Power Supplies

[jokkemokke]

Hello.

I recently became owner of a few Sinclair machines with which a have no experience with at all (I am normally a Commodore user. Please don't hate me). Among those machines there is a ZX Spectrum and a ZX Spectrum+. The machines only came with one power supply: a Sinclair UK 1400 with DC barrel plug.

Before I connected anything I decided to measure the output of this supply, and it measured 12V DC with positive in the center and negative on the rim. The machines are marked with 9V DC, but I read on this forum that it is not unusual that the Sincalir supplies measures 12V when they are unloaded.

But what really confuses me is the polarity of this supply. Shouldn't both of the machines have negative in the center and positive on the rim? At least that was what I could find from googling around.

It seems like a strange defect on the power supply. I mean why would it still work, but suddenly change polarity?

Can anyone clarify this mystery?

[Ast A. Moore]

Yes, 12V and even more is typical for a Spectrum PSU without a load. However, the swapped polarity is definitely an oddity. Are you sure you haven’t accidentally swapped the leads on your multimeter? Can you double-check with, say, another known-good PSU?

I recommend getting a modern switch-mode, regulated 9V 2A (or higher) PSU and use that instead of the original one with your Spectrums, anyway. The internal voltage regulator won’t have to dissipate all the extra energy as heat as much, and the machines will run so much cooler.

[jokkemokke]

I already double checked the leads of the multimeter to make sure that I was not crazy

Maybe an explanation could be that one of the previous owners has fiddled around with it. I will open it one of the days to check it inside.

I will see if I can find an universal PSU that can deliver 2A. Probably a good idea anyways, since there is only one supply for the two machines.

[Seven.FFF]

This one works well. Just make sure you get the centre-negative version, because northpada also make a centre-positive model.

https://www.amazon.co.uk/dp/B072PRLV26

Byte Delight also sell a good one.

[jokkemokke]

Thanks, I will have a look at that one.

I was just thinking if it would be possible to just make an adapter cable for a PC power supply. I have plenty of those lying around and they should be able to give sufficient current.

Would it be a problem to supply the machine with 5V instead of 9V? Or maybe better with 7V, if I use 12V minus 5V. I guess there is a probably a LM7805 or similar inside the machine, transferring the voltage down to 5V anyways. Would that work?

[Ast A. Moore]

Would it be a problem to supply the machine with 5V instead of 9V? Or maybe better with 7V, if I use 12V minus 5V. I guess there is a probably a LM7805 or similar inside the machine, transferring the voltage down to 5V anyways. Would that work?

1. Most LM 7805 voltage regulators will require at least 7 or 8 volts of input voltage to operate reliably. I’d say feeding it 7 volts is really pushing it.
2. The 48K/128K Spectrum feeds the unregulated +9V from the PSU directly to Pin 4 (lower side) of the edge connector, and some peripherals expect it to be there. If you’re only supplying 7V, even if the regulator works, you’ll end up running into potential issues with peripherals.

In short—not a good idea.

[1024MAK]

The Sinclair ZX Spectrum 16k, 48k and Spectrum+ machines use DRAM chips that need three DC supplies:

• +5V
• +12V
• -5V

The video circuitry also needs a +12V supply.
The rest of the machine runs off the +5V supply.

So how did Sinclair do it, given that the external power supply unit (PSU) is only rated for a nominal +9V?

The answer is that for the +5V, they used a standard series pass element 7805 voltage regulator IC. This requires at least 7V or more. It's best to supply it with a minimum of 7.5V so that it has a bit of headroom to allow for voltage drop in the cable and the power connector.

For the +12V and the -5V, Sinclair designed a DC to DC converter/inverter circuit. This takes the nominal +9V and produces the required outputs. The recommended input range is between 8V and 11.9V (on-load voltages).

Later issue machines can work with higher input voltages. As the a DC to DC converter/inverter circuit was revised to work correctly with higher input voltages.

Now if you have an external PSU that can supply +5V, +12V and -5V, you can ignore the Sinclair 9V PSU. But as pointed out above, some expansions use the +9V edge-connector supply. Some manufacturers used the +9V to supply extra 7805 ICs in the expansions. Others use it in other ways.

Mark

[jokkemokke]

Thanks for the explanations.

Just an update from my side. I opened the Sinclair supply, and yes; it looked like someone had been doing some repair work on it and for some reason swapped the polarity. I swapped the cable back to normal polarity, plugged it in to the machine and it worked

One of the machines seemed to work without problems, the other one seemed to have maybe some RAM issues (garbled screen on startup). But I guess this be a good fixing/debugging project for the winter.

[silencer82]

Hello everybody,

I am working on a project with a 48k Spectrum
which I have found in trash. Its case is almost completely
destroyed by previous owner but the computer works fine.
I am trying to make a laptop with this mainboard and a small LCD
screen that is using 12V as power supply. I have an old laptop housing that has a brand new big battery. The battery provides 5V and 12V on its output port. Would it be a big problem if the Spectrum ran on 12V instead of 9V?
I am building this just for fun.
Thank you for the help.

[Mad Fritz]

The 9-ish Volt supplied to the Spectrum are AC, not DC, and converted to +5V, -5V and +12V internally IIRC.

BR / Andy