ZX Spectrum issue 2 12v low missing bits of copyright logo

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andrewpurkis
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Joined: Mon Sep 20, 2021 7:15 pm

ZX Spectrum issue 2 12v low missing bits of copyright logo

Post by andrewpurkis »

Hi

I have dug my zx spectrum out of the loft after 25 yrs and thought id give it a go

ok so power on and RF output is very wonky

so i did the RF mod and then tried again

now boots up and I get the Sinclair logo and its stuck there

looking at the logo its seems to be missing various bits of it

so i dug my multimeter out and tested the rails on the lower ram chips

and it seems the +5v and +9v are there

but the 12v rail is low at 10v

any ideas?
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1024MAK
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Re: ZX Spectrum issue 2 12v low missing bits of copyright logo

Post by 1024MAK »

The most important supply rail as far as the ‘lower’ RAM (made up of 4116 or equivalent DRAM chips) is concerned is the -5V rail. If this is out of specification (-5.5V to -4V) or missing then there is a problem with TR4 (and maybe a problem with TR5).

You can check the -5V rail on pin 1 on IC6. See the pin out here.

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