ZX Spectrums problems : diagnostic software ?

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dizzy33
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Re: ZX Spectrums problems : diagnostic software ?

Post by dizzy33 »

To load the TZX I am using a Divmmc card.

dizzy33
Posts: 108
Joined: Mon Jan 13, 2020 3:24 pm

Re: ZX Spectrums problems : diagnostic software ?

Post by dizzy33 »

bola_dor wrote:
Tue Feb 25, 2020 7:18 pm

One thing you can do is check them one by one to see if there is a difference.. or
With machine off measure resistance between equivalent pins in different IC.. you can do this in every pin NOT marked as A0 to A6..
How do I measure resistance ?
My RAM chips are known working (and have been replaced twice), if measuring resistance is for trying to check which RAM IC is defective, I think it's useless.

bola_dor
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Re: ZX Spectrums problems : diagnostic software ?

Post by bola_dor »

dizzy33 wrote:
Wed Feb 26, 2020 7:33 am
bola_dor wrote:
Tue Feb 25, 2020 7:18 pm

One thing you can do is check them one by one to see if there is a difference.. or
With machine off measure resistance between equivalent pins in different IC.. you can do this in every pin NOT marked as A0 to A6..
How do I measure resistance ?
My RAM chips are known working (and have been replaced twice), if measuring resistance is for trying to check which RAM IC is defective, I think it's useless.
I am sorry I get lost when you said some software were reporting a bad low memory ..
So.. what was the problem then? :roll: ..
I posted about my own experience with that earlier..

dizzy33
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Re: ZX Spectrums problems : diagnostic software ?

Post by dizzy33 »

ZX Diagnostics reported bad IC's. I replaced all of them (all the 8 low RAM chips), but the problem remains :
after one minute of use, my Speccy displays bugs.
All RAM tests software say I have a problem with low RAM, but they never show the same IC's as defective ...
So I am sure that a component on the motherboard is failing and does impact the low RAM. But which one knowing that I already replaced :
- TR4
- TR5
- 7805 by a TRACO
- Z80
- All low RAM chips
- ULA
- all capacitors

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1024MAK
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Re: ZX Spectrums problems : diagnostic software ?

Post by 1024MAK »

Better late than never, even if it’s not a video but instead some static photos...

Please note that this is an issue 1 board and is a bit different to the later board versions. However, the DC input connector, the modulator and the 4116 (or equivalent, in this case they are NEC D416C DRAM chips) ‘lower’ DRAM chips are in similar places.

A good place for the negative (black) meter probe (0V/GND connection)
A good place for the negative (black) meter probe (0V/GND connection)
.
Testing the +9V input
Testing the +9V input
.
Close up showing testing the +9V input
Close up showing testing the +9V input
.
Testing the +12V on pin 8
Testing the +12V on pin 8
.
Close up of testing the +12V on pin 8
Close up of testing the +12V on pin 8
.
Testing the -5V on pin 1
Testing the -5V on pin 1
.
Close up of testing the -5V on pin 1
Close up of testing the -5V on pin 1
.
Testing the +5V on pin 9
Testing the +5V on pin 9
.
Close up of testing the +5V on pin 9
Close up of testing the +5V on pin 9

Mark

dizzy33
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Re: ZX Spectrums problems : diagnostic software ?

Post by dizzy33 »

Thanks a lot Mark ! In the meantime "Bola_dor" helped me with the voltages measurements. They are all good.
But now something new happens : running the same memory test now always end up with "IC25 failure" ...
IC25 is a SN74LS157N which seems to be involved in the memory management. I am now trying to find out how to get a new working one ...

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1024MAK
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Re: ZX Spectrums problems : diagnostic software ?

Post by 1024MAK »

Not having had time to read all the posts made today, and at the risk of answering questions that have already been answered, nevertheless, I shall give my answers... :mrgreen:
dizzy33 wrote:
Tue Feb 25, 2020 1:43 pm
I own a multimeter which I can put in "20v DC" as you request, but it's not clear to me how to do the tests you are requesting.
At first, I guess my Speccy should be powered on, right ? So I am very afraid of making a shortcircuit, that's why I am asking so much questions.
Yes, the computer should be powered on. If you can hold one probe (the black/negative one) with one hand on to either the modulator case, or the RF/UHF output connector, then you can concentrate on holding the red/positive probe on the chip pin or other point.

Even better if you have a suitable crocodile clip or other clip that can be used to attach the meter black/negative connection to the the modulator case, or the RF/UHF output connector. Then you don’t have to hold a probe on this point. There are other 0V/GND points on the ZX Spectrum circuit board, but the position varies between different issue boards.

For other users who have not replaced the 7805 series pass voltage regulator, the best and easiest place is the 7805 heatsink.

Yes, you have to be very careful when using a probe on the pins of a chip. It is easy to slip if you don’t concentrate.

If you have difficulty, I can only suggest asking someone for assistance. Either to hold the meter black/negative connection in place, and / or to read the voltage displayed on the multimeter. Then you can concentrate on probing the relevant chip pins.

It is also possible to practice with the computer switched off ;)
dizzy33 wrote:
Tue Feb 25, 2020 1:43 pm
Then, as I don't have a 7805 regulator but a Traco regulator, should I put the negative probe of my multimeter on the modulator case ? (I did a composite mod, is this still working ?) are where else exactly ?
Yes, the best point is the RF/UHF output connector, or on the lower case. Sometimes the lid does not always make good contact with the case of the modulator.
dizzy33 wrote:
Tue Feb 25, 2020 1:43 pm
Is testing my new Traco regulator necessary as I just replaced it ?
It’s not about just testing the Traco DC/DC converter, but checking that the correct voltage makes it to the digital and memory circuitry. And to check that the Traco DC/DC converter is not running in protection mode due to a short circuit or overload.
dizzy33 wrote:
Tue Feb 25, 2020 1:43 pm
To be sure I understood well the ram chip tests : I put the black probe on pin 14v, then alternatively the red probe to pin 1,8, and 9 ?
Because it’s tricky to hold two probes on the pins of chips, I recommend using a 0V/GND point as described above. Then use the red probe on pins 1, 8 and 9 in turn.

Mark

dizzy33
Posts: 108
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Re: ZX Spectrums problems : diagnostic software ?

Post by dizzy33 »

Thanks a lot again Mark ! In the meantime "Bola_dor" helped me with the voltages measurements. They are all good.
But now something new happens : running the same memory test now always end up with "IC25 failure" ...
IC25 is a SN74LS157N which seems to be involved in the memory management. I am now trying to find out how to get a new working one ...

bola_dor
Posts: 122
Joined: Wed Oct 02, 2019 5:32 am

Re: ZX Spectrums problems : diagnostic software ?

Post by bola_dor »

dizzy33 wrote:
Thu Feb 27, 2020 9:26 pm
Thanks a lot again Mark ! In the meantime "Bola_dor" helped me with the voltages measurements. They are all good.
But now something new happens : running the same memory test now always end up with "IC25 failure" ...
IC25 is a SN74LS157N which seems to be involved in the memory management. I am now trying to find out how to get a new working one ...
Wich software are you testing with?.. I am at a similar point with some low memory problem..
This is my case https://www.sinclairzxworld.com/viewtop ... =21&t=3586

dizzy33
Posts: 108
Joined: Mon Jan 13, 2020 3:24 pm

Re: ZX Spectrums problems : diagnostic software ?

Post by dizzy33 »

I am using the software which is in ROM of the Smartcard V2 (homebrew from the Smartcard maker).

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