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 » Mon Jun 18, 2018 1:23 pm

By connecting +5V to pin 5 on IC23, you are overriding the signal from elsewhere in the circuit.This point was deliberately chosen, as the existing signal comes via a resistor (R70) which limits the current flow so that the logic gate that feeds resistor R70 is not damaged. Pin 5 is an input to a logic gate.

With a +5V feed applied to pin 5 on IC23, it causes the output of the logic gate in IC23 to output a logic 1 (logic high). This is a control line to all of the 'upper' RAM chips (called "/CAS"). With this control line held at logic 1 (high), it means that all of the 'upper' RAM chips will ignore the Z80 CPU and will not respond. Hence they are disabled.

In most (but not all) RAM chip failure modes, telling a faulty 'upper' RAM chip to ignore the CPU means the ZX Spectrum will start up thinking it is a 16k model.

I see the thread has moved on since I started writing this :mrgreen:

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Re: Blank screen issue...

Post by 1024MAK » Mon Jun 18, 2018 1:31 pm

rps wrote:
Mon Jun 18, 2018 12:04 pm
Ok, so you think lower ram possibly. Does this mean you can rule out upper ram? Or could it still be upper ram?

Also, if I was to get parts to make repairs and see where I get with it. Is there ways to test a chip when you remove it from the board?

Thanks
RPS
At this stage, it looks like the ROM, the CPU and the ULA are all at least partially working. Most likely suspects remain any of the RAM chips. So far no RAM has been eliminated.

The only easy way to test a suspect chip, is either with specialist test gear, or in a known good working ZX Spectrum.

Can you please post a photo of the wire that you soldered across IC23 please.

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

Post by Ast A. Moore » Mon Jun 18, 2018 2:47 pm

rps wrote:
Mon Jun 18, 2018 12:04 pm
Ok, so you think lower ram possibly. Does this mean you can rule out upper ram? Or could it still be upper ram?

Also, if I was to get parts to make repairs and see where I get with it. Is there ways to test a chip when you remove it from the board?
No, you can’t rule out upper RAM completely. Then again, since the screen is part of the lower RAM, and it looks like the screen isn’t corrupted, it might not be lower RAM either. I just suggest a method of eliminating potential causes one by one. Technically (albeit, very unlikely) the problem might be with the ROM itself, or, possibly one of the address or data lines.

There’s no easy way of testing ICs outside a circuit, and each IC would require a special test rig in most cases. Sometimes, it’s advantageous to have a spare test “known-good” board with all ICs socketed. Then you can take individual ICs out of the board you’re woking on and putting them in the test board. Conversely, you can take out ICs from the “known-good” board and use them in the board under investigation, thus ruling out chips one by one.

That just covers the ICs. The fault may be with some of the passives (capacitors may affect timing), but diagnosing it would definitely require an oscilloscope.
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Re: Blank screen issue...

Post by Ast A. Moore » Mon Jun 18, 2018 2:47 pm

1024MAK wrote:
Mon Jun 18, 2018 1:23 pm
I see the thread has moved on since I started writing this :mrgreen:
Bah. Tell me about it! We need to coordinate better. :lol:
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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 » Tue Jun 19, 2018 9:20 am

Hi,
Here is the image of IC23 and a wire from pin 5 to 14 as mentioned. Sorry it’s not a great image, I cracked the glass on my camera lens...

So when I powered on this morning, no red lines. Turned off and plugged in again and got red verticle lines. I dont think it knows what is going on!

Thanks
RPS
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Re: Blank screen issue...

Post by Ast A. Moore » Tue Jun 19, 2018 9:39 am

Interesting. I don’t recall any 48K Spectrums that came with a Zilog’s Z80 originally. They were usually either a NEC or an SGS. Does it look that the board has been worked on before to you? (Any soldering job look different, say, visible flux residue, etc.?)
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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 1024MAK » Tue Jun 19, 2018 10:12 am

Although not as common as the NEC D780C-1 CPU (a Z80 clone), Sinclair did use Zilog Z80A CPU's in both ZX81's and ZX Spectrum's. And going by the date codes and wave soldered joints, these are the original chips that were fitted.

@rps do you have any expansions? If yes, please list them.

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Re: Blank screen issue...

Post by rps » Tue Jun 19, 2018 10:31 am

Hi,
No expansions to my knowledge and I can’t see any solder work that doesn’t look original either.

I will take some more photos for you...
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Re: Blank screen issue...

Post by rps » Tue Jun 19, 2018 10:43 am

Here’s a few photos. If you want anything specific let me know...
Image

Image

Image
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Re: Blank screen issue...

Post by Ast A. Moore » Tue Jun 19, 2018 1:06 pm

1024MAK wrote:
Tue Jun 19, 2018 10:12 am
Sinclair did use Zilog Z80A CPU's in both ZX81's and ZX Spectrum's.
ZX 81s yes, 128Ks virtually exclusively, but for 16/48K it was a rare spectacle. Hence, my suspicion. But the board does look like it’s all original parts. On the first pic, the soldering job on the CPU had a suspicious shine to it, though.

Would be great to see the other side of the board, too.

Still not convinced the CPU or the lower RAM quite have all their marbles.
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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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