Blank screen issue...

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rps
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Blank screen issue...

Post by rps »

Hello,
I recently acquired a zX Spectrum + (plus) as a friend had a spare. I was told there was an issue with it however looking online, the only thing I could find was a suggestion that it could be an issue with the upper ram.

Powers on with black paper and grey surround. There is no text, cursor or spectrum text, nothing at all.

Just wondering if anyone knows what the fault may be before I start buying new chips etc and de soldering a board!

Thanks in advance!
Rps
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Ast A. Moore
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Re: Blank screen issue...

Post by Ast A. Moore »

White border is (usually) a good sign. Ideally, I’d try to reset the CPU (rather than simply power-cycle the whole computer), and see if anything happened on the screen. Can you do that?

Oh, wait, it’s a Plus, you say, so it has a dedicated reset button. Press it briefly while looking at the screen. Describe what you see.
Every man should plant a tree, build a house, and write a ZX Spectrum game.

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and zasm Z80 Assembler syntax highlighter.
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Re: Blank screen issue...

Post by rps »

Thanks for the reply, do all the plus versions gave that? I thought it was just the plus 2 or plus 3? If so, where is that located as I definately see no buttons!

It is the one on this page... http://blog.tynemouthsoftware.co.uk/201 ... d.html?m=1
just to make sure we are talking about the same on.

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

Post by Ast A. Moore »

By “Spectrum+” I mean the original 48K Spectrum in the hardshell case (as depicted in the link you posted). There were two distinct “versions” of the Plus: an upgrade (Sinclair sold the cases, and the owners of the original rubber-key Speccies could take the logic board out of the old case and put it in the new one), and factory-made Spectrums that came in this new case. The latter came with the reset button fitted on the left side of the case:
Image
Image
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 »

Ah that! I completely forgot that was there when I originally looked over the unit. :roll:

I pressed the button and you can hear a change in the static noise briefly, then you get a split second where there are red vertical lines on the black paper.

Thank you
Amy
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Re: Blank screen issue...

Post by Ast A. Moore »

That’s also a good sign. Means that the RAM test runs, at least partially. The CPU and the ULA are likely okay. The issue might be with one or more of the RAM ICs. Whether it’s the lower or upper RAM depends on how long it takes for the RAM test sequence to complete. That’s more of a feel thing (although you can use an emulator to get the feel for it).

After the ROM completes the RAM test, it proceeds to set up some system variables and then clears the screen and prints the Sinclair Research copyright message. Since it fails to do so on your machine, it’s probably the lower RAM that’s faulty.
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.
rps
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Re: Blank screen issue...

Post by rps »

Oh ok. The place I looked at suggested upper ram. So can I show you the link that shows where it says that? Just curious to know what you think of what they have written there?

Here is the link and I am looking at the 6th one down...
http://hardware.speccy.org/reparar-i.html

If it is the lower ram as you say, is there a way to test it or is it a case of just replacing it?

Thabks
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Re: Blank screen issue...

Post by Ast A. Moore »

rps wrote: Wed Jun 13, 2018 9:19 am Oh ok. The place I looked at suggested upper ram. So can I show you the link that shows where it says that? Just curious to know what you think of what they have written there?
Perhaps it makes more sense, since the video is working fine. Upper RAM ICs are cheaper and easier to source, anyway.

Finding the faulty one (ones) is kind of tedious, and requires a digital oscilloscope. What you could do instead, is cut the +5V legs of each upper RAM IC and bend the stubs away to make sure they don’t make contact with the other halves. If the problem is indeed in one of the upper RAM ICs, the Speccy should boot up normally and work as a 16K machine.

Then it’ll be easier to replace the whole lot.
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.
rps
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Re: Blank screen issue...

Post by rps »

Hi guys, thanks for all the info and all the links. Think I will have to wait til the weekend to loook over everything.
Then I’ll be back on here and let you know how it goes!
FINGERS CROSSED :)

Thanks
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Re: Blank screen issue...

Post by 1024MAK »

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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Re: Blank screen issue...

Post by Ast A. Moore »

1024MAK wrote: Thu Jun 14, 2018 5:23 pm follow the instructions here over on World of Spectrum, to disable the upper RAM chips.
Ha! Didn’t know about this trick. Awesome.
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 »

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 »

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
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Re: Blank screen issue...

Post by 1024MAK »

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.

Mark
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Looking forward to summer later in the year.
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Re: Blank screen issue...

Post by rps »

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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Re: Blank screen issue...

Post by Ast A. Moore »

rps wrote: Sun Jun 17, 2018 4:19 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?
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.
rps
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Re: Blank screen issue...

Post by rps »

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

Thanks
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Re: Blank screen issue...

Post by Ast A. Moore »

rps wrote: Sun Jun 17, 2018 7:40 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
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 1024MAK »

rps wrote: Sun Jun 17, 2018 7:40 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 11:41 pm, edited 1 time in total.
:!: Standby alert :!:
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Re: Blank screen issue...

Post by 1024MAK »

Ast A. Moore wrote: Sun Jun 17, 2018 8:02 pm
rps wrote: Sun Jun 17, 2018 7:40 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
Okay, how much beer have you drunk? :lol:

Mark
:!: Standby alert :!:
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :dance
Looking forward to summer later in the year.
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Re: Blank screen issue...

Post by rps »

Hi,
It is a 4s model.
So I did pins 5 to 14 and nothing happened. I did pin 14 to ground (7 was it?) that shuts the machine off and then comes back on when removed.

I accidentally touched one of the other pins last night and it brought up lots of colou blocks which then disappear when power is removed.

Does any of this mean anything? I read the rest of the post but it started to go over my head. So I am hoping this can mow tell me what part needs replacing!!!

Thank you
Amy
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Re: Blank screen issue...

Post by Ast A. Moore »

1024MAK wrote: Sun Jun 17, 2018 11:40 pm
Ast A. Moore wrote: Sun Jun 17, 2018 8:02 pm Pin 8 is VDD, pin 16 is ground: http://pdf1.alldatasheet.com/datasheet- ... 4164B.html
Okay, how much beer have you drunk? :lol:
Oh, crap, it’s a bloody OR gate. I misread the number as IC22. No beer. Otherwise I’d have stayed within odd numbers. Just a long day. ;)
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 Ast A. Moore »

rps wrote: Mon Jun 18, 2018 8:56 am So I did pins 5 to 14 and nothing happened.
You’ll need to solder a wire across these pins and then power up the machine. If you bridge them after the ROM has finished checking for the presence of upper RAM (which happens almost immediately after a reset/power on), you’ll either get no effect, or it’ll be unpredictable.
rps wrote: Mon Jun 18, 2018 8:56 amI did pin 14 to ground (7 was it?) that shuts the machine off and then comes back on when removed.
That would short the entire 5V rail of the machine. If it was brief enough, it’s not too critical for most of the components if you’re lucky (not particularly great, either), except perhaps the lower RAM ICs. Those tend to fail if one of the three power rails goes down.
rps wrote: Mon Jun 18, 2018 8:56 amI accidentally touched one of the other pins last night and it brought up lots of colou blocks which then disappear when power is removed.
There’s a small chance you fried the MREQ line of the CPU.
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.
rps
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Re: Blank screen issue...

Post by rps »

Oh dear :roll: can’t believe I did that! Well I will progress anyway and solder the wire to pins 5 and 14, then start up the machine.

Thanks
!!!!
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