Help needed! 2\/mnbc message on cpc6128

[con6128]

Hello my friends!
I facing a problem with my cpc 6128, when I turn the switch on I get the message "2\/mnbc" and the cursor doesn't move at all. ICs have 5v voltage normal temperature. I get this message with the keyboard membrane connected or disconnected. Any idea what could be wrong? Thank you all any advice would be appreciated.
Kostas from Greece

[robcfg]

Hello and welcome to the forum!


Maybe I'm not the best one to answer you, but I think that maybe the problem lies on the AY-3-8912 sound chip (which reads the keyboard) or the PPi8255 chip, which is the one through you read the sound chip data.


Also, If you could take some pictures of your motherboard, we could check if there's a broken track on the board or something like that.


Best luck!

[ralferoo]

Yeah, looks like bit 6 is shorted to ground of the keyboard input port.

In terms of connections, this could be a short to ground on the trace that runs between joystick connector (pin 5), the AY-8192 chip (pin 8 ) or the keyboard connector (the 8 pin one, 2nd from one side). Less likely, it could be a short to ground between the AY-8192 chip (pin 22) and the PPI chip 8255 (pin 38).

[TFM]

A virus from the aliens...


"2\/mnbc" = Too Victory of MacroNuclear BreedChamber.

That means the aliens use your CPC to breed a new generation of Virus.



What can you do? Trace faithfully all tracks on the PCB, one of them is probably broken/scratched.

[con6128]

thank you my friends for your quick response!
I'm impressed! You all detected the faulty area of the pcb. I traced all the wiring traces and i ended up that ralferoo is right. Indeed there is a short to ground of the trace that runs between joystick connector (pin 5), the AY-8192 chip (pin 8 ) and the keyboard connector (the 8 pin one, 2nd from one side). Now, how will i isolate this track from ground?
I tested the traces with a magnifing glass, i can't find any scratch or broken wiring.
Maybe i missed something, i'll try more thoroughly again.
Thanks again for pointing me to the right direction of the problem.

Kostas
                        

[Bryce]

I doubt it's a short on the PCB or tracks. There's a very good chance that the AY is internally damaged. You could temporarily cut pin 8 of the AY to check if the rest of the track (to the Joystick and Keyboard ports) are no longer connected to ground. If this is the case, then you'll need to swap the AY.

Bryce.

[robcfg]

Better bend it, right? 

[con6128]

Hi folks, I'm back and followed the advice of Bryce and I finally figured that the AY-3-8912 chip is faulty (pin 8 and 6 are connected and then connected ground).
I remove the AY-3-8912 and I ordered one from ebay to replace it. I think i will place a 28 pin base for the chip so it will be easier to replace in the future if it happens again. Hopefully this is the only problem on board and after this it will work.
Thank you all for your help!

Kostas

[Bryce]

Good to hear you've found the problem. I'd definitely add a socket for the AY. I doubt there's anything else wrong with the CPC. The AY tends to fail all by itself without much help (in reality it's most likely caused by static discharge through the Joystick port) and rarely effects anything else on the board, so you should be fine.

Bryce.

[con6128]

Well I finally recieved my new AY chip, so I solder a socket and a placed the new AY chip on it.
The  2\/mnbc message no longer exists, but I noticed there is no response if I press any key from keyboard. I tried all the keys.
I swapped the membrane of keyboard with one from a friend (who also has an amstrad and kindly borrowed me his), and everything works fine!.
So the problem now is on membrane. Why all keystrokes give no signal?

At first glance it seems ok with no broken lines. Membrane inspection in progress. I will notify you the results.

Thank you all again for your help!


Kostas

[TFM]

That electical shock which fried your old PSG, probably also fried tracks on the keyboard membrane, probably a track which supplies all key with power / signal. Can you check (or better get a new one)?

[gerald]

Well I finally recieved my new AY chip, so I solder a socket and a placed the new AY chip on it.
The  2\/mnbc message no longer exists, but I noticed there is no response if I press any key from keyboard. I tried all the keys.
I swapped the membrane of keyboard with one from a friend (who also has an amstrad and kindly borrowed me his), and everything works fine!.
So the problem now is on membrane. Why all keystrokes give no signal?

At first glance it seems ok with no broken lines. Membrane inspection in progress. I will notify you the results.

Thank you all again for your help!


Kostas

Either the AY is not working at all, or the line selection path is not working.

The line selection path is done by the PIO (8255) driving a 4bit to 10 line decoder (74LS145, IC101).
If ICI101 chip is dead, no line will be pulled down during the keyboard scan process.

You can check that the AY is working by pulling to GND any of pin 7 to 14 on the AY with the keyboard matrix unplugged. If the AY is OK, you should get series of char (like when your AY had damaged pins).
If you do not get any char doing that,  should should also check that you did not damage the PCB when replacing the AY by checking that all connections are OK.

[Devilmarkus]

Either the AY is not working at all, or the line selection path is not working.

The AY seems to work ok now... As far I read it... He swapped the keyboard and with the other keyboard it worked.
But he wants to fix his original keyboard.

I would also look with a good lens over the lines and contacts.
I guess, the plugs broke.

[Bryce]

I suspect you've creased the membrane where it goes to the PCB and have cracked all the connections.

Bryce.

[robcfg]

If that's the case, would it work to cut a small portion of the membrane connections and try connecting them again?

[Bryce]

Depends where the crack is. If it's right at the connection then probably.

I'd also check that you haven't put the membranes back in the wrong order (ie: the top one on the bottom).

Bryce.