Does anyone know when UK700 PSUs shipped with ZX81s?

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Flatulentia
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Does anyone know when UK700 PSUs shipped with ZX81s?

Post by Flatulentia »

Hi peeps,

I'm asking this because I've just bought the best looking ZX81 I've ever seen in an auction and it came with a UK700 PSU, so I'm guessing it's fairly early?

This one was mis-listed on eBay so probably didn't attract as many bidders as it could have, so I got it for £31 delivered. :D

The ZX81 itself looks factory fresh without a single mark or blemish anywhere. The only sign of use is very light marking on the edge connector.

I did have a look through the expansion slot case hole and can see that the keyboard tails are mildly fractured, so that may or may not be saveable, but this one is definitely worth a new keyboard if it needs one.

It came in the original polystyrene packaging (no slip cover sadly), and the poly had bonded itself to the PSU mains cable, so I guess it had been kept in the box for a number of years. That cleaned off easily enough, so no problem there.

The original user manual is in there too and in close to mint condition. :)

So, whadda ya reckon? Anyone care to guess how early this one is before I crack it open and take a look for myself to see what clues I can find?

The winner gets a free virtual cookie! :lol:
ZX81 with ZiLOG Z84 CMOS Z80, 32KB battery-backed Toshiba 55257 SRAM with charge status LED, Wilf's "Why Wait?" mod and switch-mode 5V regulator. 50/60Hz mode switch, composite video output with Zigg's back porch mod and 1080p60 HDMI output.
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1024MAK
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Re: Does anyone know when UK700 PSUs shipped with ZX81s?

Post by 1024MAK »

UK700 PSUs are, in my experience, more common than the UK1200 PSUs.

The details in the manual are a clue (I’ll let you find the topic about different manual versions).

Mark
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Flatulentia
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Re: Does anyone know when UK700 PSUs shipped with ZX81s?

Post by Flatulentia »

Hmm. The manual is a Second Edition reprint from 1982, although I'm not sure whether it belongs with this ZX81 or was put together with it later.

I'm guessing it has something to do with the arrival of the 16K Rampack or the ZX Printer as I wouldn't be happy running either of those from a 700mA power supply.

This is the first manual I've had my hands on since the original manual with my first ZX81 in 1981, and I'm finding it a strangely interesting read after all these years.

I'll investigate further. Thanks.
ZX81 with ZiLOG Z84 CMOS Z80, 32KB battery-backed Toshiba 55257 SRAM with charge status LED, Wilf's "Why Wait?" mod and switch-mode 5V regulator. 50/60Hz mode switch, composite video output with Zigg's back porch mod and 1080p60 HDMI output.
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1024MAK
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Re: Does anyone know when UK700 PSUs shipped with ZX81s?

Post by 1024MAK »

A Zeddy and a 16K RAM pack are perfectly fine in terms of power consumption when running from a UK700 PSU.

The UK700 is rated at an output current of 700mA and a ZX81 requires between 300mA and 400mA (although most less than 400mA), leaving 300mA at least, available for expansions.

Sinclair supplied the UK1200 for use with a ZX Printer and a Zeddy, especially when used together with a 16K RAM pack.

I don’t have a figure for a 16K RAM pack. There being various designs/makes/versions will also affect things plus I don’t actually have a working Sinclair 16K RAM pack (just two faulty units). Doing the calculations, eight 4116 DRAM chips requires approximately 300mA peak current when active (they are not active when not being accessed, if not being accessed the current consumption is rather less).

Another (reliable) web site lists the current requirements of a ZX Printer as about 210mA.
The same site says the ZXpand interface requires about 120mA.

Using modern parts, the current and power consumption of a Zeddy can be drastically reduced. Fitting a DC/DC converter / switching regulator has the most effect. But also replacing the Z80 with a CMOS version and replacing the 1K SRAM chip(s) with a 32K CMOS type. Further reductions are possible by replacing the ULA with a vLA81 and finally, replacing the ROM with a CMOS EPROM/EEPROM or flash ROM chip.

More details on current and power consumption in this post (note that I listed the maximum current consumption of a ZX81 back then as 450mA, but given that most are under 400mA and only one I tested was just over 400mA, I now consider 400mA (as a round number) to be more representative).

Mark
ZX81 Variations
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Lardo Boffin
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Re: Does anyone know when UK700 PSUs shipped with ZX81s?

Post by Lardo Boffin »

Flatulentia wrote: Thu Jun 08, 2023 12:13 am Hi peeps,

I'm asking this because I've just bought the best looking ZX81 I've ever seen in an auction and it came with a UK700 PSU, so I'm guessing it's fairly early?

