Meet Flatulentia's American Friend

General Chit Chat about Sinclair Computers and their Clones
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Flatulentia
Posts: 93
Joined: Sun May 14, 2023 3:58 pm
Location: Cambridgeshire, UK

Meet Flatulentia's American Friend

Post by Flatulentia »

Hi guys,

I've recently taken delivery of this US version ZX81 from a UK eBay seller for £25 delivered. Bear in mind that I'm based in the UK, so this was quite an exciting find for me.

It was listed as "for parts or not working", but all ICs are socketed, so there may be some hope of resurrecting it if it actually has croaked.

The case is in salvageable condition with almost perfect paintwork on the ZX81 logo and only one missing foot due to leprosy. It just has the usual "ch2 ch3" switch on the bottom, so I didn't bother taking a photo of that.

Even the keyboard tails haven't aged too badly on this one, so it looks as though the keyboard may be fully (re)usable.

P1010378.JPG

And here's the PCB

P1010374.JPG
P1010375.JPG

I haven't tested it yet beyond a quick check of the 7805 regulator that shows a 'hot' 5.37V, so I won't power it up again before swapping that out for a switch-mode jobby I have in my spares box.

The internal photos are just to check if anyone else can see any unusual ICs in there. They all look standard enough to me, but there are far more knowledegable people here on historic variants than I, so it seemed best to share photos before doing anything drastic with it.
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.
Moggy
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Joined: Wed Jun 18, 2008 2:00 pm

Re: Meet Flatulentia's American Friend

Post by Moggy »

Nothing out of the ordinary there apart from the non UK modulator.

Second issue NEC ROM (622), Ferranti second issue ULA (184), NEC CPU and a Mostek 1k RAM chip, I always thought the US computers had a 2k chip fitted but maybe that was just the Timex versions.
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Flatulentia
Posts: 93
Joined: Sun May 14, 2023 3:58 pm
Location: Cambridgeshire, UK

Re: Meet Flatulentia's American Friend

Post by Flatulentia »

Thanks Moggy. I wasn't sure on the 1K/2K RAM situation either as this one is clearly 1K.

At least it's in a socket so I can upgrade it to 16K/32K internal RAM easily enough if it turns out to be a runner.

EDIT: Having said that, I've just found 3x NOS Toshiba TC55257APL-85L on eBay for £10 delivered. 8-)
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
Posts: 93
Joined: Sun May 14, 2023 3:58 pm
Location: Cambridgeshire, UK

Re: Meet Flatulentia's American Friend

Post by Flatulentia »

I finally got around to having a proper look at this one. It was just the dodgy 7805 regulator that needed replacing, so it now has a switch-mode jobby.

I've already slapped a big heatsink on the ULA and given it a Zigg's composite video mod inside the gutted UM1082 modulator can.

Next up will be a CMOS Z80 and 16KB internal RAM mod with a select-on-test timing adjustment capacitor for correct hi-res operation if it needs it.

The case has cleaned up nicely and the keyboard was slightly worse than I thought, but salvageable by cutting back the tails.

It's great to see what could have been a bin job alive and kicking again. :)
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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