back to article Sinclair ZX Spectrum FAILS latest radio noise rules SHOCK

A pal of mine suggested a short while back that it might be fun to obtain the blueprints for Sinclair Research’s ZX Spectrum and have a batch built up to sell to fans of retro computing. It’s a good job this plan never made it out of the pub: the dear old Speccy would have immediately fallen foul of modern electromagnetic …


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  1. Paul Crawford Silver badge

    EMC who cares?

    Really, the BT power line modems are also an abject EMC failure but due to the money behind them ofcom, etc, don't care. The solution? They re-draft regulations to allow more noise...

    1. Steve Evans

      Re: EMC who cares?

      You should try the Home Hub 3's standard PSU.

      When I first got Infinity I couldn't work out why my previously reliable network of power line kept failing to link.

      A few tests later, and I found it's the cheap PSU BT supply chucking too much noise back down the mains!

      Binned it.

    2. Anonymous Coward
      Anonymous Coward

      Re: EMC who cares?

      The point of this is.................

  2. Steven Davison

    Rebuilding a Speccy...

    If you did get the blueprints, i'd be interested in doing it as a project...

    maybe with modern components behind it, it's EM output can be reduced to 'regulated' levels..

    1. JimmyPage

      Re: Rebuilding a Speccy...

      Blueprints ?

      Surely a simple circuit diagram ?

      My memory may be playing tricks on me, but I'm pretty certain there was a very short-term release of DIY spectrum kits* so there might be something already out there. Trying to build a modern compliant version strikes me as the IT equivalent of fitting wheels to a tomato - time consuming and utterly pointless.

      *The genius that was Sir Clive realised that you could get your customers to build the unit, thus avoiding unrealistic delivery schedules *and* sell it as a "feature". Certainly the ZX80, and early ZX81s ....

    2. Paul Crawford Silver badge

      Re: Rebuilding a Speccy...

      I think (but don't have clear memory or facts) that the ZX series were cheaply made and used a double-sided PCB and not multi-layer boards with power & ground planes. That, if true, is probably the #1 reason for the poor EMC performance.

      Also note they tested it without cables/peripherals, so real-world use would be significantly worse that observed in El Reg's article.

    3. Anonymous Coward
      Anonymous Coward

      Re: Rebuilding a Speccy...

      With modern components behind it, you could likely put the entire thing in a single FPGA, with the only signal lines coming out being the cassette interface line, the video, and the keyboard scan - and in today's designs, those would all be 3.3V rather than 5V, reducing the RF even more.

    4. blemblem

      Re: Rebuilding a Speccy...

      the spectrum contained a ULA , an early (OTP) programmable logic for the address decoding and some other stuff', so no good just copying drawings

      1. ThomH Silver badge

        Re: Rebuilding a Speccy... (@blemblem)

        Then I guess the solution is to buy 'The ZX Spectrum ULA: How to Design a Microcomputer' (ISBN-10 0956507107; published in 2010 so still widely available) as the ULA is fully documented and imaged within. You could definitely build an entire new ZX Spectrum with that and even have the correct horizon on Aquaplane, the correct multicolour text on Uridium, etc.

  3. Chris 3

    It was a feature

    I can't be the only one who remembers making music through a nearby transister radio by coding different length loops on the Speccy and tuning the loop frequency to play a note on the radio.

    1. Steve the Cynic

      Re: It was a feature

      If you did it on a Speccy, you were a bit behind the times. See for a reference to doing it with a PDP-8 in the 1970s.

      Or even with recordings...

      But good work, nonetheless.

    2. jjk

      Re: It was a feature

      I can't remember making music on the Spectrum this way (it had a speaker after all), but I do remember a piano program for the ZX80 that worked that way.

      I did use a radio to monitor long-running programs - in fact I can still remember the "melody" for one of them. The trig functions had very distinctive sounds.

      1. JeffyPooh

        Re: It was a feature

        The TRS-80 Model 3/4 had an available action game that included *actual music* (intentional!) that could be heard by placing an AM radio next to the computer. The booklet that came with the game included directions explaining this absolutely-intentional feature.

