ZX Spectrums problems : diagnostic software ?

[dizzy33]

I don't have a jailbar effect on this Spectrum but that's interesting, thanks a lot. I had difficulties to find 47nf caps, so now I guess I will order 220nF instead (or 1uF ? Some say it also works fine).

[1024MAK]

Hi

Sorry I’ve not been around as much, real life (work) getting in the way.

If there is any doubt about the condition of C5 to C8, it’s worthwhile replacing at least two of them. I recommended 1uF multilayer ceramic because that is what was used when the jail-bar issue was investigated. But if you don’t have the jail bar pattern on screen, then any 100nF to 470nF ceramic capacitor should be fine.

If the lower RAM work okay for writing and reading during the ROM based RAM test, but fail in the code execution test, that points to one of them being slightly too slow (slower access speed).

Mark

[1024MAK]

The 4116 (or equivalent) DRAM should have an access time of 150ns or faster (quicker).

So if using
NEC parts, D416C-3 (D416-3)
MOSTEK parts, MK4116P-2
TI parts, TMS4116-15
Motorola parts, MCM4116AL-15 or MCM4116AC-15

Mark

[dizzy33]

Mark, thanks for your answer, I will try to re-launh the code execution test with some other RAM, trying to get all of them at the same speed, 150ns minimum. Will let you know about my result.

[dizzy33]

Mark, as said earlier, the problem is that with any set of RAM chips (I tried 3 different sets of working RAM, each set with same brand and same model of RAMs), but it always ends up with one or more RAM chip said as "bad" by my Diag ROM.
So I am 100% sure that my problem does not come from the RAM, but from a component on the motherboard which let the RAM working bad.
That's why I will now try to replace the RAM caps ...
So I finally give up with testing more RAM sets.

[dizzy33]

I confirm :
I took the whole RAM set from my none working Spectrum (the RAM set which fails when doing the code execution with my Diag ROM), and on the working Spectrum the code execution RAM test works fine.
So there is definitly a component on my motherboard which let the RAM not working fine.
Will try to replace all the RAM caps when I will receive them. If you have an idea of any other component (othen than an IC as they are all working fine), please let me know.
Thanks !

[1024MAK]

Well, I’m stumped

The chances of two or more ceramic capacitors failing and causing this kind of fault are low. Normally ceramic capacitors either fail short circuit or open circuit. And (in low voltage circuits) they normally only fail if they have been physically damaged by something.

I know you have already said which parts have been changed / renewed or tested, but can you please post up a new list to remind me.

Mark

[dizzy33]

No problem Mark (thanks again a million to you for trying to help me !).

Symptom :
after one minute of use, the Spectrum displays some bugs on the very left side of the screen (colors or characters), sometimes resets itself, and a lot of games do not load. The Smartcard Diag ROM sometimes say one or another RAM chip is failing (but all RAM chips are known working as they work fine on another Spectrum).
There seem to be a kind of sensitivity issue about the RAM chips, by testing every RAM chips I have, I could find a set of 8 RAM chips (a mix of 3 different kind of brand and models) which is quite working fine. I said "quite" because with this particular set of RAM chips, the bugs still appear on the left side of the screen but the Spectrum allows to me to load any game and to play with it.
Note : with this particular set of RAM chips, my Diag ROM don't find any chip failing, but the code execution test fails. You said there maybe a RAM speed issue, but I tested this RAM chips set, with the same Diag ROM, on another Spectrum and then the code execution test works fine.

Here is the list of parts which have already been replaced :
TRACO regulator
Z80
FULL RECAP
TR4/TR5
All lower RAM
All upper RAM
ULA (IC1)
ALL 74LS (IC3,IC4,IC23,IC24,IC25,IC26).
TR3 (ZTX313)
Tried to let it run with only 16Ko of RAM, but no change.
Tried to do the diag tests without the motherboard ROM, but no change, so the ROM is not involved in this issue.

So all the lower RAM chips are definitivly working fine, but something on the motherboard let the RAM chips not working well. As I already replaced all IC's, I guess it maybe a small electronic component like a resistor or capacitor.

My next step will be to replace all the RAM capacitors (C1 to C5 and all the other lower and upper RAM capacitors).

[dizzy33]

I replaced all the lower RAM capacitors, it did not solve the problem.
Marc, any idea of what I could try now ?

[dizzy33]

YEEEEEEEEESSSSSSSSSSSS !!!! Another Spectrum alive !!!!!
But I'm not totally satisfied ...
Here is he situation : this Spectrum was displaying some graphical bugs on the very left side of the screen, and the Smartcard was sometimes showing some lower RAM chips as failed (never the same one, and these RAM chips are known as working).
So I replaced all the IC's and RAM chips (plus RECAP, TR4, TR5, TR3, C1 to C8 ...). And still no go.
So I thought that maybe something I replaced was dead. As there were some graphical bugs, I decided to begin with the Z80, then the ULA.
I replaced the Z80 (which was a modern Z80, the one which does not produce heat) by an old one from an issue 6A board.
Guess what, now the Spectrum works perfectly !!!!

So I thought my modern Z80 was bad. I decided to replace it with another modern Z80, and still the same issue !
So for an unknown reason, this issue 2 motherboard does only accept old Z80's, not modern ones ... As modern ones, I tested these two kinds :
Z84C0008PEG, and Z84C0020PEC, both from ZILOG, they have been tested on an "issue 6A" motherboard with success, so they are working.

Is this a known issue with "issue 2" motherboards ?
What is not satisfying me with this is that if the Z80 fails, I will have a lot of problems to find a "new" old one with a working "M1" line ...
Note : thank you very much Mark for all the help you provided to me for this issue (and for the other 2 Spectrum !)

I just noticed another small issue on this Spectrum : there some some kind of parasites on the screen, I can see them not on all the screen, but in some specific places like on some colors. not sure if TR1 or TR2 may be the culprits (knowing that a full recap has been done) as the colors are good, and luminosity is good too. EDIT : I just solved this problem by adjusting the TC1 trimmer, thanks to the ZX Spectrum Service Manual !