This one was mis-listed on eBay so probably didn't attract as many bidders as it could have, so I got it for £31 delivered. :D
Ah, so you outbid me? Glad it went to a forum member. :D

The feet don’t look like the soft sponge types so its unlikely to be very early. But it is in lovely condition.
ZX80
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TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
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Moggy
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Re: Does anyone know when UK700 PSUs shipped with ZX81s?

Post by Moggy »

A rough rule of thumb re manuals are if it's a spiral bound version then it belongs with an issue one board.
For the later issue three boards they tended to have what I call, the normal paper back book spine, again just a rough and ready rule.
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Flatulentia
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Re: Does anyone know when UK700 PSUs shipped with ZX81s?

Post by Flatulentia »

1024MAK wrote: Thu Jun 08, 2023 10:18 am A Zeddy and a 16K RAM pack are perfectly fine in terms of power consumption when running from a UK700 PSU.

The UK700 is rated at an output current of 700mA and a ZX81 requires between 300mA and 400mA (although most less than 400mA), leaving 300mA at least, available for expansions.

Sinclair supplied the UK1200 for use with a ZX Printer and a Zeddy, especially when used together with a 16K RAM pack.
Yes. I was basing my guesstimate on my very first ZX81 (450mA) and homebrew 16K Rampack (300mA) so my first 700mA PSU used to run very hot. I never owned a genuine Sinclair Rampack so had nothing to compare it to at the time.
Another (reliable) web site lists the current requirements of a ZX Printer as about 210mA.
The same site says the ZXpand interface requires about 120mA.
My original UK700 PSU always smelled of melting transformer even without a ZX Printer, so I'm not surprised they upgraded. :lol:
Using modern parts, the current and power consumption of a Zeddy can be drastically reduced. Fitting a DC/DC converter / switching regulator has the most effect. But also replacing the Z80 with a CMOS version and replacing the 1K SRAM chip(s) with a 32K CMOS type. Further reductions are possible by replacing the ULA with a vLA81 and finally, replacing the ROM with a CMOS EPROM/EEPROM or flash ROM chip.
My limit so far has been changing the CPU, RAM and regulator. I think the last time I measured the one in my sig it was around 300mA including 10mA for the rechargeable backup battery, so well worth it for the wasted heat it avoids.
ZX81 with ZiLOG Z84 CMOS Z80, 32KB battery-backed Toshiba 55257 SRAM with charge status LED, Wilf's "Why Wait?" mod and switch-mode 5V regulator. 50/60Hz mode switch, composite video output with Zigg's back porch mod and 1080p60 HDMI output.
Lardo Boffin
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Re: Does anyone know when UK700 PSUs shipped with ZX81s?

Post by Lardo Boffin »

A few things to look out for in very early zeddies:

Soft spongy feet - not solid rubber.
Black ZX80 PSU - not a UK700.
First Edition manual.
Different tape cables:

These:
IMG_2163.jpeg
rather than these:
IMG_2164.jpeg

Old style TV cable:
IMG_2165.jpeg
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
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Flatulentia
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Re: Does anyone know when UK700 PSUs shipped with ZX81s?

Post by Flatulentia »

Lardo Boffin wrote: Thu Jun 08, 2023 12:15 pmAh, so you outbid me? Glad it went to a forum member. :D
Probably only by a few pence. I'm sneaky like that. ;)
The feet don’t look like the soft sponge types so its unlikely to be very early. But it is in lovely condition.
Correct. They're hard feet, not squidgy. It's in such amazing condition though that I'm torn as to whether to leave it alone or upgrade it. I'll have a think about it for a few days before doing anything drastic. :)
ZX81 with ZiLOG Z84 CMOS Z80, 32KB battery-backed Toshiba 55257 SRAM with charge status LED, Wilf's "Why Wait?" mod and switch-mode 5V regulator. 50/60Hz mode switch, composite video output with Zigg's back porch mod and 1080p60 HDMI output.
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Flatulentia
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Re: Does anyone know when UK700 PSUs shipped with ZX81s?

Post by Flatulentia »

Moggy wrote: Thu Jun 08, 2023 12:37 pm A rough rule of thumb re manuals are if it's a spiral bound version then it belongs with an issue one board.
For the later issue three boards they tended to have what I call, the normal paper back book spine, again just a rough and ready rule.
Thanks Moggy. I found the thread on ZX81 manuals. I had no idea there were so many variants.
ZX81 with ZiLOG Z84 CMOS Z80, 32KB battery-backed Toshiba 55257 SRAM with charge status LED, Wilf's "Why Wait?" mod and switch-mode 5V regulator. 50/60Hz mode switch, composite video output with Zigg's back porch mod and 1080p60 HDMI output.
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