ZX Fail

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1024MAK
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Location: Looking forward to summer in Somerset, UK...

Re: ZX Fail

Post by 1024MAK »

Yes on the second photo, the pin you circled on IC23 is pin 5. You can use any +5V point, so the logical place is pin 14 on the same chip. Which is next to the +5V pin that you have highlighted on the Z80 microprocessor.

So IC23 pin 5 —— wire—— switch —— wire —— +5V point (IC23 pin 14).

If you want me to double check, post a photo after you have made up this test arrangement, but before powering up. Then I can check it.

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
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Joined: Mon Aug 10, 2020 11:12 am

Re: ZX Fail

Post by mtullock »

ok thanks will give that a go. Do you recommend soldering to the underside or chip side? I am concerned that I might melt the chip if I o it top side, but don't want to run into any trace issues underside.
mtullock
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Joined: Mon Aug 10, 2020 11:12 am

Re: ZX Fail

Post by mtullock »

just tried it with a chip socket over the chip and it seemed to work. However, same result. I can enter a keyword like print or load or the like but as soon as I press another character the coloured screen appears and the system restarts.
mtullock
Posts: 18
Joined: Mon Aug 10, 2020 11:12 am

Re: ZX Fail

Post by mtullock »

think I may send it in for repair. I am a computer guy by trade, but I have had this baby since 1985 and I would hate to damage it beyond repair......think its time for an expert in this! :)
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1024MAK
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Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX Fail

Post by 1024MAK »

Ahh, to answer your earlier question, solder to the solder pads on the underside.

If you decide to continue, we will help as much as we can. Fault finding can take a while, it’s far from an exact science...

Obviously if you send it to an experienced repairer, hopefully they will quickly diagnose the problem and fix it.

If disabling the upper RAM has no effect, then the most likely cause is one or more of the lower RAM chips (type 4116 or equivalent) is faulty.
The quickest method of diagnosing this, is with a diagnostic / RAM test program that runs entirely from ROM (and hence does not need any working RAM).

There are a couple of choices:
  • DiagROM which is part of the SMART Card package (see here)
You either need an interface that can take EPROM or EEPROM chips, or which can use ROM files (and store them in EEPROM or flash memory), or you can use a EPROM in place of the Sinclair BASIC ROM (if your machine has a socketed ROM chip, but it looks like yours is soldered). To program an EPROM you need a suitable programmer.

Alternatively try multiple power ups and take photos of the screen display and it may be possible to identify which bits in the screen display aren’t working properly. It helps if there is know content on screen, so try to get it to display long keywords.

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 »

sorry for resurrecting an old post, so I have my Retrolium, and so finally have a means to test the ZX 48K.......

Image

hmmmmm doesnt look healthy! is it even worth replacing 12 ICs? or does there come a time where there are so many things wring they are not salvageable?

cheers
M
hatman72
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Joined: Fri Feb 12, 2021 12:59 am

Re: ZX Fail

Post by hatman72 »

I got a similar result from a RAM test and once I replaced one RAM chip all the other errors went away. Check if any of your RAM chips are hotter than the others.
mtullock
Posts: 18
Joined: Mon Aug 10, 2020 11:12 am

Re: ZX Fail

Post by mtullock »

will have a look over the weekend at that - thanks :)
mtullock
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Joined: Mon Aug 10, 2020 11:12 am

Re: ZX Fail

Post by mtullock »

Hi, me again
I have posted a video of my retrolium tests here if anyone could help. https://www.youtube.com/watch?v=1biVeZvNzTA

firstly I have tested my regulator and it is showing 4.95V which is down - could it be that?

Diag starts and I get the low buzz and then 7 red for lower RAM tests telling me that all lower RAM and that IC3 and IC4 are bad, it then tests and states IC25 and IC26 are also suspect. It then black screens and hangs (39s in vid). If I press the reset on the retrolium (1:10 in vid) it comes up with IC1, IC3 and IC4 suspect before going back to show the same RAM issues. However, this time rather than hanging on a black screen it goes to NMI and shows green on all RAM chips?! then it reboots and the cycle starts again......

If the NMI had red bars I would think all RAM, but starting to wonder if it is Voltage or IC3 and IC4 causing all the issues on their own?
mtullock
Posts: 18
Joined: Mon Aug 10, 2020 11:12 am

Re: ZX Fail

Post by mtullock »

Have made some significant progress. Replaced IC3 and IC4, the regulator, TR4, TR5 and recapped. All of my errors have gone but I do have an upper RAM error on suspect IC25 and IC26. However, the image is now stable :)

Image

Also managed to fix my +2, I ended up replacing the regulator, the power socket and 4 lower RAM (which I socketed as well). However it all went pear shaped it got a lot worse....with a bizarre TV output of it was like the image on screen was there times 4 but offset on the horizontal and vertical making it impossible to read. .then I noticed a tiny track had lifted from IC25 to the ULA. Replaced it (read bodged it) with some fuse wire from one side of the board to pin 5 of IC25 and voila! :D
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