Dead CPC6128

mike76 · 10:00, 27 August 22

Hi,
I bought recently an Amstrad CPC6128 ("AZERTY" Ver.) with a CTM644 color monitor.
Sadly, the CPC won't boot.
The power LED stays dark. (The monitor works, I tested it with my another CPC )
I opened it and the mainboard actually looks pretty well and clean,
no visible damages also the capacitors look good.
I tested the power switch with a multimeter and it's OK.
Any idea, what else can I check?

Also:
I watched some YT-repair videos already and spotted a difference to my CPC:
It has a shield over the whole mainboard (never seen it on other machines in the internet)
The shield is connected to the 5v connector with a ceramic capacitor with "2232" on it (-> see pics below)
I'm not sure, if I can completely remove the shield and desolder the capacitor,
to get a better access to the mainboard.

Short:
- Dead CPC6128 , power switch works.
- may I remove the shield (and the capacitor leading to the 5v connector) ?
- what else should I test (got only a multimeter) ?
- any other ideas how to bring it to life?

I really appreciate your help.
Mike

Pictures of the shield and capacitor:

pic-1

pic- 2

Bryce · 12:11, 28 August 22

Hi Mike,
remove the shields and the capacitor. Neither are needed for the CPC to work. Check the solder joints of the power socket and that the socket contacts (at the back of the computer) are clean (and present! I've seen a CPC where the centre pin was missing). If all is good, measure the voltage across the electrolytic capacitor right behind the power socket. This value will tell you how good the socket and power switch are performing.

Bryce.

mike76 · 14:57, 28 August 22

@Bryce
Thank you very much, I did what you said and:
the capacitor (C136) gives me 0v
but on the switch I'm getting 5,2v
Any ideas?
EDIT: ok, now i'm not getting these 5.2v... It could be the switch , indeed..
Is it safe to short its two pins (going from/to the switch) to test if it's faulty?
The CPC shoud stay constantly ON then, right?

pelrun · 15:21, 28 August 22

Yes, bridging the pins is all the switch does, it's perfectly safe. A pin jumper fits perfectly onto the connector too, if you have any lying about.

mike76 · 17:01, 28 August 22

ok, I have the said 5v on the pin of the power-switch (so, the joints are ok) but after I turn the switch on (or just short the pins) the voltage drops to 0.2v

(Measured at the power switch pin)
Doesn't look good...
And ideas?

GUNHED · 17:13, 28 August 22

Tried an external 5 V psu?

mike76 · 17:15, 28 August 22

Quote from: GUNHED on 17:13, 28 August 22Tried an external 5 V psu?

I don't have any, but I tested it on 2 different Monitors
and they both work with my other CPCs.

GUNHED · 17:16, 28 August 22

Well, then it's not the monitor.

Bryce · 07:55, 29 August 22

Quote from: mike76 on 17:01, 28 August 22ok, I have the said 5v on the pin of the power-switch (so, the joints are ok) but after I turn the switch on (or just short the pins) the voltage drops to 0.2v
(Measured at the power switch pin)
Doesn't look good...
And ideas?

That would mean a dead short on the power rail. You need to go and find that.

Bryce.

TotO · 08:31, 29 August 22

Quote from: GUNHED on 17:13, 28 August 22Tried an external 5 V psu?

DON'T! The CTM can protect the CPC against the short circuit, not an external PSU.

mike76 · 15:34, 29 August 22

Thank you for your help and suggestions.

Any idea, where should I start with the checks for shorts?
What parts of the motherboard are problematic in particular-
-as I said, all the capacitors look good, there are no other visible damages on the motherboard.

As of the shield, there was something odd:
As I said in my first post, the shield was bridged to the mainboard with a small capacitor,
to my surprise, the capacitor went from the shield to the *MIDDLE* pin of the power connector (5v)
- not to the ground (the other two pins),
I don't get it, very odd...
Of course, I removed the capacitor (and the shield as well).

Bryce · 08:36, 30 August 22

The shield is grounded, so of course the capacitor should be connected to the positive of the connector. Capacitors are bridged across plus and minus to filter out spurious spikes in the rail voltage.

Bryce.

willow3411 · 17:29, 30 August 22

Hi all
I have exactly the same problem.
Short circuit on the 5v

thanks

Bryce · 08:43, 31 August 22

Shorts inside IC's don't usually make it to 0V, there's almost always at least a "diode drop" (0.7V) and some IC getting very hot.

I'd recommend measuring the resistance across the 5V rail (with the power disconnected!). If it really is almost 0 Ohms, then I'd look for a physical short, ie: bent pins on a capacitor that are touching a different trace or similar stuff.

