A Guide to Over-voltage on a CPC

[VincentGR]

Thanks again for your help mate.

[Bryce]

Btw: The 8638 on your chip just means it was produced in 1986 - Week 38.

Bryce.

[robcfg]

Hi guys! Take a look at the CPC+ mainboard pictures. They usually carry the Microchip version of the AY-3-8912.

[dragon]

Hi guys! Take a look at the CPC+ mainboard pictures. They usually carry the Microchip version of the AY-3-8912.

And the cost down.

[gerald]

FYI, Microchip is a spin-of  some of General Instrument.

[Bryce]

FYI, Microchip is a spin-of  some of General Instrument.

I know, but I didn't think they had ever produced it under that name. But I never even spotted that on the Plus I've never had to swap one.

Bryce.

[VincentGR]

At last, I got the time to test my FIRST CPC machine back from 1988.
This is the one I'll be buried with. I made a small mod to connect an SMS joypad too. 

SMS Joypad to Amstrad - YouTube

Thank you Bryce for reviving him.

[Bryce]

Ah cool, you finally got around to testing it, glad you're happy with the results

Bryce.

[VincentGR]

Well, last easter a friend gave me his dead 6128 as I told you before.
The CPC was almost destroyed, so many traces were blown and it was repaired with a blowtorch...
The case is a mess, the metal plate on top of the drive is cut in half 
All screws were glued.

Finally I fixed it.

I replaced the capacitor next to the power switch which was one lower than the original.
(in this photo is his first power up, I solder it properly after this  )
 
Also two RAM chips were dead.

I am very happy with the result.
I gave it to a very good friend here plus a tomahawk joystick because he wants to get in the retro community and to start programming an old machine like this.

We also built a scart and tommorow we will make a pc psu.
Also a 3.5 drive and a tape cable as soon as we found the parts.

[||C|-|E||]

It is nice to see the little guy back to life  .

Funny enough, I remember one night that I was modding my 6128 with a 3.5" floppy drive and then I reversed the voltage in the main board. I still do not know if it was a miracle, but after switching on the computer and seeing that nothing was on the screen I realized my mistake and I switch it off assuming that it was dead. However, nothing happened at all. I guess that I was incredibly lucky!

[TFM]

It is nice to see the little guy back to life  .

Funny enough, I remember one night that I was modding my 6128 with a 3.5" floppy drive and then I reversed the voltage in the main board. I still do not know if it was a miracle, but after switching on the computer and seeing that nothing was on the screen I realized my mistake and I switch it off assuming that it was dead. However, nothing happened at all. I guess that I was incredibly lucky!



If you power from the monitor iirc it has a protection, which switches power supply off for a while. 

[||C|-|E||]

But it was a PC PSU 

[TFM]

Guess you used up a few of your guardian angels!?! 

[Bing]

Hi,

I did it, I reversed the polarity... 
 I was surprised not getting any image...  

Then I checked again (third time) just to find I did THE BIG MISTAKE. And now I'm guilty of amstradcide.

What could I do? 
Where could I start testing for damages?
(Ram voltages are fine, 5v in pin

Yes, I know... 

[Bryce]

Oh dear, reverse voltage is worse than over-voltage. What's your level of electronics knowledge and what test equipment do you have? This could be a long journey. The fact that the RAM is at 5V means you don't have a direct short, but also means that finding the damaged components can prove to be more difficult.

As a start I would check for a clock signal from the base clock and clock signals at the CPU and AY chip. Then I'd start probing data and address lines to see if any of them are stuck high or low.

Bryce.

@Gryzor: This needs to be moved to a separate repair thread though.