ZX Spectrum - recommended upgrades?

Trevor_B
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ZX Spectrum - recommended upgrades?

Post by Trevor_B »

I've just bought a 48k Speccie from Ebay - my 3rd (though the other 2 haven't been out of their boxes in ~8 years..!)

This one is a rebuild / renew project to tide me over until I'm allowed to visit a pub with friends again (currently 3 months, 11 days - not that I'm counting!)

So - first things first; it's just a bare machine with no power supply. I bought a power supply for my previous lockdown project - a Tynemouth Minstrel. I believe it's a similar power supply to the Atari VCS (9v output - I need to check the polarity at the plug). I also use this on my recently upgraded ZX81 - I built a convert wire to make it fit the standard ZX81 socket. Question is - would this be adequate for the Speccie - I have the option to make a second convertor wire to switch polarity if needed.

Secondly - Composite mods; I've seen articles that range from v.simple to unexplainably difficult - can anyone recommend a particular reliable composite mod?

Third - I've swapped the voltage regulator on the '81 for a modern 7805 - is this something recommended on the speccie? Judging by the size of the aluminum plate I assume there was / is quite a lot of heat generated by the standard regulator?

..and that's all I've come up with. Any other recommendations?
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1024MAK
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Re: ZX Spectrum - recommended upgrades?

Post by 1024MAK »

48k ZX Spectrum computers require a DC voltage of between 8V and 11.9V (8 to 9V nominal) at a current of around 700mA. The power supply can be a regulated unit or an unregulated (but smoothed) type. However it’s recommended that you use a power supply rated at 8V to 9V DC with a current rating of 1.4A.

The power socket is a 2.1mm barrel type. Positive (+) on the outside, the inner being the negative (-).

Do you know which issue board is in it? Issue 1 to issue 4A boards (unless modified to the latest Sinclair modification state) have a problem if the input voltage goes above 11.9V. The required modification details are here.

It’s recommended to renew the electrolytic capacitors in the DC-DC power supply converter circuitry. As a failure of this can kill the ‘lower’ DRAM chips. But as all of them are old, and some affect the quality of the video picture, it’s normal to renew all of them.

For the composite modification, I recommend the version that uses an electrolytic capacitor in series with the output to the centre pin of the socket.

Yes, you can fit a RECOM, TRACO POWER or similar DC-DC converter in place of the 7805. In fact, I strongly recommend it.

You may also need a new keyboard membrane.

A reset switch is handy, but the trouble is finding a position on the rubber key case...

Mark
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Trevor_B
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Re: ZX Spectrum - recommended upgrades?

Post by Trevor_B »

Thanks Mark

I've got a capacitor kit on order, along with a new keyboard membrane (it needed one - and a new aluminum keyboard "cover"), and a Traco Power TSR 1-2450. The rubber keyboard itself wasn't pleasant, but fairy liquid and a toothbrush got rid of the grime. The case got the same fairy liquid and toothbrush treatment too.

It's a 4a board so I'll be looking closely at your modification page - it looks like a serious bit of work.

I'm not to bothered about a reset switch - I'll modify one of the inline powerswitch adapters to suit the speccie. The power supply I have is a stable 9v - it doesn't seem to fluctuate by more than 1/10th volt from what I've seen from watching a voltmeter. The much modified '81 seems to b very stable with it and doesn't appear to get too warm. I do have an original Sinclair power supply but I don't trust it - after 35-40 years I'd rather not plug it in.

So - just the composite mod to sort out (or at least order some parts for.

Thanks again - I appreciate the recommendations

Trev
Lardo Boffin
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Re: ZX Spectrum - recommended upgrades?

Post by Lardo Boffin »

The cap kit from Retroleum comes with some extra bits for the composite mod:

https://www.retroleum.co.uk/zx-spectrum-capacitors
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1024MAK
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Re: ZX Spectrum - recommended upgrades?

Post by 1024MAK »

The original Sinclair Power Supply Units (PSU) type UK1400 are actually a conventional mains transformer, a thermal fuse, a bridge rectifier (using individual diodes) and one or two electrolytic capacitors.

The units were built to British Standards so the transformer is a split bobbin type. With this type, the mains (primary) winding kept totally separate from the output (secondary) winding by a plastic separator/barrier.

As long as they have not been abused (dropped and damaged) this makes them as safe, if not more safe than a lot of modern Switch Mode Power Supply Units (SMPSU) that are available today. And it’s definitely far more safe than poor quality cheap SMPSU from the Far East.

