UK700 PSU running at 18.1 volts

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Lardo Boffin
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UK700 PSU running at 18.1 volts

Post by Lardo Boffin » Mon Oct 23, 2017 5:04 pm

Hi all

I have just bought a zeddy with a PSU that runs at 18.1 or so volts with nothing attached. Needless to say I have not plugged this PSU into the zeddy! Is this a safe operating voltage for one of these? Most of the ones I have tested have run at around 14 to 15V without load.

Lardo
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gozzo
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Re: UK700 PSU running at 18.1 volts

Post by gozzo » Mon Oct 23, 2017 7:24 pm

hmm, may have shorted turns on the primary winding, does it get warm/hot after a while when on no load? if so it could have. also test voltage with a reasonable load...

gozzo
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Re: UK700 PSU running at 18.1 volts

Post by gozzo » Mon Oct 23, 2017 7:26 pm

and ..what mains voltage is it rated for and what is it running on. if its designed for 220v (but that would normally be a EU700? or something like that?) and being run on 240v , it will be high....maybe has a 220v transformer fitted in a uk/240v case???

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1024MAK
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Re: UK700 PSU running at 18.1 volts

Post by 1024MAK » Mon Oct 23, 2017 8:27 pm

I have tested a number of UK700 and UK1200 Sinclair PSUs. The maximum unloaded output voltage is normally 16V.

The electrolytic capacitor(s) inside the PSU is normally rated at 16V, so this may not be happy with the over voltage.

As to a ZX81, the 7805 voltage regulator has a maximum input voltage of between 30V to 35V (depending on manufacturer). However the manufacturer recommendation is to operate them at no more than 25V.

As the "9V" input does not feed anything else inside a ZX81, it should not harm a ZX81.

However, if the "rogue" PSU outputs a higher than normal voltage when connected to a ZX81, the 7805 heatsink will run significantly hotter than normal, and this PSU may damage some third party 16k byte RAM expansions.

Mark

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1024MAK
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Re: UK700 PSU running at 18.1 volts

Post by 1024MAK » Mon Oct 23, 2017 8:30 pm

gozzo wrote:
Mon Oct 23, 2017 7:26 pm
and ..what mains voltage is it rated for and what is it running on. if its designed for 220v (but that would normally be a EU700? or something like that?) and being run on 240v , it will be high....maybe has a 220v transformer fitted in a uk/240v case???
Sinclair PSUs have been found with the wrong label (or case) fitted before...
Just like everything else "made" by Sinclair, the production of their PSUs was done by a number of different companies. I have found four designs so far just for the UK700 version...

Mark

Lardo Boffin
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Re: UK700 PSU running at 18.1 volts

Post by Lardo Boffin » Mon Oct 23, 2017 8:51 pm

1024MAK wrote:
Mon Oct 23, 2017 8:27 pm

However, if the "rogue" PSU outputs a higher than normal voltage when connected to a ZX81, the 7805 heatsink will run significantly hotter than normal, and this PSU may damage some third party 16k byte RAM expansions.

Mark
That’s an interesting point about the plug ins. This zeddy has hardly been used - it is in superb condition. Even the paint on the PSU is pristine. It came bundled with a memotech 16k RAM pack which is also pristine but does not work. It either produces a black screen or various patterns on other zeddys.
I wonder if the over zealous PSU killed it? Regardless I guess I will need to find another one to go with this zeddy before I sell it.
If this PSU is any use to anyone they are welcome to it for p&p.
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Various Spectrum 48ks plus a DIVMMC future and SPECTRA

Lardo Boffin
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Re: UK700 PSU running at 18.1 volts

Post by Lardo Boffin » Mon Oct 23, 2017 8:56 pm

On the plus side a dead memotech RAM pack means my ZXpand finally has a home! Although it is an original plus AY so that complicates it somewhat. :roll:
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Various Spectrum 48ks plus a DIVMMC future and SPECTRA

Lardo Boffin
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Re: UK700 PSU running at 18.1 volts

Post by Lardo Boffin » Fri Oct 27, 2017 12:26 pm

I took the lid off and took some photos. It does have a 16V capacitor so presumably it is the correct internals just incorrectly built?
7EBB1B2B-9A96-4175-BACA-ADC15CFBC595.jpeg
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4BBE0174-3D72-45AC-A491-729234C25B84.jpeg
(506.92 KiB) Downloaded 327 times
B02992E5-F3E0-472D-93C9-378EB898BA11.jpeg
(548.3 KiB) Downloaded 327 times
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Various Spectrum 48ks plus a DIVMMC future and SPECTRA

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1024MAK
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Re: UK700 PSU running at 18.1 volts

Post by 1024MAK » Fri Oct 27, 2017 2:03 pm

Most likely, the transformer has been wound for a 220V mains supply.

Unloaded, a small mains transformer can produce an output voltage that is greater than the rating by between 15% and 25%.

If your local mains voltage is slightly high (although officially our mains is now 230V AC, in practice the power distribution companies have not changed most of the existing transformers, and these often give a voltage of between 235V and 250V).

Combined, these two factors alone can result in an output that is 20 to 40% higher.

Then take account of the 2200uF capacitor, which with no load, is charging up to the peak rectified secondary voltage (AC voltage x 1.414 gives the peak DC voltage), and you can see why the unloaded DC voltage can be much greater than the nominal 9V DC voltage. Often between 12V and 16V, and in this case, 18.1V...

Mark

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gammaray
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Re: UK700 PSU running at 18.1 volts

Post by gammaray » Fri Oct 27, 2017 10:14 pm

If anybody cares... it is rogue as in outlier not rouge as in makeup :?
5-TS1000,UK ZX81<-Sheelagh, US ZX81, 2-TS1500/KDLX , 3-TS2040 printer, 2-TS2020 cassette decks, ZXPAND+AY, ZeddyNET, ZXBlast, UDG, ZX8CCB, AERCO, BUILDS/REPAIRS ZX Spectrum, ZX80 Minstrel, ZXMAX48 v1 v2, 2-TS-2068, ROM, 16kRAM

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