop and spent many hours doing repairs on them as well as many other computers of the era, Its worth a mention on these old EPROM chips to cover the erase window with a sticker to prevent it maybe loosing its data if exposed to light, its unlikley with it being tucked in the case it would get enough UV light to erase any data but its good practice to do this once programmed, That Spectrum looked like it had spent most of its life in a box unused, we used to get some in for repair after 6 months with some keys unreadable, the keypad membrains were a common problem on these as well as the -5V and 12V power supplys for the dynamic RAM IC's, we kept a large stock of ZTX650 and ZTX213/313 transistors for these repairs and the early ULA's were also a common problem, most early versions had a bodge fix transistor mounted on the top, I spent many hours on the Spectrum when i was young, all programming in BASIC and Z80 machine code, i was never into games at all, Great videos, Dave.

[@IanScottJohnston]

My own Issue 2 Spectrum is an early one so has an IC bodge on top rather than the transistor bodge. I'm guessing it's ULA was pretty early. I was never into games either....:-)

[@DaveCurran]

IanScottJohnston The issue 1 and 2 boards also had a different arrangement for the colour timing on the video output, with two variable capacitors. Over time they tend to develop yellow or blue tinge to the screen, and it's sometimes tricky to get back to white. Issue 3 onwards had auto adjustment. You've got to be gently with those keyboard membrane tails, they get fragile with age, and bend easily. I still do ZX Spectrum repairs (and other vintage computers) and get many of those where the owner has opened it up to see what's inside and snapped the membrane tails.

[@DaveCurran]

@@ZXSpectrum128K Possibly, but there's not much inside one of those other than that big irreplaceable custom chip.

[@ZXSpectrum128K]

R u interested?

[@blackcountryme]

I love the idea of rom switching on the old speccys. Great stuff.

[@TheOnlyPommyman]

Nerdiest thing I've seen on KZbin for a long time, but I watched the whole lot and loved it! You have an engaging style and that made it so easy to watch.

[@RichSzerman]

Nice vid....Why not add a power on switch also?

[@drgr33nUK]

Don't use screwdrivers to pry chips off boards, use a plastic prying tool :D

[@mkolt]

Hi Ian, could you explain how exactly you soldered the diodes with the resistor? Are the diodes both threaded through the hole and the resistor is trimmed and soldered onto their wires? Thanks!

[@steve_ancell]

Where can I get one of those desoldering guns?

[@lbaeza]

Nice job! I was afraid that when you removed the ROM some damage could be done to the underlying traces, but fortunately the chip was cleanly removed. I will proceed and give it a go to a faulty Spectrum from a friend of mine and see how it goes by replacing the original ROM with a proper Diagnostics EPROM. Thanks for sharing.

[@repetto74]

Hi, I am willing to do this mod but need some clarification for the diodes. As I can see from your video one diodes has both leads inserted into the holes on the N markings while the second diode on the H marking has its cathode soldered to the pull-up resistor or both diodes and the pull-up resistor are soldered together? Thanks and great mod!

[@PhilReynoldsLondonGeek]

What's the desoldering tool you use?

[@asmrjunkie6613]

Is there any reason it has to be a dual throw switch? Can it just be pulled down with a high value resistor and a single throw switch to Vcc?

[@ThePeter7071]

Hi Ian, nice video. Where are you from, you sound east coast?

[@RetroComputerShack]

Hi Ian, Excellent video, Thanks. Where can I download the rom file used in your video? Regards, Ian Priddey, Retro Computer Shack.

[@sinephase]

What a great little computer. You could totally make a sort of laptop with that thing :P

[@IanScottJohnston]

Yup, a few folks have done just that......Ben Heck for one.

[@diegoterront7975]

I have a ZX 48K with an Hitachi ROM and I'd like to do the same replace operation but using an EEPROM AT28C256. I think It would be easier because I'd just need a high value on pin 27 (*WE) connecting it directly to pin 28 (Vcc) and keeping pin 20 as it is (*CE as *CS) and pin1 (A14 as NC). Thanks in advance for your opinions.

[@ToddFun]

Nice to know how to do this, Thanks. PS: Shouldn't you put a a sticker over the UV window on the prom so the die doesn't get slowly zinged by stray light over the years?

[@IanScottJohnston]

Strictly speaking yes....... However, the eprom window in the spectrum is in a pretty dark space. Stats indicate that exposure to direct fluorescent lighting would take 3-years or 1-week via direct sunlight. I'm 99.9% confident there will be no issue......but yes, I should have put a sticker on it.

[@pcuser80]

my z80 eprom has no sticker and the sun shines now and then on the chip, see avatar ;) for 7 years now, code is still there.

