ZX Spectrums problems : diagnostic software ?

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dizzy33
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Re: ZX Spectrums problems : diagnostic software ?

Post by dizzy33 »

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 ...
I ordered some new components to replace all 74LS.
But as IC25 is there to manage the upper RAM, and my main problem in on the lower RAM (managed by IC3 and IC4), I had an idea :
My Speccy is a 16K extended to 48K one. So I removed all IC's managing upper RAM, but my software still shows, randomly, some bad low RAM IC's.
So I guess the IC25 failure message was false.
I am now waiting for my new 74LS to arrive to replace both IC3 and IC4 and eventually IC25/26

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1024MAK
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Re: ZX Spectrums problems : diagnostic software ?

Post by 1024MAK »

Sorry, real life / work has kept me busy... still catching up...
dizzy33 wrote:
Thu Feb 27, 2020 7:37 pm
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 ...
There are two 74LS157 (sometimes shown as ‘LS157) chips used to access the ‘lower’ RAM, these are IC3 and IC4.
And on 48K issue 2 to issue 4S boards there are another two 74LS157 chips used to access the ‘upper’ RAM, these are IC25 and IC26.

The 74LS157 chips are multiplexer chips. The DRAM chips require the address information to be supplied as 7 or 8 bit row and 7 or 8 bit column information, one after another. But to save chip pins, both the row address and the column address use the same pins.

Whereas the CPU address bus is 16 bits wide. The multiplexer chips act like a bunch of 2 position switches, to route the correct CPU address line to the correct DRAM chip row/column address line.

If a 74LS157 chip becomes faulty, all kinds of strange symptoms can occur, as often the result is that one memory location can be written to and read from at one than one unique address. And this happens for many, many addresses / memory locations. But the CPU and the software can’t tell that this is happening.

If IC25 and IC26 are in sockets, and / or the ‘upper’ RAM chips are in sockets, remove all of them. Then power up and see what happens. The machine should power up as a 16K model. If there is no change in symptoms, then the fault is elsewhere.

If none of these are in sockets, there is another way to attempt to disable the ‘upper’ RAM. I will leave that for another post.

As you have tried removing all the chips for the ‘upper’ RAM, the next logical thing to do is to replace IC3 and / or IC4 as you have already suggested. If you fit good quality sockets, you can try one of the 74LS157 chips that were in the IC25 or IC26 positions.

Mark

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1024MAK
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Re: ZX Spectrums problems : diagnostic software ?

Post by 1024MAK »

A couple of questions, which issue is your board? Can you please upload a photo of your board.
Are you good at soldering? Do you much experience at soldering?

Apart from a multimeter, do you own or have access to any other test equipment (logic probe, logic analyser or oscilloscope etc...).

Mark

dizzy33
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Re: ZX Spectrums problems : diagnostic software ?

Post by dizzy33 »

Thanks a lot Mark, your informations confirm the one I had.
My board is an issue 2.
My soldering skills are good (I also own a desoldering station).
I put IC3 and IC4 on a socket.
No I do not have any oscilloscope.
While waiting for my new 74LS to arrive, I did some tests :
strangely, putting back my Speccy to 16K does not work anymore, it does not startup (displays bugs).
If I put back all chips for 48K (all 4 x 74LS), then it startup, the diag test says IC25 is faulty.
So I tried to swap IC25 with IC3 or IC4, then the speccy does not start correctly.
So I think my IC25 is definitivly faulty.
I did no tests using IC26 as this one has a strap on it, to use it for IC3/IC4/IC25 I would need to desolder the strap.

dizzy33
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Re: ZX Spectrums problems : diagnostic software ?

Post by dizzy33 »

This Spectrum is making me totally mad !!!!
I have replaced all six 74LS, but still the same issue: the smartcard V2 diagnostic ROM shows some lower RAM chips as defective, and never the same ones. Sometimes it does not show any RAM chip defective. But the Spectrum crashed and reset after one or two minutes of use.