Be aware however that they are of the smoothed type and do not have regulation circuitry. So although the nominal output is 9V DC, if the output is not connected, or the load is not drawing the rated current (the UK1400 is rated at a maximum of 1.4A) the output voltage will be higher than 9V. With the output not plugged into a ZX Spectrum, it can be between 12V and 15V. With a ZX Spectrum connected, the output voltage is typically between 10V and 12V.

If you are still unsure, open up the UK1400 and have a look inside, it only three screws. Obviously do make sure that it’s not plugged into the mains!

If a UK1400 PSU does need repair, there are typically only four common faults:
  • Output cable faulty due to the copper conductor breaking either near the PSU or at/near the barrel connector - replace the cable or cut out the bad section and fit a new barrel connector.
  • Electrolytic capacitor(s) inside the PSU low in value, replace with a 4700uF 25V 105°C type.
  • Rectifier diode failure - replace all the diodes.
  • Thermal fuse failed - this can be detected if there is no output voltage at all, and the resistance between the line (live) brown and neutral blue wires is infinity / open circuit. As the thermal fuse is normally embedded in the transformer, this makes it impractical to repair it.
The only real safety concerns with the original Sinclair PSUs, is how well the previous owner fitted the mains plug. Do ensure a plug with sleeved L and N pins is used, that a genuine 2A or 3A mains fuse to British Standards is fitted (yes, there are some fake fuses that have found their way into the country), that the cord grip is firmly holding the black outer sheath, that the correct colour wire goes to the correct terminal, and that where the conductor wire goes into the terminal, it’s just copper wires and there is no sign of any silver coloured solder. In other words, the solder tinned ends have been cut off.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
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Trevor_B
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Re: ZX Spectrum - recommended upgrades?

Post by Trevor_B »

All the new components have arrived. As always I've tested the machine before I start (albeit with a new membrane so the keys work!).

All looks good prior to starting...

However - looking at the board I spotted something unusual, or at least something that made me look at pictures of a normal "cream" 4a motherboard on the web.

So - is this normal? The soldering looks to be factory finished, so I don't think it's a user mod...

It's also not obvious from the picture but the 2 diodes and the capacitor are in mid air
Capacitor.jpg
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1024MAK
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Re: ZX Spectrum - recommended upgrades?

Post by 1024MAK »

Trevor_B wrote: Mon Mar 22, 2021 4:32 pm However - looking at the board I spotted something unusual, or at least something that made me look at pictures of a normal "cream" 4a motherboard on the web.

So - is this normal? The soldering looks to be factory finished, so I don't think it's a user mod...

It's also not obvious from the picture but the 2 diodes and the capacitor are in mid air
That is part of the modification I talk about here:
1024MAK wrote: Thu Mar 11, 2021 1:05 am Do you know which issue board is in it? Issue 1 to issue 4A boards (unless modified to the latest Sinclair modification state) have a problem if the input voltage goes above 11.9V. The required modification details are here.
Hence although it says issue 4A on the silkscreen, it’s actually effectively an issue 4B board.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
Trevor_B
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Re: ZX Spectrum - recommended upgrades?

Post by Trevor_B »

Thanks Mark. Maybe things aren't so bad after all then!

I now need to pick your technical brain...


One thing I couldn't try before doing the composite mod was the colour; my only analog TV is black & white. All looked ok.
Having made the composite mode (with capacitor, a 100uf 16v- as per retroleum) and plugged it into a colour composite adapter and also via a composite input on a normal telly, there is a distinct green tinge - a yellowy green kind of colour. It might have been like that beforehand, but I had no way of finding out.

Any clues? I'm thinking about taking out the capacitor, if that could be a cause?

Trev
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1024MAK
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Re: ZX Spectrum - recommended upgrades?

Post by 1024MAK »

The coupling capacitor that you added (100µF 16V) is not the cause of this. The colours are determined by the U and V outputs from the ULA, then the transistors TR8 and TR9, and IC14 (LM1889) and the surrounding resistors and capacitors, plus crystal X2.

Does the colour change at all as the Spectrum warms up over about half an hour?

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
Trevor_B
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Joined: Thu Nov 05, 2020 12:31 am

Re: ZX Spectrum - recommended upgrades?

Post by Trevor_B »

tbh I haven't left is powered up for that long yet; I'll have another go tomorrow and leave it plugged in for a while.

I'll let you know how it goes.
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