The +2 grey and +3 PSUs

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
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BenJWard
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The +2 grey and +3 PSUs

Post by BenJWard » Wed Jan 15, 2020 3:41 pm

Hi, I'm a UK based Retro computing newbie, and over the holiday period I acquired a +2 grey and a +3 spectrum, as well as a C64.

With the C64 I've had to get a new power supply with overvoltage protection because the majority of them are basically guaranteed to kill your machine eventually.

I was wondering if the grey +2 and the +3 speccy PSUs have a similar "ticking timebomb" problem? I've seen people talk about PSU's for some other old computers say things like "Well, unlike the C64, the worst that will happen with this PSU is it will fail and just not work".

The PSUs I have for my grey +2 and +3 are the original chunky grey brick (+2), and even chunkier massive black brick + red lettering (+3).

As a newbie before I start using the machines much I'd like to find out what are the concerns - if any - with these PSUs and are there any recommended actions I should take?

Thanks!
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Guesser
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Re: The +2 grey and +3 PSUs

Post by Guesser » Wed Jan 15, 2020 7:57 pm

the Spectrum +2 (grey) like all previous models just uses an unregulated 9v (nominal) DC supply with regulation and switching supply inside the computer. Being just a transformer, rectifier, and a bit of smoothing, there's very little to go wrong in the power unit. The +3 has a multi-voltage regulated supply so there's a bit more to go wrong in there, but they're still a fairly simple circuit.
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1024MAK
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Re: The +2 grey and +3 PSUs

Post by 1024MAK » Sat Jan 18, 2020 5:40 pm

The PSU for the grey +2 is very reliable. It is highly unlikely that a failure will kill the ZX Spectrum+2. So no need to replace it.

The PSU for the +3 is less reliable, and it is in theory possible for a regulation failure to occur (that’s the problem that occurs with the C64 PSU), but this failure mode is uncommon (or at least, reports of this failure mode are rare). Note that this PSU is a completely different design compared to the C64 PSU. When they do fail, by far the most common problem is loss of power to the computer, but with no damage to the computer. Repair should be a reasonable price. So again, no need to replace it.
Please note, unlike the earlier models, don’t remove the DC (DIN style) power plug while the PSU is powered up.

The problem with the C64 PSU is basically a design problem. It uses a conventional series pass voltage regulator chip, but the manufacturer encapsulated it in resin. So over time it cooks itself. And sometimes the failure mode is that the +5V DC output rises up to around 7 to 8V. Because the electronics are not accessible, servicing or repair is not practical. Not all will kill an attached C64 computer. But it’s not worth the risk (unless you fit a protection circuit between the PSU and the computer).

I hope that helps.

In terms of how to kill a ZX Spectrum (any model), or rather, how not to kill it, NEVER attach or remove and interface or expansion to/from the edge-connector while the computer is powered up.

Mark
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