All RAM chips destroyed??

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Cristian
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Re: All RAM chips destroyed??

Postby Cristian » Sun Feb 04, 2018 9:32 pm

martyn_hill wrote: it does seem likely that one further RAM chip replacement is worth the effort.
I'm afraid the title of this topic is very close to the truth :mrgreen:

martyn_hill wrote: It was your efforts and the conversation in this thread that prompted me to explore a bit more deeply - thanks for that!
I could say the same! I think you've underlined one of the main benefits of this forum indeed.
[/quote]


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Cristian
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Re: All RAM chips destroyed??

Postby Cristian » Tue Feb 06, 2018 10:22 am

Dave wrote: repeat the loop until you get to the end (it boots properly, or you run out of chips to replace)....

I'm just waiting for 15 new Dram chips... let's hope they are enough :mrgreen:


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Re: All RAM chips destroyed??

Postby Cristian » Wed Feb 14, 2018 5:43 pm

The brand new RAM chips have just arrived....
Processing... please wait


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Re: All RAM chips destroyed??

Postby martyn_hill » Wed Feb 14, 2018 7:14 pm

Good luck! Looking forward to hearing whether this finally solves the issue!


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Cristian
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Re: All RAM chips destroyed??

Postby Cristian » Thu Feb 15, 2018 10:13 am

martyn_hill wrote:Looking forward to hearing whether this finally solves the issue!

I've won the RAM battle! Now I have to win the war...
After replacing 5 RAM chips, finally the initial RAM test runs without error codes: after the normal stipple screen, the Minerva logo appears.
Then all becomes slow: after several seconds the Minerva menu (F1/F2 etc.) appears and disappears a couple of times. Then appears the red screen, slowly overlapped by the blue screen.
Then the gremlins start their job: the keyboard is unconnetted, but random characters are typed automatically...

Here below is the boot sequence:


Image

The replaced RAM chips are: IC1, IC4, IC7, IC9, IC14 (I have some suspect that IC1 might have not been faulty, anyway it's replaced).


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Re: All RAM chips destroyed??

Postby Pr0f » Thu Feb 15, 2018 11:04 am

I think I would be tempted to unplug both the ZX8302 and the 8049 or equivalent chip next to the Microdrives at the front of the PCB, then check the ic sockets for any damage, dirty contacts, broken or badly bent pins in the sockets, then if all ok, reseat both the chips, making sure they are evenly and firmly pressed back into the sockets...


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Re: All RAM chips destroyed??

Postby martyn_hill » Thu Feb 15, 2018 11:45 am

Hi Cristian

Well done - replacing those DRAMs is tedious, so say the least.

Pr0f's advice below is certainly sensible.

I had a question, however - in your photos, there appears some 'snow' on the display - is that actually present, or just an artifact of the photography - or perhaps a poor RGB cable?


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Cristian
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Re: All RAM chips destroyed??

Postby Cristian » Thu Feb 15, 2018 12:30 pm

Yes I agree 100% with Pr0f.
The "snow effect" is real and generated by the QL itself. It usually starts some seconds after the appearance of Minerva logo, and tends to increase. But sometimes it does not appear.
Since the RAM test succeeds, I suppose the Ram contacts should be OK; so I'll check the other chips as suggested.


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Re: All RAM chips destroyed??

Postby Pr0f » Thu Feb 15, 2018 12:56 pm

That's a real effort with that machine - but rewarding when you get one going again...

I think these machines have been subject to a mixture of mechanical and electrical abuse over the years, so the fact that some components are now putting up their hands and saying adios is no real suprise. You expect some electroyltic capacitors to fade over time, particulary in areas where there are high ripple currents, but there seem issues with the resilience of the custom logic chips - pins that interface to the outside world directly (such as the RGB lines for instance on the ZX8301).

I find myself wanting this machine to succeed more and more :-)


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Re: All RAM chips destroyed??

Postby Dave » Thu Feb 15, 2018 7:21 pm

We can't tell from the photos, but are the pixel noise patterns fixed (thus stored in RAM) or shimmery (thus happening after RAM)?



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