Zx 16k not booting and distorted picture

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Yesterdaystec
Posts: 4
Joined: Sat May 28, 2022 5:51 pm

Zx 16k not booting and distorted picture

Post by Yesterdaystec »

Hi,

First technical post,bought a zx16k which has been languishing in a loft for a lifetime...bought off fleabay,from a new seller so what could go wrong!!anyhoo...

Firstly it wouldn't power up,found a poor connection in the main barrel on the motherboard due to a buildup of crud. Sorted that but when it turns on there is just a garbled screen with bars and pixels generated at random. I hope I have attached pictures so please see below,also does everything look standard,I feel it has had a 48k upgrade and there is a random PCB sticking out of a IC up top? Any ideas.

Thanks in advance
Attachments
IMG20220528164027.jpg
IMG20220528173801.jpg
IMG20220528173808.jpg
IMG20220528191003.jpg
sP1d3r
Posts: 118
Joined: Mon May 16, 2022 9:20 am

Re: Zx 16k not booting and distorted picture

Post by sP1d3r »

Yesterdaystec wrote: Sat May 28, 2022 8:50 pm Any ideas.
It's looks like an issue 2 motherboard, upper ram fitted.
The screen displays likely show ram faults and/or a duff ULA, the little board goes in a 74LS157 socket but I think that it's a factory fitted mod on some early or upgraded iss.2 motherboards.

I recall sorting out problems like the stripes from Spectrum motherboards I bought and repaired in the past and IIRC it may mean that the UL A isn't the source of the problem, which may be one or more duff capacitors.

Mind you I replaced all capacitors and diodes on the motheboard because those sort of displays are typical with rubber key Spectrum cases which have a lack of ventilation, causing excess heat.
Yesterdaystec
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Joined: Sat May 28, 2022 5:51 pm

Re: Zx 16k not booting and distorted picture

Post by Yesterdaystec »

Thanks for the comprehensive reply that is very informative,so basically my thoughts were to renew the caps as a matter of course so that ticks off one if the suggestions. I will grab a ula from somewhere and some new ram and go from there. I am quite lucky as the ram is socketed so should be relatively easy,do you have any suggestions regarding which manufactures offer the most reliable ram chips?I think mine are OKI if I recall?

Thanks
Yesterdaystec
Posts: 4
Joined: Sat May 28, 2022 5:51 pm

Re: Zx 16k not booting and distorted picture

Post by Yesterdaystec »

Also to note there is no sound,maybe because it's not booting?
sP1d3r
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Re: Zx 16k not booting and distorted picture

Post by sP1d3r »

Yesterdaystec wrote: Sun May 29, 2022 4:39 pm I am quite lucky as the ram is socketed so should be relatively easy,do you have any suggestions regarding which manufactures offer the most reliable ram chips?I think mine are OKI if I recall?
You'll notice at the left of the expansion slot there's links corresponding to the type of memory installed in the upper ram sockets, but AFAIK 4164 chips can be used whatever the links are set for.
The lower ram chips can be Mostek or STC, mainly.

A picture of an iss.4 motherboard showing a mix of chips.
Mostek & STC lower ram with TI 4164 upper.
Mostek & STC lower ram with TI 4164 upper.
sP1d3r
Posts: 118
Joined: Mon May 16, 2022 9:20 am

Re: Zx 16k not booting and distorted picture

Post by sP1d3r »

Yesterdaystec wrote: Sun May 29, 2022 4:39 pm I will grab a ula from somewhere and some new ram and go from there. I am quite lucky as the ram is socketed so should be relatively easy,do you have any suggestions regarding which manufactures offer the most reliable ram chips?I think mine are OKI if I recall?

Thanks
BTW this seller has some NOS ULAs as well as these tested ones;

https://www.ebay.co.uk/itm/394045443201 ... SwqEpiUWRv

The ram chips, capacitors or a more modern type of ram replacement can be got from Retroleum.co.uk
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1024MAK
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Re: Zx 16k not booting and distorted picture

Post by 1024MAK »

The black and white stripes indicate either a loss of the +12V and -5V supplies (especially if the picture is monochrome), one or more faulty DRAM chips, or a faulty Z80A microprocessor.

The extra PCB is to enable the issue two board to use the OKI DRAM chips to provide the ‘upper’ 32k bytes of (expansion) RAM. Issue two boards were only designed to use Texas Instruments DRAM.

So before doing anything else, test the +12V and -5V supplies on IC6 (or any other of the 4116 or equivalent ‘lower’ DRAM chips).

It’s less likely to be the ULA, as it’s clearly generating a video image, which proves at least 75% of it is working.

For testing purposes, as IC15 to IC22 (the OKI DRAM chips) can be carefully removed. Then the board will be equivalent to a 16k ZX Spectrum.

What ROM chip is fitted? Only the coloured wire (right hand side of the ULA) in your picture is unusual.

Mark
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