I have a ZX Spectrum+ and it didn't came with it original power supply.

I bought a 9 Volts and 2.25 Amps power supply with switchable connectors.

There was a mistake and the polarity was inverted, the barrel connector had positive in the center, but from what I read, it must be negative in the center.

I checked the ZX Spectrum+'s mainboard and nothing looks in bad shape, even after the polarity was reversed.

The problem is that it didn't boot before and it still doesn't boot with the negative in the center. What happens in both cases is that the internal regulator starts to heat really quickly (I read that it was normal for it to get hot) and nothing shows up on screen (I user a SCART interface).

Could the ZX Spectrum+ have been damaged by the reversed polarity, if yes, what components could have been damaged, and how can I fix it?

  • 1
    Anecdotally it's always a good idea when trying an experimental power source to double check input requirements like AC/DC, voltage, and current draw. That's your first line of defense. Your second line of defense is to turn the device on and count 3 mississippi's. Generally these devices are robust enough to survive a short time span of reversed voltage, but its a really bad idea to let them sit and cook that way. My instinct is to advise you that whatever problem you were having before the misadventure with the power supply is probably still the problem. Good Luck!
    – Geo...
    Commented Oct 8, 2016 at 15:59
  • You've most likely damaged the power circuit and possibly more. One useful thing is the Spectrum has a speaker on board so you don't need a TV. With the correct PSU try holding down a key and see if you hear the clicks. Commented Oct 10, 2016 at 11:03

1 Answer 1


You have damaged your Spectrum for sure. The first casualty is the 7805 regulator. These normally explode and release a "black smoke of the death" clearly visible, but yours doesn't seem to be the case. It's curious that this component is designed to survive overvoltage and short circuit conditions, but it is very vulnerable to a polarity inversion. The second casualty is likely to be TR4 (the ZTX650 transistor that forms the DC-DC transformer inside the machine). TR5 might be damaged as well.

With a little luck, no memory/logic/ULA/Z80 chips will be damaged and these two components will be the only ones to replace.

  • Thank you for the answer, I'll see what I can do to fix it (I don't think I can replace the components myself). Also, why did they chose to put negative in the center? From what I know, 90% of electronic equipments with barrel power connectors have the center set to be positive, even back in the 1980s. Is there any know reason for this weird design choice? Or could it be just to prevent users from using power supplies that were not manufactured by Sinclair? Commented Oct 9, 2016 at 23:34
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    Not every manufacturer put the negative in the outside. I have some 80's era Casio musical keyboards and all of them use the same polarity as the Spectrum. Commented Oct 9, 2016 at 23:37
  • Historically, centre negative was the norm. Probably because it was useful when the socket was switching between battery supply and external power ("battery eliminator" as the power bricks used to be called). EEVblog has a brief episode on this: youtube.com/watch?v=ix2fR-rh1vc
    – user6576
    Commented Feb 9, 2020 at 15:50
  • In those days, there was no standard or convention for the barrel connector polarity. I had plenty of equipment that used either, such as portable hi-fi and cassette recorders. Because of this, universal mains adaptors that with adjustable output voltage and selectable polarity for their barrel connector could be bought, certainly from the late 80's onwards - I've still got one.
    – TonyM
    Commented Jul 21, 2020 at 7:20

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