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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 20th Jun 2022, 9:21 am   #1
sparkymike
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Default Sinclair ZX Spectrum polarity

On the power supply input socket of the Sinclair Spectrum, which is positive ? The outer ring or inner ?
Mike.
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Old 20th Jun 2022, 9:53 am   #2
SiriusHardware
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Default Re: Sinclair ZX Spectrum polarity

I'm away from base so I can't measure mine but all the hits I get on Google say centre NEGATIVE and nominal 9VDC (unregulated) at 1.4A.

You should be able to verify this first by looking for continuity between the centre connection on the socket and the metalwork on the modulator.

Last edited by SiriusHardware; 20th Jun 2022 at 10:04 am.
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Old 20th Jun 2022, 10:32 am   #3
Dave Moll
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Default Re: Sinclair ZX Spectrum polarity

Although the ZX power supply rather unhelpfully fails to indicate the polarity of its (supposedly) 9VDC output, a quick test with a multimeter confirms it to be centre negative - at just under 14V with only the DMM across it.
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Old 20th Jun 2022, 10:53 am   #4
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Default Re: Sinclair ZX Spectrum polarity

Somewhere in a cluttered corner of the back of my memory, I seem to remember hearing that some Sinclair Spectrums had turned up with the opposite polarity to usual.

Some checking might be worthwhile.

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Old 20th Jun 2022, 11:54 am   #5
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Default Re: Sinclair ZX Spectrum polarity

Hate barrels with centre negs and positive outers! why would they do that?
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Old 20th Jun 2022, 3:56 pm   #6
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Default Re: Sinclair ZX Spectrum polarity

Because the same man invented the Sinclair C5 and the Sinclair digital multimeter that blows up when connected to the mains even though it was rated for it. Sinclairs motto "Why be difficult when you can be B dangerous ".
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Old 20th Jun 2022, 4:05 pm   #7
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Default Re: Sinclair ZX Spectrum polarity

Worthwhile modifications on the ZX Spectrum are to ditch the 7805 voltage regulator and heatsink and replace it with one of the Traco Power switch mode replacements, the machine runs a lot cooler. Composite video mod is easy to do too, you can even re-use the RF output socket as composite providing you don’t need RF - it’s an easily reversible mod anyway.

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Old 20th Jun 2022, 4:54 pm   #8
Graham G3ZVT
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Default Re: Sinclair ZX Spectrum polarity

Quote:
Originally Posted by Phil__G View Post
Hate barrels with centre negs and positive outers! why would they do that?

I have two very similar Casio music keyboards, each requiring 9v dc using the same barrel connector....

....You guessed it, opposite polarities!
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Old 20th Jun 2022, 10:34 pm   #9
SiriusHardware
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Default Re: Sinclair ZX Spectrum polarity

I think one reason why centre negative came out (not applicable to the Spectrum) is that the switch built into the socket switches the outer connection and not the centre connection.

If you have an arrangement where you want the power from batteries to be disconnected when an external DC source is plugged in, something very primal in you wants it to be the positive which is disconnected / switched over rather than the negative, even though it makes no difference which leg of the power is switched. So this may be one reason why some providers opted for outer positive / centre negative, so that it would be the positive power feed which was switched.

Casio (esp. music keyboards) were certainly one of the well known centre-negative brands.

I think, but can not check just now, that the ZX81 may also have a centre negative power connector although in that case it is a 3.5mm mono jack plug instead of a barrel type connector, so is more correctly described as 'tip negative'.

Last edited by SiriusHardware; 20th Jun 2022 at 10:41 pm.
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Old 21st Jun 2022, 7:24 am   #10
sparkymike
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Default Re: Sinclair ZX Spectrum polarity

Thanks for the info.
I had reversed the power supply many years ago to power up a keyboard but as it was so long ago I thought it prudent to double check.!!
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