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Spectrum ZX 128k - complete newbie

S

stevedias_uk

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Hi all,

I'm a complete novice to these vintage computers as i just unearthed my childhood computer - a spectrum ZX 128k.

Please help as i can't seem to get a consistent load screen as it seems to differ every time i press the reset button however the screens with the most info is attached

Any ideas to get this going and relive some memories from over 30 years ago.

Thanking you all in advance
 

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Hi Steve - how do you have this connected to your TV / display - via RF and tuned in, or via an RGB cable plugged into the monitor-out socket?
 
stevedias_uk said:
Hi all,

I'm a complete novice to these vintage computers as i just unearthed my childhood computer - a spectrum ZX 128k.

Please help as i can't seem to get a consistent load screen as it seems to differ every time i press the reset button however the screens with the most info is attached

Any ideas to get this going and relive some memories from over 30 years ago.

Thanking you all in advance
Click to expand...
That's a really weird screen to get. I have a +2A too and have never seen that before. It might be video ram, but it looks like it's managing to do quite a lot and I would think you'd get more random artifacts on the screen...

You could try running the built in self test by holding down Break key whilst Resetting it and on the test pattern screen, press and hold QAZMLP for a few seconds and itll take you through some simple tests. That might uncover some more issues for you...
 
That's good info about the test modes, I wasn't even aware of those.

It looks to me as though the display can't really decide how to frame the old-school video signal coming out of the Spectrum - in one image it's put the initial prompt right at the bottom of the screen and in the other it is 'torn'. My guess would be that it would look absolutely fine on any old CRT TV about the same age as the Spectrum.

I'm guessing the OP is using either an RF or composite video connection between the Spectrum and the display and if so I think he might have more luck using an RGB connection to the monitor, if the monitor has RGB-in available (for example on a SCART socket). RGB generally looks sharper and crisper anyway, so I'd always favour RGB over any other type of connection.
 
A Spectrum 128K has a menu that comes up in the middle of the screen on boot. Last I hit a problem like this, it was caused by a faulty AY-3-8912 chip. Cost a small fortune to find a replacement.

The border color is related to how far it gets through it's POST.


Anyway, you can get a test ROM that you can put into the socket to assist in troubleshooting.

Google Retroleum.

BTW, what revision 128K is it?

There is the original Toastrack, the Grey +2, the Black +2 and the +3.
 
Hi all and thanks for the very useful replies.

Yes its connected via a RF cable with the modern LCD TV being tuned on an analogue signal.

I think i will try an old TV first as i can manage that.

Any ideas how much its worth sold as described assuming i cant get hold of an old TV?
 
Ah, I missed the obvious +2A bit.

These are the schematics to assist.

Diagnostic Rom: https://www.nightfallcrew.com/wp-co...ds/2020/03/ZX-Spectrum-Diagnostics-GitHub.pdf

Service Manual: https://speccy4ever.speccy.org/doc/2b3bsm.pdf

Roms: https://speccy4ever.speccy.org/_SIN.htm

If you run the diagnostics ROM and it finds no faults, then it can be something hanging it in the ROM, though usually it should hang at the same place every time, so the diags rom should help find the issue.

The 2A has the Amstrad Gate Array which are reputed to be fairly solid, but they suffer from memory related issues with a couple of 41464's in each 64K block. So check IC5 and IC6. Also, depending on the signals from the AY-3-8912 it will hang in software looking for a status signal on a test which can cause lockups and failure to boot into the menu.

Also, check both roms are working and have the correct Checksum/data since the boot software switches between 4 possible images during boot and if one of the roms, or part of the rom, is missing, it can cause issues.

General testing aside from that. Check the address lines etc are working at the correct levels. Look for corrosion or dry joints on the PCB ( especially around the gate array ).

David
 
Second image in the OP has +2A on the on screen image, presumably that makes it one of the Grey (Amstrad) 128s. (Edit: cross posted with David above.)

I would not be in too much of a hurry to look for a hardware cause for this, it has all the hallmarks of a modern flatscreen looking at the video signal, trying to work out how best to fit it on the LCD screen, and not doing it very well. Definitely try it on another TV first, ideally not a modern flatscreen.
 
Any ideas how much its worth sold as described assuming i cant get hold of an old TV?
Click to expand...

For likely worth, my answer is always that you should check SOLD listings on ebay and similar to see what they have sold for recently, and bear in mind that it would be ones sold as 'for spares or repair' that you would need to check because obviously ones stated to be fully working will have sold for a bit more.

But don't give up so easily ;) Ask around to see if friends have anything - even other flatscreen TVs are worth a try. Even in the here and now it is surprising how many current TVs still have the ability to tune for analogue signals when (In the UK at least) there have been no analogue TV transmissions for quite some years now.

If it looks OK via an RF connection to other TV sets, just not on yours, then it is probably worth trying the modest outlay for a ready-made +2A to SCART RGB connection lead - that may work better with the TV you already have and if it does it will give you a better picture anyway.
 
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