White Screen, no Microdrives Activity

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Flocke1967
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White Screen, no Microdrives Activity

Postby Flocke1967 » Wed Nov 22, 2017 11:56 am

hi Guys

I have a problem with a QL, white screen. The Microdrives show no activity after pressing F1 / F2. I have already swapped all socketed components (ULA 1, ULA 2, 6808) without improvement. So I am typing on a RAM error. I have ordered a Minerva switch board at SMR and wonder if the RAM test will be started automatically if the QL does not power up properly after powering up. Or do I have to install any program there before? And when, how, if the QL gets stuck in the white screen?

Please excuse my spelling, the Google translator was at work. Maybe someone can answer me in German.



Thanks and regards
Stefan


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Cristian
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Re: White Screen, no Microdrives Activity

Postby Cristian » Wed Nov 22, 2017 8:34 pm

Hi,
with Minerva the test starts automatically. No software needed ;)
You'll get a sequence of screens showing alphanumeric codes


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Whopper
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Re: White Screen, no Microdrives Activity

Postby Whopper » Fri Nov 24, 2017 8:33 pm

Flock1967,

Does the white screen have vertical black / colour dotted lines displayed. If so then that shows a memory error. Pure black lines = a short between the data output of a chip & colour dotted lines show a chip fault.

To diagnose which chip(s) are affected sort out the memory chips (TMS4164) VSS and Q pins. Pine 16 and 14. On good memory chips this should produce another black line on the screen. When you short out a memory chip without the screen image changing that is one of the affected chips. The memory are always in pairs, so find the partner to the bad chip and replace both.

HTH


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Cristian
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Re: White Screen, no Microdrives Activity

Postby Cristian » Fri Nov 24, 2017 10:10 pm

Whopper wrote:Does the white screen have vertical black / colour dotted lines displayed.

If Flock1967's description is accurate, it matches the symptoms of a QL of mine: a pure white screen, no lines nor dots.
Will your procedure works anyway?


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Whopper
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Re: White Screen, no Microdrives Activity

Postby Whopper » Sat Nov 25, 2017 11:33 am

Cristian wrote:
Whopper wrote:Does the white screen have vertical black / colour dotted lines displayed.

If Flock1967's description is accurate, it matches the symptoms of a QL of mine: a pure white screen, no lines nor dots.
Will your procedure works anyway?


Cristian,

I don't THINK it will harm the computer. If you don't get any response from the first couple of memory chip, then stop and srew the lid back on.

Peter


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martyn_hill
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Re: White Screen, no Microdrives Activity

Postby martyn_hill » Sat Nov 25, 2017 4:38 pm

In my (too frequent!) experience troubleshooting dead QLs at start-up, my conclusion was that a pure white (or black) screen indicates that the Display ULA (8301) is simply not seeing any memory accesses at all and is typically (in my experiments) due to some faulty device holding some part of the bus high or low - faulty ROM or EPPROM ICs for example.

Another interesting faulty display is a black/white chequerboard pattern. I think this might be a 'default' diagnostic display from the ULA in absence of successful memory access.

The ROM routines may or may not be running, but as Minerva will simply stop at the memory error - then restart again some seconds later, you would never see the memory diagnostic displayed due to the above bus-error, and thus no MDV activity nor anything else.

In my case, it was usually one of my dodgy, home-made interfaces knocking-out DSMC or DTACK or bad connections.

A faulty bus-driver (LS245) or those two 8 to 4-line decoders (LS257) might also mask any memory writes from actually hitting the DRAM. However, many times I thought of replacing these chips, only to then resurrect the QL by re-seating my home-made hardware...

Good luck!


tcat
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Re: White Screen, no Microdrives Activity

Postby tcat » Tue Dec 05, 2017 5:14 pm

To diagnose which chip(s) are affected sort out the memory chips (TMS4164) VSS and Q pins. Pine 16 and 14. On good memory chips this should produce another black line on the screen. When you short out a memory chip without the screen image changing that is one of the affected chips. The memory are always in pairs, so find the partner to the bad chip and replace both.

Hi,

This is an ingenious way of probing for faulty RAM chips. Not having any experience, I would have thought of using known good chip and try to piggy back on top of each onboard chip.

Reading forum here, I am lead to believe that Iss#5 boards are prone to suffer RAM faults more than any later revisions, is that right?. Generally what is the source of a RAM fault? Also what specification the spare RAM chip should have?

Many thanks
Tomas


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Cristian
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Re: White Screen, no Microdrives Activity

Postby Cristian » Wed Dec 06, 2017 3:38 pm

tcat wrote:Also what specification the spare RAM chip should have?

If my "personal cereblal Ram" is still OK, the chips are 64 Kbit dynamic RAM at 150 ns.
For more details, you can easily find specific datasheets in the internet: you have only to read the data printed on your memory chips.



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