ZX Fail

mtullock
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Joined: Mon Aug 10, 2020 11:12 am

ZX Fail

Post by mtullock »

Hi All,
Thanks for letting me join! I have just dusted off my ZX spectrum 48K rubber key computer. Having had to replace one or two bits and pieces (membrane for example) I was all excited to try one of my old games. However, even though the machine powers on and gets to the (c) Sinclair Research Ltd, whenever I try to enter a command (and it can be any command) when I press return it goes white and then reboots and comes back to the prompt. Any ideas? I was wondering if it was a RAM issue?

cheers
Magnus
RWAP
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Re: ZX Fail

Post by RWAP »

I had this before and the problem is that when you press the enter key, it causes a short somewhere, or twists the heatsink against the 5V regulator. I resolved it by putting a small pad of felt (or a spare rubber key) on top of both the TV modulator and the heatsink
mtullock
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Joined: Mon Aug 10, 2020 11:12 am

Re: ZX Fail

Post by mtullock »

Thanks for the advice. Was just away to try it and the fault seems to be deeper rooted. Typing more than a few characters ends up with this:

Image

cheers
Magnus
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1024MAK
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Re: ZX Fail

Post by 1024MAK »

Looks like a RAM fault. But do try putting an insulator between the top of the heatsink and the tail of the membrane, do the same between the top of the case of the modulator and the other tail of the membrane. Sinclair used either self adhesive foam pads or self adhesive rubber feet. But for testing, insulation tape, masking tape or even a sheet of paper will do.

Which issue board is it?

Do you want to try fixing it yourself?

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
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mtullock
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Joined: Mon Aug 10, 2020 11:12 am

Re: ZX Fail

Post by mtullock »

cheers, yeah it was doing it with the case secured down and also tried it floating so that no downward pressure was on mobo but still happened. It is also a new membrane for the keyboard. Seems odd that I can add a keyword like LOAD or NEW but as soon as I add another character it starts to glitch. Would that be upper ram? The black screen on boot seems quite quick which also made me wonder about the RAM. I am keen to try and repair it myself but don't have a huge amount of specialist tools.

I gave it to a relation of mine years ago which is when it started to fail, before he gave it back to me he took it to a repair shop and they said it was not repairable...... i find that doubtful? (This was very long ago) Would a video of it booting and failing be helpful?

The board is issue 3

Cheers
Magnus
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1024MAK
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Re: ZX Fail

Post by 1024MAK »

It could be the upper RAM that has a problem. But at this stage, it’s too early to be sure.

When powered up, the code in the ROM first sets the border to white. Then it fills all RAM with a pattern. This is a test to see how much RAM is fitted, it’s not a proper memory test. So it can think that it has a full 48k of working memory. Next it locates the Z80 machine stack in high memory. If the upper RAM is faulty, the return addresses pushed on the stack will become corrupted, hence any manner of weird symptoms can be the result.

Unless there is a large gaping hole in the PCB, it’s repairable. It all depends on what is wrong and how much time and effort is needed. Sometimes, ‘professional’ repairers don’t think something will be economically repairable.
  • Basic tools and equipment needed to continue are:
  • Reasonable digital multimeter,
  • Decent soldering iron,
  • 60/40 lead based solder (don’t buy the cheap stuff from China),
  • A good quality solder sucker / solder pump and good quality desoldering wick/braid (don’t buy the cheap stuff from China).
A logic probe will also help (or you can make your own at very low cost).

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
mtullock
Posts: 18
Joined: Mon Aug 10, 2020 11:12 am

Re: ZX Fail

Post by mtullock »

Thats good, I have all those :) (bar the probe) I followed a video on YT earlier and checked the lower chips for resistance, i think they were fine. Not sure where to go next tho i admit.

Thanks for all the advice till now, I will try not to become too much of a pain in the backside :lol:
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1024MAK
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Re: ZX Fail

Post by 1024MAK »

So the next step is to disable the upper RAM and see what happens.

There is simple way to fool the machine into thinking it has no ‘upper’ RAM and therefore it’s only a 16K machine. That is, we temporarily disable the ‘upper’ DRAM chips. Please note this only applies to issue 2 through to issue 4S boards. Issue 5 and issue 6A boards use different circuitry.

To disable the "upper" 32k of RAM, on a issue issue 2 through to issue 4S board, take a piece of wire and connect pin 5 on IC23 (a 74LS32) to +5V. (link).

I suggest you make a temporary solder connection with an SPST on/off switch in series. Start with the switch set to on. Then if you get to the copyright screen and BASIC works, after that, you can turn the switch off so that some simple BASIC commands can access and therefore test the upper RAM.

This is done because the ROM code tries to see how much RAM there is at start-up. After this, the BASIC ROM will ignore any RAM that is not present (or in this case, is disabled)(well, unless you reset the limits using CLEAR n). But the BASIC POKE and PEEK commands have no limits and work across the whole Z80 address range.

Double and triple check that you have the correct pin on the correct chip, or you WILL damage something. This temporary modification will disable the "upper" RAM (when the switch is ON), as the CAS signal won't reach the RAM chips.

Note that if a faulty DRAM IC is jamming one of the data lines, this won't really help.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
mtullock
Posts: 18
Joined: Mon Aug 10, 2020 11:12 am

Re: ZX Fail

Post by mtullock »

so does that mean I am to connect these two pins together using an SPST?

Just wanted to make extra sure that I was understanding you correctly

Image

cheers
mtullock
Posts: 18
Joined: Mon Aug 10, 2020 11:12 am

Re: ZX Fail

Post by mtullock »

actually after looking at issue 3 schematics and layout diagrams, I think I had the wrong chip, this image is probably more accurate?

Image

cheers
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