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General Category => Technical support - Hardware related => Topic started by: DrDoom on 22:19, 14 June 22

Title: CPC 6128 not working, hangs after a few seconds
Post by: DrDoom on 22:19, 14 June 22
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.
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: Bryce on 08:55, 15 June 22
The cable to the monitor probably has breaks in it or the pins in the monitor socket are full of dirt.

Bryce.
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: GUNHED on 15:06, 15 June 22
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).
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: MaV on 15:37, 15 June 22
Is the first line in the screenshot a normal message for a 6128?

"Amstrad 128k Microcomputer (s3)" <- I'd expect "(v3)" here.
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: DrDoom on 18:05, 15 June 22
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
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: MaV on 18:14, 15 June 22
Ok. Seems the Spanish versions are a bit different. I didn't know that.
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: DrDoom on 19:14, 15 June 22
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.
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: GUNHED on 09:54, 16 June 22
5V 3A PSUs with appropriate plugs are cheap to get.  :)
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: TotO on 10:24, 16 June 22
- "hangs after a few seconds"
- "red border"

You have an Amiga inside your CPC! :-\
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: DrDoom on 11:50, 18 June 22
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 (https://www.cpcwiki.eu/forum/hardware-related/another-faulty-6128-is-it-ram/msg180734/#msg180734)

.(https://i.imgur.com/WqZwy2d.jpg)

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



Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: DrDoom on 11:52, 18 June 22
I forget to mention that once I reboot, I have to wait several minutes to get the boot text again (capacitors?)
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: Bryce on 15:19, 18 June 22
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.

Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: DrDoom on 16:08, 18 June 22
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 (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?
(https://i.imgur.com/T7lXQfW.jpg)
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: Bryce on 13:14, 19 June 22
Never seen a 6128 with a MOSTEK Z80, so most likely changed. It also looks suspect, possibly a re-labelled 2.5MHz version?

Bryce.
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: gerald on 14:12, 19 June 22
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.
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: GUNHED on 15:21, 19 June 22
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.
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: Bryce on 15:50, 19 June 22
Quote from: GUNHED on 15:21, 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.
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: GUNHED on 18:25, 19 June 22
Interesting case...
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: DrDoom on 21:39, 19 June 22
This a MC0020F board. Like this one:
https://www.cpcwiki.eu/index.php/File:CPC6128_PCB_Top_(Z70290_MC0020F).jpg (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/ (https://auamstrad.es/taller/calibrar-imagen-en-monitor-amstrad-ctm-644/)
There are RGB wheels that I think I have to check.
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: DrDoom on 17:54, 28 June 22
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?
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: Bryce on 10:28, 29 June 22
You need to find out which part is broken before you start ordering anything.

Bryce.
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: DrDoom on 11:36, 02 July 22
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?
(https://i.imgur.com/EpJdh9b.png)
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: Bryce on 09:47, 04 July 22
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.
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: DrDoom on 12:16, 09 July 22
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!
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: Bryce on 11:25, 10 July 22
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.
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: DrDoom on 13:20, 10 July 22
Ok, I see this is the CRTC: UM6845:
(https://i.imgur.com/APtabjr.png)
pin 40 (VS) gives me this:
(https://i.imgur.com/Z3VWzF2.jpg)
but pin 39 (HS) nothing as you can see :
(https://i.imgur.com/iASoPFR.jpg)

Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: Bryce on 14:05, 10 July 22
Then there's a very high chance that it has failed.

Bryce.
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: DrDoom on 21:36, 13 July 22
Now I see that my oscilloscope has a 200kHz bandwidth only,  :-[ so I will need a better one at least 8MHz? Could you suggest me one just for this task?
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: Bryce on 09:46, 14 July 22
How much are you prepared to spend?

Bryce.
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: DrDoom on 10:15, 14 July 22
About 200€
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: Bryce on 11:57, 14 July 22
You don't get much scope for €200, but this one might be a step up from what you have: https://www.amazon.de/Hanmatek-Oszilloskop-HO52-Oscilloscope-Multimeter/dp/B08VRDG94N

Bryce.
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: DrDoom on 10:09, 23 July 22
Ok, now I've got an FNIRSI 1014d "100MHz" oscilloscope for 200€! and got some measurements:
Crystal oscillator: 16MHz
(https://i.imgur.com/Fu3gJKi.jpg)

Z80 pin 6 (clock): 4MHz
(https://i.imgur.com/YlTQjTx.jpg)

Gatearray pin 4 (clock): 1MHz
(https://i.imgur.com/Y5NxVv9.jpg)

CRT pin 21 (clock) 1MHz
(https://i.imgur.com/1PW5TK0.jpg)

CRT pin 40 (Vsync) 5v
(https://i.imgur.com/cubeFk8.jpg)

CRT pin 39 (Hsync) 0v
(https://i.imgur.com/1lIpM3b.jpg)

I also have now a faulty Amstrad PCW board to get components from, and I changed the Z80 with same results. I hope no PCW fans will get angry.
(https://i.imgur.com/reK29t5.jpg)

How can I be sure what is wrong? the CRT? am I measuring it right?
Thanks!
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: DrDoom on 16:28, 30 July 22
I keep trying.
I disoldered two 6845 (CRTC) from some old CGA/Parallel Port boards.
Can I piggyback a 6845?
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: Bryce on 11:49, 31 July 22
Are you sure you have the timebase set properly when reading pins 39 and 40 on the CRTC?

Bryce.
Title: Re: CPC 6128 not working, hangs after a few seconds
Post by: DrDoom on 18:14, 31 July 22
I tested again and no activity reading  39 and 40 on the CRTC. Tryed on diferent timebases.
But, I have now a Dandanator CPC mini, and when connected there's activity in pin 40, but not on 39.
Anyway no image and no keyboard sounds when the Dandanator is connected.
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