GX4000 almost dead. Could we fix it?

[Querubin]

Hi,
I found a broken GX4000 board and maybe it could be repaired.

If 9,5 volts and SCART nothing on screen. Maybe a fast blink on screen but black.
If 9,5 volts and RF nothing on screen. Allways garbage. Maybe when power on the garbage signal/sound changes in other way but other king of garbage.
If 5 volts and Amstrad monitor connector appears that on screen.



ASIC dead?. Maybe RAM?. How to start to check?.

Thanks in advance.

EDIT: Changing the number 12V by 9,5V because was that I used.

[00WReX]

If you have a C4CPC board, you can check the RAM...

http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/quick-and-dirty-ram-test-for-cpc/

Cheers,
Shane

[Querubin]

Thanks but don't have C4CPC.

[gerald]

Hi,
I found a broken GX4000 board and maybe it could be repaired.

If 11 volts and SCART nothing on screen. Maybe a fast blink on screen but black.
If 11 volts and RF nothing on screen. Allways garbage. Maybe when power on the garbage signal/sound changes in other way but other king of garbage.

ASIC dead?. Maybe RAM?. How to start to check?.

Most likely RAM.
But if it's not giving the same screen with the 11V, it could be worst.
Either the 5V regulator is not able to provide engouh current and the voltage is droping low to the point the Z80 ad al does not work, and you get a balck screen.
Or the 5V regulator is dead.

So I would 1st try to replace the RAM chip, and keep testing with the 5V input. If it's OK, I would then check the 5V regulators.

[Querubin]

Thanks Gerald,

So I would 1st try to replace the RAM chip, and keep testing with the 5V input. If it's OK, I would then check the 5V regulators.

Replace IC10 and IC11, aren't they? Could I to take advantage and replace it with higher ones or is not as easy?. Or needed to do something like this?(shame schematics are out).

I'd try to make a PCB to do this memory expansion if someone provides me an schematic or detailed instructions. Or adding 2 more 64K ICs or extracting the 64K ram and replacing it for a PCB with the propper expansion(haven't a clue as you can see).

I don't have 41464 RAM ICs. While I receive them I will test the IC181 7805 and IC17 78L05 5V regulators.

EDIT: Checked the voltage regulators.

with 9,5V input - 7805 pin 1=4,49 / pin3=2,97
                       78L05 pin 1=4,42 / pin3=3,26
with 5V input   - 7805 pin 1=4,15 / pin3=4,92
                       78L05 pin 1=3,04 / pin3=4,12

Says this something to you?.

EDIT2: Changing the number 12V by 9,5V because was that I used.

[gerald]

Thanks Gerald,
Replace IC10 and IC11, aren't they? Could I to take advantage and replace it with higher ones or is not as easy?. Or needed to do something like this?(shame schematics are out).

I'd try to make a PCB to do this memory expansion if someone provides me an schematic or detailed instructions. Or adding 2 more 64K ICs or extracting the 64K ram and replacing it for a PCB with the propper expansion(haven't a clue as you can see).

As a 1st step, jus replace IC10 and IC11. Put them on socket.
As for increasing to 128k, the only solution is to piggy-pack the additional devices.
I've recently tried to make a PCB for this, but the power switch just above the RAM and DAC just prevent doing so.

Checked the voltage regulators.

with 12V input - 7805 pin 1=4,49 / pin3=2,97
                       78L05 pin 1=4,42 / pin3=3,26
with 5V input   - 7805 pin 1=4,15 / pin3=4,92
                       78L05 pin 1=3,04 / pin3=4,12

Tell this something to you?.

It seems that the 5V rail is overloaded, If the RAM are the culprit these must be quite hot. This is a simple check, but mind your finger
At least the 5V regulators does not seem dead short.
Can you check the voltage at the output of both 5V regulator with 11V input and power switch off ( the power switch is after the 5V regulator). These should be 5V +/- 5%. The we can assume these are working.

[Querubin]

The 78L05 was blown and I'm making measures with a 7805 on its place while new one arrive.

Any link to see how to make this piggy-pack issue to expand the RAM?

My mistake writing 12V instead 11V. But to be sincere I'm using 9,5V. Don't mind, isn't it?.

Now the measurements with power off but current plug in.

with 9,5V - 7805 pin 1=10,69 / pin3=5,06
               78L05 pin 1=8,23  /  pin3=10,70
with 5V  -   7805 pin 1=4,23 /  pin3=5,14
               78L05 pin 1=3,17 /   pin3=4,23

And the finger test has a curios result too. With 9,5 volts and power on no warm is detected but when doing the same with 5V current both IC10 and IC11 get very hot. I have to quit the finger so not be injuried.

Thanks for your help Gerald.

[Querubin]

I built a multicart with nilquader method and burnt the diagnostic ROM 00WreX suggested and it appears this directly.



Confirms that is a fail on the RAM. Video test didn't run so maybe something more will be wrong.
I'm still waiting for the new RAM modules. I'll keep you informed.

[Querubin]

Two months later the new RAM modules came from long and after replacing the old with socketted ones the GX4000 works fine again.

Thanks for the help.

Does anyone know an easy guide to upgrade the GX4000 to 128K? Or maybe first... Does it worth?.

[Shaun M. Neary]

Two months later the new RAM modules came from long and after replacing the old with socketted ones the GX4000 works fine again.

Thanks for the help.

Does anyone know an easy guide to upgrade the GX4000 to 128K? Or maybe first... Does it worth?.

I wouldn't bother. All the converted games are 64k and load just as fast. The rare enhancements such as speech aren't worth it in my honest opinion.

[Skunkfish]

Would be good to have 128KB to play games such as The Addams Family on a GX4000. I recall that Piotr Bugaj upgraded his GX4000 to 6128+ spec a couple of years back.

The thread is here:
http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/gx4000-converted-to-6128-plus-job-almost-done-)/

[Querubin]

Yes but I don't like this kind of thread where someone shows what he did but not how he did.


There are instructions here but are a bit cryptic to me because there isn't any pic or schematics.

[parodius]

Yes but I don't like this kind of thread where someone shows what he did but not how he did.


There are instructions here but are a bit cryptic to me because there isn't any pic or schematics.

Are you referring to the 464+ upgrade instructions ?
https://web.archive.org/save/_embed/http://cpctech.cpc-live.com/docs/mods/464p128k.html

[Querubin]

Are you referring to the 464+ upgrade instructions ?
https://web.archive.org/save/_embed/http://cpctech.cpc-live.com/docs/mods/464p128k.html

No, to this https://web.archive.org/web/20020107215353/http://andercheran.aiind.upv.es:80/~amstrad/docs/mods/gx2plus.html
It has pictures.

Now I need time to do it.

Thanks.

[Chinnery]

No, to this. It has pictures.

I get an error on that url
Name Error: The domain name does not exist.

[Querubin]

Fixed. Try now.
Thanks.

[parodius]

Cheers ! Will definitely try that, sooner than later hopefully.

[gerald]

No, to this https://web.archive.org/web/20020107215353/http://andercheran.aiind.upv.es:80/~amstrad/docs/mods/gx2plus.html
It has pictures.

Now I need time to do it.

Thanks.

Did you find time to do it ?
For just adding 128K to a GX, you only need to:
- piggy-pack the ram devices except pin 16 which is connected to pin 91 of the ASIC with the 33 ohm resistor. This connection is the most difficult part.
- connect a 10k resistor between pin 11 of IC8 and ground (IC8 pin