Disabling Upper RAM

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HappyLittleDiodes
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Disabling Upper RAM

Post by HappyLittleDiodes »

Hi all

A link was posted recently to a trick to disable the upper RAM in a 48k.
Unfortunately the link is dead now :(
Can anybody remind me what the trick was?
I think it was a case of bending a leg out of the socket so disable RAM addressing or...something along those lines?

I'd like to avoid cutting legs at the moment

Many thanks

HLD
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1024MAK
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Re: Disabling Upper RAM

Post by 1024MAK »

You could use that nice empty search box on the top right corner ;)

Anyway, I’ll repost my comments from another topic...
1024MAK wrote: Thu Apr 16, 2020 2:59 pm To disable the "upper" 32k of RAM, take a piece of wire and connect pin 5 on IC23 (a 74LS32) to +5V. (link).

I suggest you make a temporary solder connection with an SPST on/off switch in series. Start with the switch set to on. Then if you get to the copyright screen and BASIC works, after that, you can turn the switch off so that some simple BASIC commands can access and therefore test the upper RAM.

This is done because the ROM code tries to see how much RAM there is at start-up. After this, the BASIC ROM will ignore any RAM that is not present (or in this case, is disabled)(well, unless you reset the limits using CLEAR n). But the BASIC POKE and PEEK commands have no limits and work across the whole Z80 address range.

Double and triple check that you have the correct pin on the correct chip, or you WILL damage something. This temporary modification will disable the "upper" RAM (when the switch is ON), as the CAS signal won't reach the RAM chips.

Note that if a faulty DRAM IC is jamming one of the data lines, this won't really help.
Mark

P.S. The World of Spectrum site is currently off line while server maintenance is being done. So the link above may not work until the site is back up. It’s expected to be back up next week.
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Re: Disabling Upper RAM

Post by HappyLittleDiodes »

Thanks, I'll try it.

The handy search box took me to the link!

Cheers Mark
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Re: Disabling Upper RAM

Post by WDeranged »

N00b question. Why would anyone want to disable the upper ram?
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Re: Disabling Upper RAM

Post by HappyLittleDiodes »

I have a spectrum that won't boot. Very erratic, almost a different result every time. I'm eliminating various components... lower ram, upper ram, cpu. I think I'm now down to bad ROM, or IC27
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Re: Disabling Upper RAM

Post by HappyLittleDiodes »

I've also done a fair bit of continuity testing and tested for shorts, checked the entire board for splashes. Voltages good and steady enough I think. It's a very interesting one
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Re: Disabling Upper RAM

Post by 1024MAK »

WDeranged wrote: Sat May 02, 2020 3:42 am N00b question. Why would anyone want to disable the upper ram?
If you think that one or more of the ‘upper’ DRAM chips, or the multiplexers that supply the address bus to them may be faulty.

The ROM code carries out a test at power on or reset to see how much RAM is installed in the system. It is not designed as a full test to see if the RAM is working correctly. It’s job is just to work out how much RAM is available to the system.

If the upper RAM system is faulty, but the ROM code does detect that it is present, it will set the system up as a 48K system. So the Z80 stack will be placed high up in the ‘upper’ RAM area. But with faulty RAM (or a faulty multiplexer), the data stored on the stack will be corrupted. Hence as soon as the Z80 reads the data from the stack, the machine will crash.

If you disable the ‘upper’ RAM, then the ROM code will set the system up as a 16K system, so the Z80 stack will never be put in the ‘upper’ RAM. If the ‘lower’ RAM is fully functional and there are no other faults that prevent the Z80 and the ROM from working normally, BASIC should start up.

Mark
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Re: Disabling Upper RAM

Post by HappyLittleDiodes »

Slight diversion from the original thread but I just noticed that whoever recapped this board used the wrong rating of capacitor in a few locations.