    3. Anonymous Coward
      Anonymous Coward

      Re: It was a feature

      I did it with a ZX-80 in 1981 before buying a ZX-81 and subsequently (for my 18th birthday) a Spectrum.

      It amuses me that my current PC has 12582912 times the RAM of my ZX-80 (12Gbx1024Mbx1024Kb) ...

    4. stucs201

      Re: Music via radio.

      The oddest case of this I came across occurred with an Amstrad PC 1512 running Digger. The actual game music could be picked up by a radio tunned to just below Radio 1's FM frequency.

      I'm not sure if this was a deliberate feature of the game that worked with any PC or just a peculiar interaction with the Amstrad's hardware. It didn't seem to be a general occurrence with the 1512, I never managed to do the same with other games.

      1. Dr Wadd

        Re: Music via radio.

        The flip side of that was being shown once how touching a screwdriver to the right point on a BBC B's motherboard caused it to play a particular radio station from the speaker. I wish I could remember how to pull of that particular trick.

      2. Anonymous Coward
        Anonymous Coward

        Re: Music via radio.

        > tunned to just below Radio 1's FM frequency.

        Wow! It was frequency modulated?

    5. Pet Peeve

      Re: It was a feature

      Ha, funny you say that. I remember back in the 70s that my TRS80 played merry hell with radios (and sometimes UHF tv stations) whenever it was running. There were even programs you could run that would make the interference sound (kind of) like music or speech.

    6. Ilgaz

      Re: It was a feature

      I remember reading about how early developers used the RF noise to their advantage with AM radios.

      Note that they were likely far slower than z80.

      1. Ilgaz

        For debugging

        I mean for debugging, literally listening loops. Sorry.

    7. Neill Mitchell

      Re: It was a feature

      I REM this. My neighbour used to complain loudly when I ran my Speccy because he said it used to POKE his TV reception. Said it also upset his DIM CAT. The CONT.

      I got the POINT and so I went OUT to Tandy, had a PEEK and bought some metallic spray. Upon my RETURN I coated the inside of the case and then insulated with electrical tape. Sorted the problem FOR sure. CLEAR result. Try it out to VERIFY.

  4. jason 7

    Is that a MK2 board.....

    ...pictured in the article? I think it is.

    May be worth trying a MK3 board. Just for giggles.......

  5. bonkers
    Thumb Up

    "abject" ?

    Its a marginal fail, its only 4 or 5dB above the limit, and at a couple of spot frequencies. A few minor modifications would fix it - either an "R" or a ferrite bead in the clock line, or a little work on the grounding scheme. I've seen much worse initial plots than this. Oh, and the emissions above 100MHz should be fine, look a the plot, there is no energy in the higher frequencies, it is just the test equipment noise floor. OK there might be problems with spurii and harmonics of the UHF modulator, once it gets enabled again. Not bad for the time and rather exceptional for our Clive - or Jim Westwood (see Reg's passim) to give him a proper name.

    1. Steve Evans

      Re: "abject" ?

      A nice shielded case would do wonders...

      Egor, break out the evostick and tinfoil!

      1. Anonymous Coward
        Anonymous Coward

        Re: "abject" ?

        Ahahaha! Ever peer inside the case of an Amstrad PCW? Alan was way ahead of you there!

      2. Dave Bell

        Re: "abject" ?

        I remember seeing metal-loaded spray paint intended to provide screening for plastic cases. Such solutions may still be available.

        1. BenDwire Silver badge

          @Dave Bell

          It's still available, and is still used in a couple of our 20+ year old designs! (Our industrial customers are a conservative lot and still want "what they bought last time" - who am I to argue if they're willing to pay for it?)

          Mind you, the updated versions use all the techniques mentioned in the original article and end up costing far less overall, not more. In short, stop the product from radiating in the first place and you don't need to stick expensive filters everywhere.

