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Amiga computer related discussion => Amiga Hardware Issues and discussion => Topic started by: nadoom on December 22, 2020, 03:42:00 PM

Title: A1200 Dim Led, Black screen :-(
Post by: nadoom on December 22, 2020, 03:42:00 PM

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.
There is now CCK4 signal to the crystal so i am thinking budgie may be nackered?

Your suggestions would be really useful.  I am a mid level noob (although I have built a A500++ and a floppie A2000), good at sticking stuff on a motherboard but find it hard to know where to start with actually diagnosing issues.. so if any of you have some ideas where I can  make a start please let me know?

Cheers

Nad

Thanks in advance!

Title: Re: A1200 Dim Led, Black screen :-(
Post by: giZmo350 on December 22, 2020, 04:28:42 PM

Has the board ever been recapped?

Title: Re: A1200 Dim Led, Black screen :-(
Post by: TribbleSmasher on December 22, 2020, 04:34:35 PM

Is everything disconnected, even the floppy drive?
Which KickROM version is installed? v40 waits a few seconds to let the hdd spin up.
Otherwise try to get a DiagROM.

Title: Re: A1200 Dim Led, Black screen :-(
Post by: nadoom on December 22, 2020, 08:01:42 PM

Hey,

I am going to be recapping it very soon, just waiting on the caps, however nearly all the caps are related to things other than the black screen ( or so I read ) so i think i should still get something from it other than the black screen?

Its got 3.0 Roms, I've tested these in a working A1200 and they do work fine.

Diagrom hopefully will help, I am waiting for 2 x EPROM to arrive for that, I only have 1 EPROM at the moment ( for A500 Diagrom) so it doesn't work in the 1200


Thanks for your replies

Title: Re: A1200 Dim Led, Black screen :-(
Post by: TribbleSmasher on December 22, 2020, 09:08:48 PM

If you take the mainboard out of the case completely you can try to push the ROMs down to reseat them. sometimes this helps. as you see no colour code it seems the machine wants to start but is just not there yet.

Title: Re: A1200 Dim Led, Black screen :-(
Post by: nadoom on December 23, 2020, 04:17:09 PM

Hi I gave it all a good clean, and also resoldered the connection to the rom's, still getting a black screen.  Very strange, what else could it be? CIA's maybe?

Title: Re: A1200 Dim Led, Black screen [Fixed!]
Post by: nadoom on December 23, 2020, 07:25:56 PM

A chap on amigalove pointed me in the direction of the reset and halt signals on the 68020.. They were both low, so came across this helpful diagram:

https://imgur.com/hiDZrS7

So I tested U49 which is a voltage detector apparently. basically it was outputting 1V rather than 5V so this turns out to be the issue.

I tested the theory by connecting pin 1 with pin 2 so that line was 5V and the A1200 Led went bright and it the kickstart 3.0 screen was shown!!

Turns out its fairly common:

https://www.tsb.space/knowledge-base/replace-amiga-cd32-1200-voltage-detector-with-a-tc54vc/

Thanks to everyone that helped out, also to Amigakit guys for moral support :-D

Nad

Title: Re: A1200 Dim Led, Black screen :-(
Post by: giZmo350 on December 23, 2020, 08:54:10 PM

Good find! I’ve never even heard of this (common) fault. I bookmarked this under my “Amiga Repair” folder for future reference. Thanks for reporting back & with detail.  Happy Holidays!  ;D