EEVblog #557 - Retro Sinclair ZX Spectrum Computer Teardown

[Stonent]

Sinclair has always had a history of cost-cutting including their half-rom calculator.

http://files.righto.com/calculator/sinclair_scientific_simulator.html

[Zad]

This is Richard Altwasser's LinkedIn: http://www.linkedin.com/in/richardaltwasser

Current occupation is "Reader at Portishead Parish" which sounds to me like he is retired and has taken up religion. It seems a waste of his talents, but if that is his vocation now, then so be it. Might be worth approaching!

Here is a photo of the Issue One PCB, note the hand laid out traces, and the memory daughterboard:

[img[]http://k1.dyndns.org/Vintage/Sinclair/82/Sinclair%20ZX%20Spectrum/PCB%20Images%20%28angled%20views%29/ZX%20Spectrum%20PCB%20issue%201%20%2B%2032K%20RAM%20dK.jpg[/img]

RFI-wise, they weren't clean, but they weren't that noisy either. I used to listen to AM radio at the same time as playing on my Spectrum without any great problem. I year or two back, someone did some research into restarting production with the original design. As you can imagine, when they tried it in an anechoic chamber the the EMC test gear practically laughed at it.

The simplicity of the "Speccy" is what has given the UK such a big base of skilled software and hardware professionals (and true hackers) today. People forget how relatively poor the UK was at the time, most people just couldn't afford a £400 BBC Micro or a £300 C64. The Spectrum was perfectly pitched. Yes is was compromised, with crap sound, keyboard and poor graphics - but it was enough to do the job!

For anyone interested in the early UK computer industry, TheRegister regularly publishes well researched articles, including interviews with the people involved: http://www.theregister.co.uk/hardware/vintage/

[AndyC_772]

RFI-wise, they weren't clean, but they weren't that noisy either. I used to listen to AM radio at the same time as playing on my Spectrum without any great problem.

My Spectrum was the single noisiest piece of kit I've ever owned! It may have been OK on the MW band, but it made a heck of a mess of local FM stations.

For all its added complexity, the BBC model B I traded it for was much better.

[Kjelt]

i love the old spectrums and have a large collection of vintage computers.

Sorry for the small sidetopic post but I always wondered what was more important to the price of a vintage computer: if it still works OR it has the original parts.
I means electrolytic capacitors are often dead after 30+ years, so are the chances on the ROM's. So to make it still work you can replace the elco's and replace the ROM's with Eproms.
But maybe if you leave it non working original it is worth more? Would like to hear your opinion as a collector. 

[Shred]

I recall that "The Australia Tax" was heavily applied to Sinclair products.  The cheap computer in the UK was quite an expensive computer by the time it arrived here - from memory it was at least twice the price it should have cost.  My father attempted to buy a ZX81 by mail order, but the UK vendors refused to sell one to an overseas buyer.

[SLJ]

I still have a bunch of Sinclairs sitting around.  Mostly ZX-81s and 1000s.



The board of the 1000:

[deephaven]

When the Spectrum was current, someone where I worked got hold of the schematic and a source of supply of the ULA chip. Our company used dram by the bucket load, so that wasn't a problem. So we made up several 'Spectrum' units from scratch and they worked a treat.  We didn't have a source of keyboards so made our own using proper keyboard switches. I still remember playing "Attic Attack" amongst others.

[Kaptein QK]

... I did own a Dick Smith System 80 in 1983, and upgraded it to a Hong Kong made "rotten Apple II" in 1984. ...

Would that be a Multitech MPF-II?
It was my first computer, I learned 6502 assembly by just reading the datasheet

[Balaur]

Heh, story time.

Sinclair clones were quite a nice thing to have back in my home country (Romania). I had one myself and I must say that it was definitely a huge deal to me at the time.

The computers were reasonably available (YMMV), not very expensive, had a mechanical keyboard, tons of software available on pirated C45/60/90 cassettes and so on. I didn't even had to buy a cassette reader as my boombox happily connected to the Spectrum.

There was even a technical/computer learning program on national TV that broadcasted a few games and applications on TV (i.e. the audio played the software and the screen was the typical save screen from one fairly known copy program.

When in the electronics engineering school, there was still some Spectrum traffic: home grown stuff, hand-assembled units (you could buy the PCB and the keyboard) even if the whole thing was declining as the PCs and Prince of Persia was on the rise.

I've broken my Speccy twice:
The first time, I've made an EPROM programmer that required 24V or something like that. Everything was cobbled together on some PCB and the programming voltage wire touched the extension interface. It was very tough to repair it, as I was missing funds, equipment and knowledge (was 13-12 at the time?). It was a great triumph when I had finally managed to get it back working again. Only had to change the CPU, EPROM, and a handful of discrete ICs

The second and final time was when I wanted to use the computer as a personal assistant and run an agenda program made by me. I wanted to let the computer working all the time, but it interfered with the TV and radio reception. I've just tried to adjust the RF frequency a bit and broke everything; never worked again. No idea what was wrong, even to this day.

[elgonzo]

Dave should try and get his hands on an early Commodore 64 computer.

Actually, i would suggest somebody sending Dave a SX-64, so he has some fun taking it apart. But that bugger is bulky and heavy.
Anyway, back to Sinclair Spectrum...

[rsjsouza]

Dave, thanks for the teardown; it brought good memories to me!

My first program was written on a Brazilian clone of the Spectrum ZX80 (called TK82C) from a company called Microdigital (external and internal pictures) - it had the crappy membrane keyboard and the build was botched as ever!

Years later I was given a TK85 (a ZX81 clone) from the same manufacturer (external and internal pictures). This one had the chiclet rubber keyboard (much better than the membrane).

