Blank screen issue...

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
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1024MAK
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Re: Blank screen issue...

Post by 1024MAK » Thu Jun 14, 2018 4:23 pm

Before doing any further work on it, and certainly before replacing any chips, or cutting any chip legs, you should follow the instructions here over on World of Spectrum, to disable the upper RAM chips. Don't worry about oscilloscope tests that are also in that post.
Disabling the upper RAM may allow the machine to start up as a 16k model. That may then give you a better idea of the problem.

Mark
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Ast A. Moore
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Re: Blank screen issue...

Post by Ast A. Moore » Thu Jun 14, 2018 4:41 pm

1024MAK wrote:
Thu Jun 14, 2018 4:23 pm
follow the instructions here over on World of Spectrum, to disable the upper RAM chips.
Ha! Didn’t know about this trick. Awesome.
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Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.

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Re: Blank screen issue...

Post by rps » Sun Jun 17, 2018 11:09 am

Cool, just reading over everyhting now including the last one RE: Dissabling the upper ram.

I was wondering, if there is an issue with any of the ram, do the individual chips ever over heat at all? I checked them and they all seemed cold (same temp). Just wondered if that would have been an indication of an issue.

Thanks
R
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Re: Blank screen issue...

Post by rps » Sun Jun 17, 2018 1:33 pm

Also, in reading all the info, the capacitors were mentioned in relation to the ram.
Is this something I may also need to consider? Or do you think that would be unlikely?

Thanks
Rps
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Re: Blank screen issue...

Post by 1024MAK » Sun Jun 17, 2018 2:28 pm

The 'lower' RAM are 4116 DRAM (16k bits each) chips which run warm, with some running warmer than others. Sometimes when they fail, they may get hotter. But only if the supply can supply the extra power. Otherwise the switching transistor for the +12V and -5V dies.

The 'upper' RAM are 32k bits each (various part numbers). They are 4164 DRAM (64k bits each) chips which are only half used, these being cheaper at the time, as a half working 64k bit chip could be sold as a 32k bit chip. These are more advanced than the earlier 4116 DRAM design. They run much cooler and most of the time, it is not possible to detect a faulty chip by feeling the temperature with your finger.

Renewal of the electrolytic capacitors is recommended so that the DC/DC inverter/converter on the main board is reliable, and to minimise the noise on the video screen. It is less important for the upper RAM. It is unlikely that a capacitor failure will have caused a failure of the upper RAM.

But we will be in a better position to advise you after you have tried the disable upper RAM test.

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Re: Blank screen issue...

Post by rps » Sun Jun 17, 2018 3:19 pm

That's good, hoping to run that test laser this evening/ tomorrow morning.

The screen that I do have booting up does tend to have quite a bit of noise on it. I would say some of my other consoles do too but I don't think it is as bad as on this one.

Any way will try run the test and see from there.

Thanks
Amy
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Ast A. Moore
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Re: Blank screen issue...

Post by Ast A. Moore » Sun Jun 17, 2018 6:40 pm

rps wrote:
Sun Jun 17, 2018 3:19 pm
The screen that I do have booting up does tend to have quite a bit of noise on it. I would say some of my other consoles do too but I don't think it is as bad as on this one.
Is your Speccy composite-modded or are you using the original RF output?
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Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.

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Re: Blank screen issue...

Post by rps » Sun Jun 17, 2018 6:40 pm

Hi,
Do you know what number pin the +5v is on the IC23? Can’t find a pin diagram that shows it.

Thanks
RPS
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Ast A. Moore
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Re: Blank screen issue...

Post by Ast A. Moore » Sun Jun 17, 2018 7:02 pm

rps wrote:
Sun Jun 17, 2018 6:40 pm
Do you know what number pin the +5v is on the IC23? Can’t find a pin diagram that shows it.
Pin 8 is VDD, pin 16 is ground: http://pdf1.alldatasheet.com/datasheet- ... 4164B.html
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Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.

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1024MAK
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Re: Blank screen issue...

Post by 1024MAK » Sun Jun 17, 2018 10:39 pm

rps wrote:
Sun Jun 17, 2018 6:40 pm
Hi,
Do you know what number pin the +5v is on the IC23? Can’t find a pin diagram that shows it.
On IC23 (a 74LS32, quad 2-input OR gates), pin 14 is Vcc (+5V). 0V/GND is on pin 7.

I should point out that the method given in the earlier post about how to disable the 'upper' RAM only applies to issue 2 to issue 4S boards. It does not apply to issue 5 to issue 6 boards.

I've put a pin-out of a 74LS32 (IC23), and a schematic of an issue 4 on my Flickr account here ;)

Mark
Last edited by 1024MAK on Sun Jun 17, 2018 10:41 pm, edited 1 time in total.
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