A1200 Dead, Black Screen, dull led

[nadoom]

Hello Chaps,

I am having a right mare trying to find out what is wrong with an A1200 I have acquired. Its a 2B mobo, and when power is applied from a good working PSU, I am only getting a black screen , led remains dull.

Power seems to be reaching every where, +12,-12 and +5 is ok.
My oscilloscope is showing that all the address lines show no activity, so its as if it isn't starting at all.
None of the chips get hot, alice gets a little warm.

Any way and suggestions would be really useful. I have checked some of the traces from the Odd CIA . but I am a bit of a noob (although I have built a A500++ and a floppie A2000) so if any of you have some ideas please let me know!

Thanks in advance!

Nad

[EzdineG]

What are the status of the /HLT and /RST pins on the cpu? If either are low, that is the reason you have no activity on the address lines. According to the attached image they’re pins 6 and 22 on the EC version of the chip as is on the A1200.

When you say +5v is okay, do you mean it is above 4.7V? I believe that would be the absolute minimum for the machine to boot.

[nadoom]

Hey ezdineG,

both 6 and 22 are Low 0 volts , the voltage is 5.16 ish,

thanks for the reply, what should I look at next ?


I think I will have a mooch around this diagram see if I can see why it isn't starting, is that a good starting point?
https://imgur.com/hiDZrS7
Nad

[nadoom]

Did some digging around U49

It has 5V going in and 1V coming from VOUT on pin 2, that should be 5V shouldn't it?

[nadoom]

Just confirmed this on my working A1200 it should output 5V!

Also I just got a wire and shorted pin 1 with in 2 ( basically by passing the voltage detector).. and what you know?

Led Light went bright and the kickstart screen was shown!

Just need to order the part and fit it. Really pleased!

Thanks @ezdineG!!

[EzdineG]

Great work!