So far I have replaced all IC's on the motherboard (including the ULA and the Z80) except these two :
- the LM1889N which is close to the modulator.
- the ROM.

About the ROM, there is something strange ...
- sometimes, when the Spectrum crashes and reset, it then displays an Amstrad copyright ... !!!??? This is very strange knowing that my motherboard is an issue 2. When it happens, the Smartcard V2 is plugged to my Spectrum, so I do not know if this Amstrad message comes from the Smartcard or not, but I do not see no reason why it would come from the Smartcard V2.
- The reference printed on the ROM chip is : "NEC 8228P9 D23128C 057".

So I suspect now my ROM, but if my ROM was defective, would it show the same strange behavior as my Speccy has ?

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1024MAK
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Re: ZX Spectrums problems : diagnostic software ?

Post by 1024MAK »

The word ‘Amstrad’ did not make it into any Sinclair product until after Amstrad bought Sinclair Research from Sir Clive Sinclair. That was long after the issue 2 ZX Spectrum boards had finished being produced (in fact, the last issue board for the 48K and plus models was an issue 6A board).

The date code on your ROM chip is week 28 in 1982. So there is definitely no ‘Amstrad’ text in it.

I don’t know if the ROM images in the SMART Card contain the work ‘Amstrad’. The test program may contain the word ‘Amstrad’, as it can also be used on the 128K machines. And the +2, +2A, +2B, +3 and +3B all were made by Amstrad, hence have ‘Amstrad Copyright’ text in then.

It maybe worthwhile cleaning the edge-connector contacts. Use Isopropyl alcohol (IPA).

In terms of reliability, the ROM chips that Sinclair used are fairly reliable. But occasionally they do fail. I would not rush to condemn the ROM chip too fast, as if you remove it, you will find that a modification to the board is needed before a EPROM or EEPROM can be used. It is also unusual for a ROM chip to fail a few minutes after it has been working okay. Is it getting hot?

The LM1889 is purely an analogue chip and has no effect on the digital part of the computer. In fact, it’s possible to power it down or remove it and this will not affect the running of the CPU.

Mark

dizzy33
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Re: ZX Spectrums problems : diagnostic software ?

Post by dizzy33 »

Thanks Mark. So that means that now, the only possible defective components, are purely electronic ones, I mean like transistors (TR4 and TR5 are already replaced), diodes, etc ...
But how to guess which one is defective ?

I don't want to give up with this Spectrum, as I already replaced many parts on it. But I think now my competencies are out, there is nothing more I can do ... Does anyone know if I can send it to someone who could eventually repair it ?

dizzy33
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Re: ZX Spectrums problems : diagnostic software ?

Post by dizzy33 »

I just discovered something :
strangely, IC13 is getting hot quite fast. In about one minute, it's sometimes a bit painful if you touch it.
This is the only lower RAM to do that, no problem with others. Of course, I tried to replace IC13 with 2 or 3 different RAM chips and I have the same result.
Does anyone know which component on the motherboard may let IC13 getting too hot ?

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1024MAK
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Re: ZX Spectrums problems : diagnostic software ?

Post by 1024MAK »

Apart from pins 2 and 14 (data in and data out), all lower DRAM pins are wired in parallel with the other similarly numbered pins on the other lower DRAM pins.

4116 DRAM chips don’t always run with consistent temperatures between the same chips. I don’t know why. It is worthwhile testing all three of the supply voltage pins on it (the DRAM chip). If all the voltages are okay, then TR4 and TR5 are okay.

Also with the machine switched off / disconnected from power, test pin 2 to 14, and then pin 2 to the power rails: +5V, +12V, -5V and 0V. Then compare with pin 2 on the other lower RAM DRAM chips. If you get any unusual results, post up your findings.

Mark

dizzy33
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Re: ZX Spectrums problems : diagnostic software ?

Post by dizzy33 »

All my voltages seem OK ... Unfortunatly ... I think I now need to send this Speccy to someone who have better electronic competencies than me ... If you know the address of someone who can fix Speccies, please tell me !

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