[@stanmarsh14]

Great tutorial here with showing how to remove ROM's (Works for IC's too), and how to fit replacements, as well as good general soldering tips. So happens the same eprom you used, is very common in UK fruit machines from the 27c series (In this case, you used a 256k version). Would say however as others have pointed out, that the eprom window should be covered just in case, even with a small white sticker will do, which you can also write the details upon, like rom versions etc :)

[@stanmarsh14]

Just so happens I am about to order an eprom programmer (A MCUmall True USB GQ-4X, which at £80 from their official UK Distrubitor C.U.S is not cheap, but better than relying upon one of the cheaper Chinese knock off units), so I can get more in to the programming side of things. One thing I do wish to ask..... that solder gun would be perfect for the work I do repairing Fruit machines, so do you mind telling us what it is, and any possible links as to where to get one?

[@IanScottJohnston]

Duratool ZD-915 via EBay.

[@eLJaybud]

So how much does it cost to have it done for you against what the kit costs to do it yourself? Got two Spectrum + 48k that I'd like to allow either types of key input. :D

[@up2tech]

Where can I find such rom to download?

[@papa_robot]

Hello. I made the mod, but signal never goes low (2 v in zeros), do you think I should use a lower resistor ?

[@unlokia]

Superb job, Ian :) Is there a specific reason you used EPROM and not EEPROM? God bless you, thank you :)

[@IanScottJohnston]

It's not something I'm going to erase and update very often, so an eprom does the job fine. I have a UV eprom eraser to hand anyways so no issue there. I like the old school ways....:-)

[@edgeeffect]

I'd like to play around with some old EPROMS (I've got one from an OKI printer right now) Anyone got any clues as to what sort of EPROM programmer I need and where I could get one from. I could never afford that sort of thing back in the day, so never looked into it.

[@ronnyverminck2741]

I have place also a EPROM on a issue3, it works but when i run test in 48K mode, it shows errors on the multiplexers (IC25 and26) the IC are good. Does it has to do something with the bridges on top of the board? mine is just set on TI (top left switch)

[@charlieoscar09]

Another great production Ian and really well explained,super to see the retro Speccy getting a new lease of life. Can you let us know the model of the little TV/Monitor your using there,would be useful in my repair shack....cheers

[@IanScottJohnston]

No name brand......just a cheap LCD monitor (it's not a Tv) I picked up via Ebay.

[@soul-pn9rk]

Hi - how I couldbe read this PROM in programmer ? It doesn't work like eprom protocol ...

[@IanScottJohnston]

You don't need to, the original ROMs are available on the web to download as a hex file.

[@soul-pn9rk]

+IanScottJohnston thx but my ask is from interesting - how work this PROM and what I'm must doing for properly him work under parallel programmer - my ask is: where is difference between simple eprom and this one ? I'm sure so great electronic like You must know it :) regards bro

[@IanScottJohnston]

The pinout of the rom is not the same as an eprom.

[@soul-pn9rk]

+IanScottJohnston It's only the difference ? No any "additive click lo/hi" in chip select or any other ? Sth what don't do difference for eprom in acces...

[@scossa_vr]

Hi Ian, can you share the binary file of the rom? Tnks.

[@backyardbasher]

great video Ian :) could you give some details on your desoldering iron please as there are hundreads on ebay and i dont know what to buy and yours looks good!

[@IanScottJohnston]

Mine is just a cheap one - DURATOOL ZD-915

[@backyardbasher]

IanScottJohnston thank you Ian

[@unlokia]

IanScottJohnston Good old Duratool. I have a ZD-931 (anti-static, supposedly). Not the Rolls Royce of irons, but for the amount I solder, it is perfect... AND CHEAP :D

[@TheMadmagik]

what model desoldering gun are you using?

[@MrFixer1983]

Very nice video, I want to try this out ;) But with one of those GENIUS G540 can you also write eprom's for the C64 and 1541 disk drive? Mosly they using 2764,27124 and 27256 eprom for that. I'm looking for a Cheap eprom programmer to use for my ZX Spectrum's and C64's.

[@IanScottJohnston]

A better programmer is the TL866A which you'll find on Ebay. I have both and actually prefer to TL866A.

[@MrFixer1983]

IanScottJohnston Ok, thank you for the tip. I will look in to it, i'm new to eprom programing. Some years ago I order a cheap eprom programmer from ebay. But I got only problems with it. So I want to know for sure that this one can write 27xxx eproms.

[@MrFixer1983]

MrFixer1983 Hi, I got a TL866A some weeks ago. It works great and i'm very happy with it.

[@zx8401ztv]

Loverly job, smashing video :-D, i learnt on the zx81 then the 48k and 128k versions. I loved building interfaces and basic programming, hacking code was also good fun. I did the same as you did, but on the later 128k version, i had the sinclair 128/48 rom in one switch position and the amstrad 128/48 in the other, best of both worlds :-). Ive repaired loads of speccys and the buzz buzz psu and memory goes bang mostly. I allways wiggled the pins on the chips after desoldering, top of the chip as well, the cpu and ula were fun to get out LOL :-D

[@up2tech]

Where can I have one like that not on the ZX spectrum +2A?

