16K internal RAM mod on issue one ZX81

Discussions about Sinclair ZX80 and ZX81 Hardware
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mafty
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Joined: Thu Sep 09, 2021 8:14 pm

16K internal RAM mod on issue one ZX81

Post by mafty »

Hi all.
New to the forum, but old school ZX81 user (it was my first computer, back in '81)

I have just completed the internal 16K mod, using the 32K Alliance 62256 chip, as detailed here...
http://www.zx81.de/english/32k-rame.htm

When I power the computer up, all I get is a white screen.
It was working fine with the original 1K chip, and I have checked and double checked all connections.
There are no stray solder blobs or unwanted bridges that I can see, so I am at a loss.

Anyone have any ideas as to what the problem is?
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1024MAK
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Location: Looking forward to summer in Somerset, UK...

Re: 16K internal RAM mod on issue one ZX81

Post by 1024MAK »

Hi mafty

You may want to introduce yourself by starting a new topic in the Welcome section.

I’ve moved your post into the Hardware section.

Can you please post photos of your ZX81 board, both top and bottom, showing what you have done.

Mark
mafty
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Re: 16K internal RAM mod on issue one ZX81

Post by mafty »

Here is what I have done to my ZX81
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IMG_5454.JPG
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Dim_75
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Re: 16K internal RAM mod on issue one ZX81

Post by Dim_75 »

Are sure that your Z80 is not upside down ???? :shock:

it should be like that :
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Moggy
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Re: 16K internal RAM mod on issue one ZX81

Post by Moggy »

Well spotted that man, hopefully found a cure.
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BarryN
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Re: 16K internal RAM mod on issue one ZX81

Post by BarryN »

Dim_75 wrote: Thu Sep 09, 2021 10:07 pm Are sure that your Z80 is not upside down ???? :shock:

it should be like that :
Ouch! That probably fried the CPU. Hopefully it didn't cook anything else!
mafty
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Re: 16K internal RAM mod on issue one ZX81

Post by mafty »

Well spotted guys! That was the issue.
BUT, in my defence, I took the Z80 out earlier to check the pins for corrosion etc, and the socket is actually upside down!
See the pic...
Anyway, thanks so much guys, it seems to be working now.
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1024MAK
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Re: 16K internal RAM mod on issue one ZX81

Post by 1024MAK »

mafty wrote: Fri Sep 10, 2021 8:39 am BUT, in my defence, I took the Z80 out earlier to check the pins for corrosion etc, and the socket is actually upside down!
See the pic...
Ahh, you have to be careful with Sinclair ZX80, ZX81, TS1000, ZX Spectrums etc, as it’s not uncommon for the production line to sometimes fit the sockets the wrong way round. Seen this several times now.

It’s wise to make note of what is fitted and it’s orientation before removal. Better still, take some photos. If in doubt always double check by referring to the official layout diagrams where available, or checking against known good photos.

Beware that on some Sinclair boards, an electrolytic capacitor has the incorrect polarity marking on the silk screen printing.
mafty wrote: Fri Sep 10, 2021 8:39 am Anyway, thanks so much guys, it seems to be working now.
Anyway, good to know it’s now working. Have fun :D

Mark
Neurogate
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Re: 16K internal RAM mod on issue one ZX81

Post by Neurogate »

Dim_75 wrote: Thu Sep 09, 2021 10:07 pm Are sure that your Z80 is not upside down ???? :shock:
It was a brilliant case and remind me Lewis Carroll: "I see nobody on the road," said Alice. "I only wish I had such eyes," the King remarked in a fretful tone. "To be able to see Nobody! And at that distance too!". I also wish to have such eyes :)
Sinclair ZX81, Sinclair QL, Sinclair ZX Spectrum +2, Atari 130XE, Commodore 64C, Yamaha YIS503IIIR MSX2 & etc
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