CPC 6128 not working, hangs after a few seconds

Started by DrDoom, 00:19, 15 June 22

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DrDoom

Hello, I've got an Amstrad CPC 6128 (no disk drive) and color monitor. Somebody give me 20 years ago, and now I just turn it on.
There are several issues and I don't know where to start.
The screen colors are weird (see attached picture).
Once powered up, it let me type some keys, and after a few seconds it hangs freezing the screen, and I have to power it down.
Doing this several times results in not being able to see any text on the screen, and have to wait long time until it boots again.
Also there are some vertical pink lines on the screen text.
I really apreciate your help.
I will keep you posted as I fix it. Will be fun.

Bryce

The cable to the monitor probably has breaks in it or the pins in the monitor socket are full of dirt.

Bryce.

GUNHED

Let this CPC run for 30 minutes, then touch the RAM chips. If one (or few) of them is more warm (hot!) than the others, then it's broken and needs to be replaced.  :)

Alternatively there can be a problem with the ROM (Contents faulty).
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MaV

Is the first line in the screenshot a normal message for a 6128?

"Amstrad 128k Microcomputer (s3)" <- I'd expect "(v3)" here.
Black Mesa Transit Announcement System:
"Work safe, work smart. Your future depends on it."

DrDoom

This is the spanish version. Maybe (s) is for Spanish?
I can see (s3) in the Spanish manual screenshots.
http://www.cpcmania.com/Docs/Manuals/Manual%20de%20Usuario%20Amstrad%20CPC%206128.pdf

MaV

Ok. Seems the Spanish versions are a bit different. I didn't know that.
Black Mesa Transit Announcement System:
"Work safe, work smart. Your future depends on it."

DrDoom

Cleaned the monitor cable contacts but It seems there's a crazy voltage fluctuation on the 5v comming from the monitor. Now I need to find a 5v adapter for further testing.

GUNHED

5V 3A PSUs with appropriate plugs are cheap to get.  :)
http://futureos.de --> Get the revolutionary FutureOS (Update: 2022.03.09)
http://futureos.cpc-live.com/files/LambdaSpeak_RSX_by_TFM.zip --> Get the RSX-ROM for LambdaSpeak :-) (Updated: 2021.12.26)

TotO

- "hangs after a few seconds"
- "red border"

You have an Amiga inside your CPC! :-\
"You make one mistake in your life and the internet will never let you live it down" (Keith Goodyer)

DrDoom

Now I've got a 5v 2A stable PSU.
At boot all goes very slow and does not get to Ready state.
I've also tryed to boot from the second memory bank with the hack I found on this forum (see atached picture) and same results, so may be it's not RAM related.https://www.cpcwiki.eu/forum/hardware-related/another-faulty-6128-is-it-ram/msg180734/#msg180734

.

So there's a mini video I posted, i hope someone could get me on the spot:




DrDoom

I forget to mention that once I reboot, I have to wait several minutes to get the boot text again (capacitors?)

Bryce

That could be several things, but not RAM. If it was only slow I'd say the clock, but the fact that the prompt never comes could point towards the CPU, GA or one of the ROM's.

Bryce.


DrDoom

The CPU is MOSTEK TC8634,MK3880N-4,Z80-CPU
This CPU is not listed here: https://www.cpcwiki.eu/index.php/Z80#Part_numbers_used_in_the_Amstrad_CPC_during_its_lifetime
Does it mean that someone has changed it before?

Bryce

Never seen a 6128 with a MOSTEK Z80, so most likely changed. It also looks suspect, possibly a re-labelled 2.5MHz version?

Bryce.

gerald

I've seen mostek Z80 on MC0008C and MC0026D mother boards.
Oldest date code of component on mother boards was 1st quarter of 87.
That seems to match the OP motherboard from date code point of view.

GUNHED

It should be 4 MHz. See here:

https://www.cpu-world.com/CPUs/Z80/Mostek-MK3880N-4.html

Does the 16 MHz Crystal work? It look like it's running at 10% or lower.
http://futureos.de --> Get the revolutionary FutureOS (Update: 2022.03.09)
http://futureos.cpc-live.com/files/LambdaSpeak_RSX_by_TFM.zip --> Get the RSX-ROM for LambdaSpeak :-) (Updated: 2021.12.26)

Bryce

Quote from: GUNHED on 17:21, 19 June 22It should be 4 MHz. See here:

https://www.cpu-world.com/CPUs/Z80/Mostek-MK3880N-4.html

Does the 16 MHz Crystal work? It look like it's running at 10% or lower.
Yes, but if it was re-labelled by some idiot reseller then you have no idea what's inside. The surface looks wrong. In black there were very smooth shiny cases or really matt black cases. The case in the picture is somewhere in between which would suggest that it has been painted over or sanded down to give it a new "upgraded" part number.

Bryce.

GUNHED

http://futureos.de --> Get the revolutionary FutureOS (Update: 2022.03.09)
http://futureos.cpc-live.com/files/LambdaSpeak_RSX_by_TFM.zip --> Get the RSX-ROM for LambdaSpeak :-) (Updated: 2021.12.26)

DrDoom

This a MC0020F board. Like this one:
https://www.cpcwiki.eu/index.php/File:CPC6128_PCB_Top_(Z70290_MC0020F).jpg
As you can see has the same MOSTEK Z80 microprocessor.
Anyway I just ordered a new one ,in a few days we'll know if that was the issue.
For the weird colors on the monitor I've found this tutorial (spanish)
https://auamstrad.es/taller/calibrar-imagen-en-monitor-amstrad-ctm-644/
There are RGB wheels that I think I have to check.

DrDoom

After testing a few times, now there's no image and no sound, it's dead, only the red ligth and the chips are just a bit hot like they are working. I tested with a multimeter and all the IC's are powered.
I've a new Zilog Z80 I changed it and the same result.
So now I have no idea how to proceed, soon I'll have an oscilloscope to do more tests.
But what do you think? at the beginning there was text on the screen I can type on the keyboard, but after a few test (turning on and off) now I've nothing on the screen and I think that the Z80 and RAM are good.
Is this the point where I need to buy more parts?

Bryce

You need to find out which part is broken before you start ordering anything.

Bryce.

DrDoom

I tested with a multimeter the NOT gate near the crystal (IC117)
And I get this readings:
1- 0v
2- 4.8v
3- 3.6v
4- 0v
5- 0v
6- 4.8v
7- GND
8- 2.4v
9- 2.4v
10- 2.5v
11- 2.2v
12- 2.2v
13- 0
14- VCC 4.9

As this is NOT gate pins 8,9,10,11 seems not working.
But do I need to test that with and oscilloscope?

Bryce

A multimeter measures the average voltage over a few milliseconds. So if those pins have a signal that's switching between 0 and 5V with a duty cycle of 50% then you will measure exactly what you are seeing on those pins.

Bryce.

DrDoom

Ok, now I've got an simple oscilloscope, tested some points using the schematic but don't know what to look for o where to start.
I can see clock signals on the ICs pins.
Can someone tell me where to start?
Remember that at the begining I saw the welcome screen and could type some characters, later on it goes slow and doesn't get ready, then I realized that the monitor +5v fluctuate, now I have an external power supply but a black screen.
Can you tell me which points to test with the oscilloscope?
It must be alive again!!!
Thanks!

Bryce

Measuring pins 39 and 40 of the CRTC is a good start. There should be sync signals on both. If one is missing it could mean the CRTC has failed.

Bryce.

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