Disabling Upper RAM

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: Disabling Upper RAM

Post by 1024MAK »

C80 - should be 22uF, was 1uF - :shock: this will affect the power output of the DC/DC converter :-(
C74 - should be 4.7uF, was 22uF - :shock: this will affect the response time of the regulation of the +12V line :-(
C27 - should be 1uF, was 4.7uF - that will just give a slightly longer reset pulse to the Z80.

Did the person who you got this from wear a stetson?

Mark
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HappyLittleDiodes
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Re: Disabling Upper RAM

Post by HappyLittleDiodes »

1024MAK wrote:
Sun May 03, 2020 12:37 am
C80 - should be 22uF, was 1uF - :shock: this will affect the power output of the DC/DC converter :-(
C74 - should be 4.7uF, was 22uF - :shock: this will affect the response time of the regulation of the +12V line :-(
C27 - should be 1uF, was 4.7uF - that will just give a slightly longer reset pulse to the Z80.

Did the person who you got this from wear a stetson?

Mark
Haha, I couldn't tell you unfortunately but it could be the case.
I was hoping this would have resolved the problem but nope :(

Border colour is changeable, it's very random, the coloured blocks flash at ~ 2Hz
Same result with lower ram removed.
Same result with upper ram disabled.
ULA is fine
Z80 swap didn't help
I think I might end up changing the ROM or the multiplexer chip I'm thinking.
No sign of RAM test on reset.

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HappyLittleDiodes
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Re: Disabling Upper RAM

Post by HappyLittleDiodes »

Hi!
Sorry, I like to share, I hope it's interesting.

After reading some of the Z80 Technical Manual (Great little book happy to have found it) I realised a silly mistake. My replacement Z80 was actually a Z84C20 with is a PIO chip.
I put an actual Z80 in and got a white border with some more familiar failure modes. What's very interesting is that it sometimes (rarely) starts with blue and yellow lines (and static noise) around the border as if it's loading something (it doesn't do this every time).
Anyway I removed the lower RAM and disabled the upper RAM and got something that looks correct (although with lots of noise).
I found that with the RAM installed I could directly influence the picture by touching IC12, the result was the same when I swapped chips around so I suspected the socket.
Continuity testing shows no continuity between A5 on IC12 and the adjacent ICs, so I'll be taking the socket off and inspecting the joint.

It's so exciting to find a fault like this, looking forward to seeing if it fixes some of the issues I'm getting

Cheers
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HappyLittleDiodes
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Re: Disabling Upper RAM

Post by HappyLittleDiodes »

It looks like a bad joint on the topside of the board as there is continuity from the socket to the underside
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dfzx
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Re: Disabling Upper RAM

Post by dfzx »

HappyLittleDiodes wrote:
Sun May 03, 2020 9:02 pm
My replacement Z80 was actually a Z84C20 with is a PIO chip.
What, and it didn't work? :lol:
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HappyLittleDiodes
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Re: Disabling Upper RAM

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dfzx wrote:
Mon May 04, 2020 9:18 am
HappyLittleDiodes wrote:
Sun May 03, 2020 9:02 pm
My replacement Z80 was actually a Z84C20 with is a PIO chip.
What, and it didn't work? :lol:
I'm impressed that anything happened.
I got the screen shown in the post above and the blocks were changing.
The chip did get quite hot though...

It got me thinking, could you make a simulation so accurate that you could predict this behaviour if you put this chip into a working spectrum?
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1024MAK
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Re: Disabling Upper RAM

Post by 1024MAK »

HappyLittleDiodes wrote:
Mon May 04, 2020 11:39 am
dfzx wrote:
Mon May 04, 2020 9:18 am
HappyLittleDiodes wrote:
Sun May 03, 2020 9:02 pm
My replacement Z80 was actually a Z84C20 with is a PIO chip.
What, and it didn't work? :lol:
I'm impressed that anything happened.
I got the screen shown in the post above and the blocks were changing.
The chip did get quite hot though...

It got me thinking, could you make a simulation so accurate that you could predict this behaviour if you put this chip into a working spectrum?
Try powering up a ZX Spectrum with the Z80 missing...
The Z80 in a ZX Spectrum is a slave :lol:

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HappyLittleDiodes
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Re: Disabling Upper RAM

Post by HappyLittleDiodes »

I found my broken track, hooray!

But I got impatient and made it worse. :oops:
Time to put it away until the copper tape arrives...
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dfzx
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Re: Disabling Upper RAM

Post by dfzx »

HappyLittleDiodes wrote:
Mon May 04, 2020 8:04 pm
Time to put it away until the copper tape arrives...
Copper tape? You mean the desoldering stuff?
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HappyLittleDiodes
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Re: Disabling Upper RAM

Post by HappyLittleDiodes »

dfzx wrote:
Tue May 05, 2020 10:54 am
HappyLittleDiodes wrote:
Mon May 04, 2020 8:04 pm
Time to put it away until the copper tape arrives...
Copper tape? You mean the desoldering stuff?
No it's a sticky copper tape for electronic repairs. You tidy up the damage, stick this over and solder it in.
I tried to make the link with a small wire but buggered it up

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