ZX Spectrums problems : diagnostic software ?

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1024MAK
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Re: ZX Spectrums problems : diagnostic software ?

Post by 1024MAK »

Yes, very frustrating!

Hence why you have me stumped as well...

If the ROM is in a socket, remove it. Power up the board with it not fitted and take a photo of the screen.

Mark
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dizzy33
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Re: ZX Spectrums problems : diagnostic software ?

Post by dizzy33 »

Here is the photo of the display without ROM :

Image
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1024MAK
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Re: ZX Spectrums problems : diagnostic software ?

Post by 1024MAK »

Leave it running for say 20 to 30 minutes, keep checking to see if the pattern stays the same.

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.
dizzy33
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Re: ZX Spectrums problems : diagnostic software ?

Post by dizzy33 »

The red, on the blue columns, moves.
I will now try to let it run for 30 minutes.
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1024MAK
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Re: ZX Spectrums problems : diagnostic software ?

Post by 1024MAK »

Yeah, it will. The Z80 is constantly executing RST38h, which pushes the contents of the PC (program counter) to the stack. But as we never take anything from the stack, the stack pointer in the Z80 continues to grow. The result is that the same two byte value ends up filling every RAM location. The Z80 will just keep doing this, constantly looping through memory until you remove power...

It’s a good quick test.

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.
dizzy33
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Joined: Mon Jan 13, 2020 3:24 pm

Re: ZX Spectrums problems : diagnostic software ?

Post by dizzy33 »

After having wait for about 25 minutes, here is the result :
the red things, which appear on the blue columns, flash every second :
the columns are totally blue for one second, then the red appears on them for one second, then the columns are totally blue for one second, and so on ...
dizzy33
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Re: ZX Spectrums problems : diagnostic software ?

Post by dizzy33 »

I finally replaced TR3, no difference (but I was not expecting much as the clock is present on the Z80).
So still investigating ... Here is the current behaviour of the Spectrum : I turn it on, it displays the sinclair copyright. If I don't do nothing with it, then, after about one minute, it resets itself and go back to the sinclair copyright. And so on ...
dizzy33
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Re: ZX Spectrums problems : diagnostic software ?

Post by dizzy33 »

I wonder if my problem could come from the RAM capacitors ...
Last edited by dizzy33 on Mon May 04, 2020 5:41 pm, edited 1 time in total.
dizzy33
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Re: ZX Spectrums problems : diagnostic software ?

Post by dizzy33 »

That's very strange, here are my new investigations :
by replacing the RAM chips by others (old and new ram chips are known as working), and mixing different brands of RAM chips, I could find a RAM chips set which is "quite" working fine.
I mean that with this set of selected RAM chips, the Spectrum does almost work fine :
when I start a game, it works fine for a few minutes then, some graphical bugs appear on the very left side of the screen. But the game still works fine.
If I run my Diag ROM, it does not find any RAM chip failing. But if I ask to my Diag ROM to run some programs execution tests in lower RAM, then the test fails ...

So I feel like a small electronic component (not an IC) let the lower RAM bug. I measured the voltages on all lower RAM chips and they are OK.
That's why I'm wondering if this problem could come from the RAM capacitors (C1-C8) .
Does anyone have an opinion about this (Mark seems to be unavailable these last weeks, I hope he is well) ?
Lardo Boffin
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Re: ZX Spectrums problems : diagnostic software ?

Post by Lardo Boffin »

Replacing those capacitors is recommended to help reduce jail-bars (https://www.sinclairzxworld.com/viewtopic.php?t=3059) so I would certainly give it a go regardless.
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