          1. cortland

            Re: @Dave Bell

            --stop the product from radiating in the first place --

            Got THAT T shirt. Odd how spending a few pennies on prevention is begrudged, but a few hundred thousand dollars may be spent to find and fix RFI later.

            I'm retired now and get the occasional job via a contracting firm when someone realizes he's ignored High School (6th Form for you lot) Physics.

            Or can't figure out his own test equipment.

            Take two ferrites and call me in the morning.

      3. Anonymous Coward
        Anonymous Coward

        Re: "abject" ?

        In Vicarage Lane, we had to wait for the tea trolley - to buy chocolate for the silver foil wrapper it came in.

  6. Mark 49

    Reminds me of my first computing job in the mid 80's.

    We had an old 'British Micro' running CP/M with an orange monochrome display. If you placed the display on top of the case of the main unit, then the display would distort as the electro-magnetic fluctuations from the unit would effect the cathode tube.

    Add some floppy access to it and the screen would become unreadable.

    To solve, we had to use a 1/8" steel sheet between the box and the display.

  7. ganymede io device

    You could debug with a transistor radio

    Tight loops produced a different noise (more whine than ticking hum) on a nearby transistor radio than main control loop of a program (presumably DRAM refresh cycle rate changed), so I was able to find errant code by listening to the radio tuned to some interfering harmonic frequency on similarly Z80 equipped Jupiter ACE.

    I always assumed the cause was the plastic case not providing any shielding.

  8. Randolf McKinley

    I wonder if the Spectrum has caused any real interference problems either with contemporary or modern electronic equipment, despite the failure? If not, what does that say about the limits? Or modern electronics?

    Abject fail icon to reflect the abject fail ...

    1. Phil O'Sophical Silver badge

      Define "real".

      If it makes your radio buzz, so you have to move the radio to the other side of the room, maybe no big deal, but it's "real".

      Then put 10 Spectrums (Spectra?) in a classroom, and discover that it causes patterning and noise on the TV upstairs where they're watching a schools broadcast. Is that "more real".

      Then you get the really awkward ones, like when it upsets the hearing aid of the deaf kid so (s)he can't use the computer lab.

      You ned to draw a line somewhere, and enforce it.

  9. Chris Walsh

    I thought the 3.5MHz clock might be one suspect.

    The clock is stepped down from 14Mhz crystal but the 3.5 was used by most of the computer and 7 and 14 used by the ULA to render the screen (pixel clock etc.)

  10. Anonymous Coward
    Anonymous Coward

    Better than expected

    I would have expected far worse to be honest. I recall my Spectrum+ would knock out Radio 1 on FM!

    Not sure about “rather fishy power supply and cable”. The brick was a simple transformer/bridge rectifier/capacitor jobbie and onboard the regulator was a basic linear 7805. Nothing noisy there.

    1. Phil O'Sophical Silver badge

      Re: Better than expected

      Oh, a 7805 can be plenty noisy if you forget the decoupling capacitor on the output. Oscillates around 3MHz IIRC.

  11. Anonymous Coward
    Anonymous Coward

    Back in the day...

    Back in the days of the ZX81 here in the US, the requirements for computers were the less rigorous Part 15A, intended for office work (since nobody would have a computer in the home where Part 15B ruled).

    There was one computer of which I am aware that would stand any chance in the modern regulatory environment, and that would be the old Atari 400 and Atari 800 - all the guts were in a potmetal shell weighing more than the entire ZX81. It would be fun to try one of them out in an RF chamber.

    Then there was the good old TRS-80 Model 1 - a machine so RF noisy that several games did sound effects by instructing you to tune an AM radio between stations and place near (within 2m) of the machine. A machine so noisy it "passed" TEMPEST because it put out SO MUCH hash you couldn't recover what was on the screen.

  12. Anonymous Coward
    Anonymous Coward

    The question on everyone's lips of course is....

    ...was the C64 more or less leaky than the Speccy? :o)

    1. Anonymous Coward
      Anonymous Coward

      Re: The question on everyone's lips of course is....

      It had a metal shield on most models. They were more compliant than Sinclair who were way too obsessed with cost and this cheapness is the reason why I think we have very little in the way of a computer industry.