........might be a good place to start!
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Re: Disabling Upper RAM

Post by 1024MAK »

HappyLittleDiodes wrote: Sat May 02, 2020 7:05 pm Slight diversion from the original thread but I just noticed that whoever recapped this board used the wrong rating of capacitor in a few locations.

........might be a good place to start!
Different capacitance? Different voltage? Higher value? Lower value?
In most applications, the next value up, either capacitance or voltage is not a problem.

Mark
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Re: Disabling Upper RAM

Post by HappyLittleDiodes »

1024MAK wrote: Sat May 02, 2020 11:12 pm
HappyLittleDiodes wrote: Sat May 02, 2020 7:05 pm Slight diversion from the original thread but I just noticed that whoever recapped this board used the wrong rating of capacitor in a few locations.

........might be a good place to start!
Different capacitance? Different voltage? Higher value? Lower value?
In most applications, the next value up, either capacitance or voltage is not a problem.

Mark
Hi Mark

C80 - should be 22uF, was 1uF
C74 - should be 4.7uF, was 22uF
C27 - should be 1uF, was 4.7uF

Changed them all, hasn't resolved anything but needed doing at least for C80 as it's right in the middle of the power circuit

Cheers

Jim
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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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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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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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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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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:
Derek Fountain, author of the ZX Spectrum C Programmer's Getting Started Guide and various open source games, hardware and other projects, including an IF1 and ZX Microdrive emulator.
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Re: Disabling Upper RAM

Post by HappyLittleDiodes »

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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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 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:

Mark
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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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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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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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Re: Disabling Upper RAM

Post by dfzx »

HappyLittleDiodes wrote: Tue May 05, 2020 11:00 am 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

Image
Something else I never knew existed. :)
Derek Fountain, author of the ZX Spectrum C Programmer's Getting Started Guide and various open source games, hardware and other projects, including an IF1 and ZX Microdrive emulator.
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Re: Disabling Upper RAM

Post by HappyLittleDiodes »

I've never used it before but it seems like a good solution, fingers crossed, if I make a mess again it's going to get harder to fix
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Re: Disabling Upper RAM

Post by speccyplus »

Don't feel bad, I managed to lift about 4 traces too, removing all the chips on my board. Most of them from chips that were already replaced by a PO and I suspect damaged them.

Its a tricky board to remove chips from. Patience is key, as in flux and re-flowing - fresh solder.
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Re: Disabling Upper RAM

Post by speccyplus »

Image

For you feel better pleasure. Top side just had a few traces, bottom was a disaster ground. I did the full nuke on removing all the upper ram and logic chips and for some reason that section gave me all the trouble (ram). Rest was 98% fine. Good news is I have a fully socketed board once I get that last two sockets in.

Not my best work or my most proud, I have never had so much trouble desoldering components than on this board being so fragile.

So many pads came off on the 32k section from previous damage. Not needed as they were not connected to anything, but still. Next time I will try the thin wire repairs hidden by the socket, that I have seen folks do. You need a steady hand...

https://www.youtube.com/watch?v=KRSQOOY67DU


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Re: Disabling Upper RAM

Post by HappyLittleDiodes »

speccyplus wrote: Wed May 06, 2020 2:26 pm Image

For you feel better pleasure. Top side just had a few traces, bottom was a disaster ground. I did the full nuke on removing all the upper ram and logic chips and for some reason that section gave me all the trouble (ram). Rest was 98% fine. Good news is I have a fully socketed board once I get that last two sockets in.

Not my best work or my most proud, I have never had so much trouble desoldering components than on this board being so fragile.

So many pads came off on the 32k section from previous damage. Not needed as they were not connected to anything, but still. Next time I will try the thin wire repairs hidden by the socket, that I have seen folks do. You need a steady hand...

https://www.youtube.com/watch?v=KRSQOOY67DU


Image
Did you use a heat gun to help with removing the chips?
I find it makes the actual removal a lot smoother, but you still have to take the time to get as much solder out as possible!

I'll let you know how I get on using the tape
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