ZX Spectrums problems : diagnostic software ?

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1024MAK
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Re: ZX Spectrums problems : diagnostic software ?

Post by 1024MAK »

I will help where I can.

First though, just play with it and get used to the controls and the effect they have.

What type of probe(s) were supplied? Do they have a switch marked 1:10 or similar?

With non-complex repetitive waveforms, adjust the signal level, timebase and trigger controls until you get a stable steady waveform. Try the CPU clock signal on Z80 pin 6. But note that the ULA sometimes pauses this when it wants to pause the CPU when it requires priority access to the screen RAM.

Unfortunately in a computer, like the ZX Spectrum most signals are complex so a different method has to be used.

If I find time, I’ll post up some example waveforms.

Mark

dizzy33
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Re: ZX Spectrums problems : diagnostic software ?

Post by dizzy33 »

I already played with it, measured some audio signals, etc ... Yes my probe has a "1x" "10x" switch.
All I need is just you to tell me on wich pin of which component I should attach my probes and which signal should I see.

dizzy33
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Re: ZX Spectrums problems : diagnostic software ?

Post by dizzy33 »

Hello Mark,

today my Spectrum does not start anymore (I didn't try it for about a month). Here is how looks the display when I power it up :

Image

As you requested I connected my oscilloscope to pin 6 (clock) of the Z80, here is the result :

Image

What do you think about this result ?

dizzy33
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Re: ZX Spectrums problems : diagnostic software ?

Post by dizzy33 »

I tested the ULA voltages. Here is my (bad) results :
on pin 13, I should have 5v. I only have 3.5v -> BAD
on pin 14, I should have 5v. I have 5v -> OK
What could be responsible of this low voltage for pin 13 of my ULA ? (as a reminder my motherboard is an issue 2).

EDIT : finally this 3.5v may be normal ... Here : http://www.breakintoprogram.co.uk/compu ... rdware/ula
pin 13 is notified with 5v ... But here : https://spectrumforeveryone.com/wp-cont ... Manual.pdf it's said we should have 3v ... So I guess the "breakintoprogram.co.uk" pinout is wrong.

So I really need help here ... :(

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1024MAK
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Re: ZX Spectrums problems : diagnostic software ?

Post by 1024MAK »

dizzy33 wrote:
Thu Apr 09, 2020 10:51 am
Image
That’s a classic RAM failure screen. The ROM code found a problem and stopped (well, it actually runs an infinite loop).
dizzy33 wrote:
Thu Apr 09, 2020 10:51 am
As you requested I connected my oscilloscope to pin 6 (clock) of the Z80, here is the result :
Image
That’s not quite right!

I’ll try to find time to post a photo of what it should look like later....

Mark

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1024MAK
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Re: ZX Spectrums problems : diagnostic software ?

Post by 1024MAK »

dizzy33 wrote:
Sun Apr 19, 2020 11:46 am
I tested the ULA voltages. Here is my (bad) results :
on pin 13, I should have 5v. I only have 3.5v -> BAD
on pin 14, I should have 5v. I have 5v -> OK
Both those are acceptable results. Pin 13 is fed via a resistor so the voltage will be lower.

Mark

dizzy33
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Re: ZX Spectrums problems : diagnostic software ?

Post by dizzy33 »

Thanks a lot Mark. But I think I have found a RAM problem.
I don't understand as at the beginning of my investigations, I already have tested the RAM voltages, and as far as I remember they were OK.
Now, here is the result I have for IC6 and IC13 (didn't test all the RAM but I guess it's the same issue everywhere) :
PIN 1 = -4.4v -> OK.
PIN 9 = +5v -> OK.
PIN 8 = 0V -> BAD -> should have 12V ...

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1024MAK
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Re: ZX Spectrums problems : diagnostic software ?

Post by 1024MAK »

Going purely on the screen display, it’s possibly IC12 in the lower RAM that has failed.

Mark

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Re: ZX Spectrums problems : diagnostic software ?

Post by 1024MAK »

dizzy33 wrote:
Sun Apr 19, 2020 12:42 pm
Thanks a lot Mark. But I think I have found a RAM problem.
I don't understand as at the beginning of my investigations, I already have tested the RAM voltages, and as far as I remember they were OK.
Now, here is the result I have for IC6 and IC13 (didn't test all the RAM but I guess it's the same issue everywhere) :
PIN 1 = -4.4v -> OK.
PIN 9 = +5v -> OK.
PIN 8 = 0V -> BAD -> should have 12V ...
Are you sure? With no +12V supply none of the lower RAM chips will work... (and loss of the +12V rail will also affect the video output).

A reminder that the pin out is here.

Mark

dizzy33
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Re: ZX Spectrums problems : diagnostic software ?

Post by dizzy33 »

Mark, yes, you are right, my diagnostic cartridge says IC12 is defective, but if I replace it, then it will say another one is.
That's my problem since the beginning of my investigations. Something is wrong on my motherboard which let the RAM goes bad.
Now I don't have no 12v anymore on PIN 8 of my RAM chips, I had it at the begining of my investigations. I guess I should test TR4, right ?
If yes, how should I proceed to check if it's good or not ?
Any other component may be related to the lack of 12v in my RAM ?

EDIT : I have found this video which shows how to test TR4 with an oscilloscope ... Will try it ...
https://www.youtube.com/watch?v=rZ641nqT8F0

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