The ZX Spectrum was called TK90X and it was manufactured only years later - the local manufacturer did not have Sinclair's license to use the Ferranti ULA and they developed one of their own.

[grumpydoc]

I had the misfortune to have repaired hundreds of spectrums in the 1980. As Chipguy says, the ZX spectrum had a heatsink. It went over the top of the edge connector almost to the modulator. It relied on the ventilation of the edge connector which was fine until something was plugged into it.
The most common problem was the ZTX650 transistor giving up, often caused by someone plugged something into the edge connector while it was powered up. The ZTX650 was used in the charged pump circuit to create -5 and +12 in addition to the +5, required for the 4116 DRAM ICs

I spent a summer doing repairs and had a fair few broken Zx80's, 81's and spectrums through my hands - the most common problem I had was dead CPUs and DRAMs - there was no buffering of the Z80 signals going out to the expansion connector so plugging anything in with the power on was almost sure to kill something. With the power off I suspect they were pretty susceptable to ESD damage as well.

The ULA seemed a bit more robust as it usually survived but we had a few that basically needed a 100% rebuild.

[IanJ]

Hi all,

Repaired lots of ZX Spectrums back in the day, my own of which was my pride and joy, as follows:-

- ROM replaced with 27128 Eprom so I could play with the code including changing the startup copyrigt message at boot on my friends Spectrums.
- DKtronics 'real' keyboard.
- Couple of Microdrives via Interface 1.
- Home built Eprom programmer (to burn 2716 & 27128), designed around a 6402 UART and operated via RS232 from Interface 1.
- Home built serial-parallel interface (another 6402) so could drive a Centronics printer without any TSR/driver. Print# rings a bell.

Note: I stupidly sold the Eprom programmer in the 90's.......tried hard to get it back a couple years ago but the guy couldn't find it..... ......so if anyone in UK happens upon it let me know (easily recognizeable in a two tone enclosure with 2off ZIF sockets on top). Would love to see it again!

Dundee (of Sinclair fame) is just 50miles from me so back in the day I knew quite a few guys that worked there. I managed to get a hold of lots of Spectrum parts/ICs including documentation such as schematics, repair guides etc etc.

Ahh memories!

Ian.

[komet]

This machine taught me everything I know about everything.

When I was 13 I built an ADC and DAC interface that I could use as a sampler. I am still stupidly proud of it. Here it is. Sadly I've lost the edge connector that used to connect to the dreadful Spectrum interface port.

[TheBay]

I was having a clear out of my garage the other day, I binned 2x ZX 128+2's (One grey one black, I think Amstrad made one), a Acorn electron, I brought an Amiga in but it committed suicide after about 2 minutes of loading a disc, then got the dreaded lights. Found an Oric 1 in a box unused, C64 with keys missing.

But for some reason I have a ZX 48k keyboard rubber just on my garage floor, I couldn't throw the thing out as I must have had that since I was young, might hang it on a wall LOL 

God knows where my ZX81, ZX48 are, I had the microdrive? like a continuous tape and a thermal printer for it too, think I set fire to the rolls of silver paper a few years back...

I really enjoyed the ZX48k, it used to crash a lot and we figured the CPU was popping out of the DIL socket when warm, we cured it with a cork jammed in the case.

One reason I like TTi instruments as their heritage is from Sinclair...

[kxenos]

I was 10yo, 1986. One day, the teacher came into the classroom and said: Kids, I have something to show you! And he presented a shining new ZX spectrum. He said: This is the future. Whoever masters this, shall master the future, or something similar. Later that day, I return home and say to my father: Father, there is this new thing called a computer and whoever masters computers will master the future and I better start now because I don't want to end up a carpenter like you. My father didn't say anything but two months later he brought me a Philips MSX with a 3,5MHz Z80 and a 3,5" floppy disk. I just had an English user manual where in the last pages there where some example programs in GWbasic. That's where it all started for me:
10 CLS
20 PRINT "Hello world"
30 GOTO 30
run

Boy, those where the days...  O0

[RupertGo]

Just a note on RFI and the Spectrum.. it failed FCC regs at first, because of the RAM bus architecture. One of the clever design features was the contended RAM below 32k, which was shared between the CPU and the video circuitry - basically, the CPU had its clock stopped when it was trying to access RAM that the video needed. The video buffer was in the CPU address space, saving the cost of two lots of chips, but at the price of performance - obviously, the video had to take priority, because when a scan line's starting on your television it needs its data there and then.

But the video buffer was fixed and small and lived below the 32k boundary. A 48k machine had 32k of memory above that , logically and physically separate, that the video circuitry would never need to see. So, by splitting the bus, it was possible for the CPU to run at full speed in the upper 32k while the video stuff could do what it liked in the lower. And - bonus feature! - you could do it with a bunch of resistors of just the right value to let both sides of the data bus be driven differently when appropriate, but track each other when one side relinquished driver duties. Cheap, fast, lovely.

Which is great, but it means when both sides are being driven independently you have two sets of PCB traces flailing around being two buses instead of one. When you have one length of conductor going one way and the other going the other, well - you have a lovely dipole And as you can see from the teardown, RFI shielding was as alien to Sinclair as juggling sand-fleas.

The FCC was unimpressed, which is why the Timex-Sinclair 2000 (and the other Spectrum derivatives) were late and had a lot more tin foil and that gritty grey conductive spray-applied coating on the inside of their cases.

[hikariuk]

Current occupation is "Reader at Portishead Parish" which sounds to me like he is retired and has taken up religion. It seems a waste of his talents, but if that is his vocation now, then so be it. Might be worth approaching!

Reader is a lay position; I suspect he's just retired and also an active member of his local church.