Like a buss I wanted a Spectrum and ended up with 3

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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zack4mac
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Like a buss I wanted a Spectrum and ended up with 3

Post by zack4mac » Tue Apr 16, 2019 6:51 pm

I dont know why I do these things and its probably best not to try and figure it out! But now I have a Harlequin, ZX 48+ and a Grey +2 all without power.

What are my best options, I would prefer to invest in a known good quality power supply with plenty of room, if extra current is needed.
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Re: Like a buss I wanted a Spectrum and ended up with 3

Post by PROSM » Tue Apr 16, 2019 7:08 pm

For the 48K Spectrum, you'll want a regulated 9V power supply. Don't bother buying an original, as these deliver a slightly higher voltage, putting more strain on the internal regulator, and they usually need refurbishment anyways. You will need to make sure that the connector is centre negative. I cannot stress that enough, and it's worth mentioning as most generic power supplies sold these days use a positive centre, so you will need to ensure that your supply uses a negative centre, or has a detachable connector that allows you to switch the polarity.

You'll need to make sure that the supply can give a high enough current also, if you are planning on using any peripherals; 1 amp is sufficient if only the computer itself is being used, and 1.4 amps should suffice when attaching things like an Interface 1.

I'm not certain about the grey +2; I know it uses the same barrel connector, but I don't know about the polarity. Here's a supply for a +2 from retrogamesupply.com (I've never bought from them, but it's one of the first results in Google).
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Ast A. Moore
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Re: Like a buss I wanted a Spectrum and ended up with 3

Post by Ast A. Moore » Tue Apr 16, 2019 7:29 pm

A 48K will be quite happy with a +2 power supply. No modifications are required. A 2A or more power rating is recommended.
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Re: Like a buss I wanted a Spectrum and ended up with 3

Post by zack4mac » Tue Apr 16, 2019 7:57 pm

I'm suspicious there seems to be a lot of expert retro sites that sell psu's that look very similar to what you would find on Aliexpress
I might try and find multi voltage adapter that follows your recommendations, that provides 9v @ 2 amps or more outputting center negative.

Thank you kindly
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Re: Like a buss I wanted a Spectrum and ended up with 3

Post by Ast A. Moore » Tue Apr 16, 2019 8:41 pm

There’s nothing special about a Spectrum-friendly PSU, aside from the (now) unusual center-negative plug arrangement. Any decent switch-mode (regulated) PSU will do. I use a fairly generic Mean Well PSU for my Toast Rack, for example. I simply opened it up and swapped the two conductors around (it was originally wired for the more usual center-positive polarity).

The Spectrum doesn’t need more than 9V. In fact, even 9V is only necessary for some peripherals that take power from the expansion connector. The Spectrum’s built-in voltage regulator will convert any input voltage over 7.5V down to 5V from which it will derive more voltages for its various circuits. The rest will be dissipated as heat and effectively lost. The higher the input voltage, the more of it will be dissipated as heat, shortening the life of the voltage regulator and other components.

Don’t bother with a multi-voltage adaptor. Spend the money on a good 9V 2A (at least) instead.

The 16/48K Spectrums, the original 128K (Toast Rack), and the +2 all operate on the same voltage and use the same center-negative barrel plug. A 2A PSU can be easily used to power either machine, so if you have all three, you can use a single PSU for all of them. The +2A/+3 use a different PSU that supplies multiple voltages and has a different style connector.
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Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.

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Re: Like a buss I wanted a Spectrum and ended up with 3

Post by patters » Tue Apr 16, 2019 9:01 pm

I bought a 1.5A tiny switched mode mobile phone charger sized thing for my 48K. I take it a DivMMC Future plus SD card won't pull much current? Haven't had any issues so far but haven't used it for long.
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Re: Like a buss I wanted a Spectrum and ended up with 3

Post by Ast A. Moore » Tue Apr 16, 2019 9:07 pm

patters wrote:
Tue Apr 16, 2019 9:01 pm
I bought a 1.5A tiny switched mode mobile phone charger sized thing for my 48K. I take it a DivMMC Future plus SD card won't pull much current?
The 48K Speccy draws around 700–900 mA typically. I doubt a DivMMC draws more than half an amp. I’m pretty sure you’re good.
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Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.

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Re: Like a buss I wanted a Spectrum and ended up with 3

Post by djnzx48 » Tue Apr 16, 2019 9:10 pm

Isn't the Spectrum PSU supposed to be unregulated? I've read in a few places online that the peaks of the input voltage are used to generate the 50Hz interrupt, but thinking about that now it seems kind of suspicious. If that were true then getting T-state accuracy on the interrupt timing would be pretty unlikely.
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Re: Like a buss I wanted a Spectrum and ended up with 3

Post by Ast A. Moore » Tue Apr 16, 2019 10:08 pm

djnzx48 wrote:
Tue Apr 16, 2019 9:10 pm
Isn't the Spectrum PSU supposed to be unregulated? I've read in a few places online that the peaks of the input voltage are used to generate the 50Hz interrupt.
Heh. No, that’s pretty ridiculous. :lol: The interrupt, as well all other intermediate frequencies are derived from a crystal oscillator (two in 48K machines, a quirk that resulted in the infamous dot crawl effect) by the ULA.

(Some digital clock designs used the AC line frequency for timekeeping. I even considered making one way back when, but opted for a more traditional version that used a crystal oscillator. It was a damn nice clock, too, with a large VFD that glowed blue in the dark . . . Ah, those were the days.)

Ahem. The Spectrum PSU is not supposed to be unregulated. It was just much simpler and cheaper to use a transformer, a diode bridge, and a smoothing capacitor, and let the 7805 voltage regulator inside the Spectrum do the rest (along with the heat sink). Besides, switch-mode power supply design in the early 80s was far from being perfect, and there were no specialized driver ICs available. Switch-mode PSUs were expensive, complicated, and not particularly reliable.

The Spectrum was all about cost cutting at every corner. Sinclair didn’t even bother to install a diode as a reverse polarity protection. Would have cost them pennies in bulk.

So, a modern switch-mode regulated PSU is not only recommended but preferred for any Spectrum. The machine will run cooler, more stable, and its life span will probably be extended. That, and the fact that it’ll save you quite a bit on your electric bills.
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Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.

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Re: Like a buss I wanted a Spectrum and ended up with 3

Post by djnzx48 » Tue Apr 16, 2019 10:21 pm

You're right, it didn't seem a very plausible theory. The main reason I thought that is the BASIC manual states the built-in clock (the FRAMES variable) runs at 60Hz in North America, and I didn't see how that could be possible without deriving it from the mains somehow. Do Spectrums still run at 50Hz in the US then? I know there were a few different models over there, including the Timex and the NTSC Spectrum.

I found this thread about it: https://www.worldofspectrum.org/forums/ ... ment/54720
And here's a guy making the claim: https://old.reddit.com/r/vintagecomputi ... um_repost/
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