Modern PSU: picture improvement?

Jim M · December 2015

I've recently seen advertised some 'replacement power supplies' for Spectrums for which the claim is made that they may make an improvement in picture.
Does anyone think this is a reasonable claim to make?
'Stable PSU, Cooler Speccy and improves display'.
They look like generic, modern black power supplies.
I ask the question from a technical viewpoint, not to be interpreted as a negative or positive remark about this particular product.

Jim

balford · December 2015

I'm not sure about any display improvements, but they definitely are:

1) More stable - modern switch mode PSU's will put out a steady 9v under or without load, unlike the linear Speccy PSU that puts out 15v off load and about 11v under load.
2) Cooler - as the 7805 voltage regulator effectively dissipates the extra voltage as heat, it stands to reason that less heat will be generated in regulating 9v to 5v, than 11v to 5v.

Personally, I don't use the original PSU's any more and instead use a switch mode 9v supply rated at 2A, which can power all Spectrums except the +2A/+3 (and ZX81's with an appropriate adapter).

B

DEATH · December 2015

I can't really see that a new PSU would improve the display unless the original supply was faulty in some way

Jim M · December 2015

I might try one of these then. It's ridiculous that the ZX power supply isn't fit for purpose (so it seems) and, presumably, unnecessarily stressing the voltage regulator in the Spectrum.

Zorn · December 2015

The original supply is fit for purpose, though it can be improved upon. As pointed out, there are some advantages to using a modern switching type of supply. However, I wouldn't expect it to cause a Spectrum to produce a better picture, unless the original supply is faulty in some way. In fact, a poorly designed switching supply would be more likely to cause picture interference than the original type.

Jim M · December 2015

I have 2 ZX power supplies and one replacement Sinclair sent me when my old one broke (c. 1983) and am confident they are not causing any problems.

I think what has disturbed me about this issue is that I have had a piece of equipment fail on me because I was using what I thought was an ok supply and apparently wasn't - a totally different piece of kit - a Sony pro walkman - which I blew up, twice, being bloody minded at not purchasing one of their Sony £16 supplies (in 1993!) and using one which was the correct voltage but a few milliamps out.

Thanks for the reassurance anyway!

RetroTechie · December 2015

Switchmode power supplies (like the small 'bricks' that come with mobile phones) tend to have more noise on their output. But that's only a problem if that noise is a) bad enough, AND b) doesn't get filtered somehow on its way to video components. Not likely to be a problem when used for a ZX Spectrum, I think.

These days, I think there's a few good methods to power a ZX Spectrum:

1) Bypass onboard regulation and feed regulated +5V (and whatever other voltages are used by the specific Spectrum model) directly from an external psu. Not the most versatile option, but minimizes internal heat.
2) Replace +5V regulator with a low drop version (like LM2940, LM1086CT-5.0 or similar), and feed with ~6V regulated DC supply. Also keeps internal heat to a minimum. Not sure if voltage conversion circuitry (16/48K models) would work with such a low input voltage, maybe someone who tried can comment on that?
3) Replace +5V regulator with a switching replacement (see Recom, Tracopower, ....). This allows a wide-range input voltage, and also keeps heat losses down. Works fine with original ZX power supply.

For both 2 and 3 you could add a (Schottky?) diode to prevent reverse-polarity mishaps. And of course you need a psu + regulator that can handle the current draw.

Other options just produce more waste heat inside the Spectrum than necessary. And thus shortens life of ULA, keyboard foil, RAM chips etc. :(

1024MAK · December 2015

balford wrote: »

unlike the linear Speccy PSU

Gggahhh! Don't use that word (linear). The Sinclair ZX PSUs as used with the ZX80, ZX81, ZX Spectrum 16k / 48k / plus / 128k+ (toastrack) and the Amstrad +2 (grey) are all just a transformer, some diodes and one or two smoothing capacitors. The correct term is "conventional smoothed DC power supply". In no way can any part of the any of the characteristics be described as "linear" X( X( X(

As said above, the original Sinclair PSUs are fit for purpose. Spectrums have been left on for days at a time using the Sinclair PSUs and suffered no problems at all. Of course, with more modern SMPSU (switch mode power supply unit) technology, you can buy something better these days... ;-)

Does using a modern PSU improve the picture? No, not if you are comparing to a healthy Sinclair PSU.
Cooler Spectrum, well if they output a lower voltage than a Sinclair PSU, yes the 7805 series voltage regulator will dissipate less power as heat, so the Spectrum will run cooler (well, slightly).

The thing to remember is that there are at least SIX different classes of PSU. In brief:
1) AC output,
2) unsmoothed DC output,
3) smoothed DC output (unregulated), [Sinclair type for computers up to and including the Amstrad +2 (grey)]
4) regulated (stabilised) DC output (+2A, +2B, +3)
5) SMPSU with regulated outputs
6) SMPSU with unregulated outputs

Plus the polarity if DC. And the voltage rating (and range)

So it is easy to plug the wrong PSU (or the wrong polarity) in to an item of equipment and blow it up
And of course watching out for polarity and the actual output voltages...

Mark

guesser · December 2015

a transformer and a FUUUULLLL BRIDGE RECTIFIER!!!! (sic) :)

balford · December 2015

Sorry, 'unregulated'. I was multitasking at the time :D

B

Modern PSU: picture improvement?

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