[@pcuser80]

I have a 64k eprom on mine zx spectrum. with 4 rom versions.

[@eLJaybud]

And they do make 256k roms.

[@pcuser80]

@@ZXSpectrum128K I made my own version its the main one, modified keyboard driver, to work without interrupts, interface 1 macros, to save typing. and some modified basic commands.

[@mike.a.irving8227]

Wow! Thanks for your great video. Nice to see this happen. I never got round to it, so thankyou!

[@AnotherUser1000]

Any ideas about the actual name of the ROM' s program? In other words: How can I have the ROM' s contents, in order to use it in an emulator? I am gonna use it in place of "48.rom". Please answer me only once. -Great video, by the way!

[@AnotherUser1000]

***** OK, I found it and it worked! No need to answer it.

[@AnotherUser1000]

Can I ask something from all of you, guys? I don' t remember virtually anything from electronics, but I think that we could use an OR gate, instead of these two BAT85S diodes. What do you say? No voltage drop, by using -maybe- an SMD gate, floating(or even being glued) on the board. Am I wrong? I, also have a ZX Spectrum + issue 4S(my first and most beloved computer, ever!) but I was a bit young back then and I short-circuited it a bit. It just blinks garbage, but I would like to see it working again, with or without a new ROM. How can I find a technician specialized in Spectrums, in order to get it fixed? With the appropriate fee, of course. The only serious modification that I did by myself, was just the RF-mod and it works fine(the mod, always...). If you want, I can also post some photos of my mod and the results of the previous problem on the TV set. I would be grateful if I got an answer. Greetings from Greece!

[@danielemele7519]

thats a good example, I respect you!

[@javilarg]

What desoldering gun are you using? BTW, I love your video.

[@repetto74]

I did the mod and works like a threat! thanks!!

[@michaelhawthorne8696]

Nice video. Could do with your solder sucker at work....

[@cthoadmin7458]

Why not take the opportunity to swap out the electrolytics for new Nitchicons?

[@IanScottJohnston]

From what I remember an ESR check showed they were fine.

[@paulb4uk]

Nice video superb work .

[@rda6899]

nice Tshirt - Saludos desde Uruguay!!!

[@paweljankowski7836]

cool T-shirt btw, I will create later something like this ^^ And motherboard with chips looks like a blueprint system from Unreal Engine 4 where u can exchange things almost the same way

[@MrSoundshark]

The Ben Heck Show has just done a great bit on the same unit including changing the video rom etc

[@webwarp2023]

It was new but he scratched it a lot with this video LOL this guy it´s just a retro computer destroyer

[@twobob]

Solid, maybe shove in a switching regulator one day...

[@Derundurel]

Strange. I seem to remember doing almost exactly this over 25 years ago!

[@ahmedalshalchi]

hahahaha .... Strange for me too as I did it in 1988 too.

[@federicocresci698]

Thanks Man!

[@jdmgarageb20a]

all is beautiful painted!could not help me I can not sew HN613128P

[@AnotherUser1000]

Can I ask something from all of you, guys? I don' t remember virtually anything from electronics, but I think that we could use an OR gate, instead of these two BAT85S diodes. What do you say? No voltage drop, by using -maybe- an SMD gate, floating(or even being glued) on the board. Am I wrong? I, also have a ZX Spectrum + issue 4S(my first and most beloved computer, ever!) but I was a bit young back then and I short-circuited it a bit. It just blinks garbage, but I would like to see it working again, with or without a new ROM. How can I find a technician specialized in Spectrums, in order to get it fixed? With the appropriate fee, of course. The only serious modification that I did by myself, was just the RF-mod and it works fine(the mod, always...). If you want, I can also post some photos of my mod and the results of the previous problem on the TV set. I would be grateful if I got an answer. Greetings from Greece!

[@AnotherUser1000]

Can I ask something from all of you, guys? I don' t remember virtually anything from electronics, but I think that we could use an OR gate, instead of these two BAT85S diodes. What do you say? No voltage drop, by using -maybe- an SMD gate, floating(or even being glued) on the board. Am I wrong? I, also have a ZX Spectrum + issue 4S(my first and most beloved computer, ever!) but I was a bit young back then and I short-circuited it a bit. It just blinks garbage, but I would like to see it working again, with or without a new ROM. How can I find a technician specialized in Spectrums, in order to get it fixed? With the appropriate fee, of course. The only serious modification that I did by myself, was just the RF-mod and it works fine(the mod, always...). If you want, I can also post some photos of my mod and the results of the previous problem on the TV set. I would be grateful if I got an answer. Greetings from Greece!