      People grew out of the speccy and its limitations and the lack of profit meant no investment in a decent 16-Bit machine, Sinclair had wasted all the money on a QL.

      1. cyborg

        Re: The question on everyone's lips of course is....

        I doubt your reasoning. We have perfectly cromulent hardware success stories coming from Cambridge including the ARM and we punch way above our weight in software terms. It's hard to argue the cheapness of the Spectrum isn't part of the reason why so many were able to get into software. It helped in my case. (Age 7).

  13. M Gale


    Why bother? You could probably emulate a speccy with a microcontroller.

    Wonder if anybody has stuck the whole thing, ULA, CPU and all, on a single chip yet?

    1. Adrian 4 Silver badge
      Thumb Up

      Re: Rebuilding?

      The answer, it appears, is yes :

  14. Sandpit


    The clue was in the name all along

  15. John Deeb


    My guess for a source of the radio noise would be the power supply section on the main board (not the adapter). At the time it was a common source of breakdown and if I remember correctly it was a cheap self-oscillating circuit generating -5 and +12 in addition to the +5 from a common regulator using the 9V main. It's hard to tell how much EMF could be generated this way or which harmonics but it was not shielded at all.

    1. Stoneshop Silver badge

      Re: self-oscilating

      You're right about the circuit. Strange thing is that if they had run 9VAC into the machine, they could have had their 12V and -5V without faffing with that oscillator, just simple diodes, caps and regulators. There would have been a larger smoothing cap inside the Speccy (maybe a few smaller ones in parallel according to available space), but apparently the way it was done was the absolute cheapest.

  16. Callam McMillan

    While I understand the need for some regulation of the EM radio spectrum, this kind of thing is disappointing. Once upon a time, anybody with the knowledge necessary to build a circuit and shove it in a box could then go and put it on sale. Nowadays it looks more like you'd need a legal qualification more than the electronics knowledge to put something on sale. You have EM Compliance, you have the god damned WEEEEEEEEEEEEEEEEE directive and no doubt another few regulations I am not aware of!

    I wonder how many innovative products are left rotting in peoples sheds because they've looked at the cost and effort of taking them to market and have turned round and said sod that for a laugh...

    1. phuzz Silver badge

      I wonder how big the pile of non-rotting obsolete products would be if we didn't have said regulations, and personally I'm quite glad that I can listen to the radio/get wireless reception without my next door neighbour spewing EM noise out with some bit of kit knocked up in someone's shed.

      The regulations aren't just there to stop people having fun you know...

      1. jason 7

        Then look at it another way, we have all these new regulations and now have electrical appliances and TVs that generally only last the warranty period of a year and then fail.

        Whats the average life of a modern LCD TV? 2-3 years? I remember a time when a TV would last 15+ years.

        RoHS has just meant brittle inferior solder that cracks at the first sign of heat/stress and then it's landfill bound. Now that's eco friendly.

        They giveth with one hand...

        1. Callam McMillan

          Unfortunately I cannot remember where I read it, but I remember once reading that in modern electronic hardware, the quantity of valuable metals is actually being reduced. Where once a circuit board would have significant amounts of gold on connectors, and lead in the solder as you pointed out, now there is very little to make recycling economical, and I have to wonder if ironically that has actually countered the increased concern in recycling old equipment?

          As for my earlier point, I am not disputing that some regulation is both necessary and good, I too expect to be able to use my laptop and phone to get reliable connections to all manner of wireless signals. However I also have to wonder how bad things would be in an unregulated environment. I mean, when these devices were being produced (literally in sheds and garages to begin with) and there wasn't huge amounts of regulation, I don't hear stories of how they destroyed TV and radio transmissions. Nor do I necessarily see the need for CE marks, since electrocuting your customers or burning their houses down tends to be bad for business!

          What I am really suggesting is perhaps a lighter set of regulatory requirements that allow inventors to come up with new products for minimal overheads, although perhaps only where these do not involve the use of Wireless RF components.

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