Bryce.

willow3411 · 16:46, 22 September 22

Thank you Brice
I will look in this direction...
because I have exactly the same error

ps:
I have no hot IC

willow3411 · 06:38, 24 September 22

Well I tested my components.
I have a net short circuit when connecting the 5v to the motherboard.
I was asking myself a question. could it be diode D101?
Because after I have trouble following the 5v line.
thank you in advance
willow

Bryce · 11:00, 24 September 22

Have you measured D101?

Bryce.

willow3411 · 12:37, 24 September 22

hi brice
Thanks for the return

yes D101 is actually ok.
I was wondering about C302 and C303 next to the 5v socket?

Summary
plus D101 OK tested
Well here it is, I have a CPC 6128 which is shorted:
When I plug it into a CTM 644. no 5 volts at the socket. the 5v power supply goes into safety on the monitor.
I specify that the screen works very well and I tested with a second 6128.
So no LED on and no beep when pressing LED . Which is logical.
Of course I changed the Z80, the 40010 still the same.
On the other hand when I connect the motherboard without any components on their supports, I have 3.2V (so always something that shitty).
I don't know what happened with this amstrad, I don't have the history.
I have the diagram in front of me but I have trouble following the path of the 5v
I also specify that I have no IC that heat.
In your opinion, what do you think of Q302 and Q303 (the two transistors next to the power supply)?
I have a donor motherboard but before doing anything I prefer to ask the right questions and especially to see if you have already had the problem.
I searched on different forums, but found nothing.
People have had the problem but no solutions posted...

mike76 · 13:43, 24 September 22

Hey willow3411,
my case is indeed very similar to yours but the voltage drops to 0.2v.

I didn't have the time to do the tests Bryce recommended,
will try to do it tomorrow and if not, then at least within the next week.
I'll post my results here.

BTW:
>> the 5v power supply goes into safety on the monitor.
What is that? how do I find out if the psu has gone into "safety" ?

willow3411 · 14:50, 24 September 22

Hi
The voltage goes from 5v to 0v on the CTM 644 monitor cable when I plug in the motherboard

I have to turn off the monitor to find the 5v on the socket (not connected to the motherboard of course)

Bryce · 14:50, 24 September 22

Quote from: willow3411 on 12:37, 24 September 22hi brice
Thanks for the return

yes D101 is actually ok.
I was wondering about C302 and C303 next to the 5v socket?

Summary
plus D101 OK tested
Well here it is, I have a CPC 6128 which is shorted:
When I plug it into a CTM 644. no 5 volts at the socket. the 5v power supply goes into safety on the monitor.
I specify that the screen works very well and I tested with a second 6128.
So no LED on and no beep when pressing LED . Which is logical.
Of course I changed the Z80, the 40010 still the same.
On the other hand when I connect the motherboard without any components on their supports, I have 3.2V (so always something that shitty).
I don't know what happened with this amstrad, I don't have the history.
I have the diagram in front of me but I have trouble following the path of the 5v
I also specify that I have no IC that heat.
In your opinion, what do you think of Q302 and Q303 (the two transistors next to the power supply)?
I have a donor motherboard but before doing anything I prefer to ask the right questions and especially to see if you have already had the problem.
I searched on different forums, but found nothing.
People have had the problem but no solutions posted...

There is no C302 and C303 next to the power socket, I assume you mean Q302/303. These are a discrete darlington pair used to switch on the 12V rail to the disk drive. You can remove both of these and the CPC should still work fine.

Bryce.

willow3411 · 15:55, 24 September 22

Yes sorry I was wrong it is Q302 and Q303....
However, I realized one thing. I have track points that are blocked?
here are the photos.
I don't have this on my other motherboard.
weird isn't it?

You cannot view this attachment. You cannot view this attachment.

Bryce · 13:06, 25 September 22

No, that's just a side effect of the wave soldering process that was used. If the temperature is slightly low it will leave solder in the vias, if the temperature is higher this solder falls back out before it has cooled.

Bryce.

willow3411 · 14:36, 25 September 22

Thanks for this reponse.

So here is my progress.
Lab power supply set to 5V (see photo)
So 5v closed circuit (OFF button)
Then 3.2v and 1.9  Amp (see photo).
It's been like this for 5 minutes and components ( rams ) are a bit hot


Here is where I am...
Thanks in advance
also posted on CPC rulez

willow3411 · 16:27, 25 September 22

Well the RAMS are very hot.
I will change them

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Dead CPC6128