Amiga 1200 Address line 8 shorted to 5V

[Gitz]

Hi,

I have my second Amiga 1200 weird short to 5V, maybe someone can point to what check next?

This motherboard had many issues and i have fix or removed damaged parts:
- Z221 shorted (removed)
- +5V line shorted cap. (fixed)
- CPU dead (fixed with new cpu)
- KS sockets broken (fixed)

But still no picture from composite, rgb or serial (putty), i have diagroms installed.

After many hour of using multimeter i did find that CPU address line 8 is shorted to +5V line.
With amiga PCB site and schematics R1 i was sure that A8 line goes from CPU to alice and both cia chip's.
- remove alice (still short)
- removed both cia chips (still shorted)
- Lift CPU pin 80 from pad, CPU pin no short but PAD gives still short.
- Board don't show any visible solder bridges with other components.

So my question is what next to check?
Is motherboard garbage and best is buy re-amiga motherboard or is there something that i can still check? (memory, some other chip's?)

 

[darefail]

Hi,

When you say shorted, you mean 0-1 Ω or 20Ω?
Just asking because both values give the beep sound.

 

[Gitz]

0.4 Ω about. board rev is 1D4.

 

[levo-ace]

U47 buffer for the PCMCIA .
Any cards on the trapdoor expansion ?

And memory chips , of course .

 

[Gitz]

Removed U47, still shorted.
No expansion, just bare motherboard at table at the moment.

 

[levo-ace]

Have you had a look at PCB explorer ? If you highlight that address line it will show you where and how it runs . You can follow the trace to check for shorts .

Amiga PCB Explorer

The Amiga PCB Explorer is a browser application that was designed to assist people in various aspects of Amiga mainboard repairs.

www.amigapcb.org

You can change the PCB color in settings to make the route clearer .

 

[Gitz]

I have, A8 signal goes CPU - KS socket's - CIA - CIA - Alice - U47
All removed and still shorted.

Measured all caps and resistor and those are good.
i need rent thermal camera if that gives some clues where short happens.
Also must read schematics more if i missed something.

Amiga PCB explorer is one nice software, helps a lot.

 

[RetroMaNi]

Just being practical:

option 1:
Cut trace A8 into sections and beep out the short. Place bodgewire instead of short part of A8

Option 2:
Place CPU - KS socket's - CIA - CIA - Alice - U47 and lift pin A8 from board.
Place bodgewires between legs. So no use of board traces.

 

[Gitz]

Hi, thank you for advice.

I forget to update this situation, i poked board a lot and found more sort's on near cia that is nearest to serial port, that also didin't make any sens.
I didin't understand what was happening so i googleted a lot ja one post on internet give me a clue.

I learned that A1200 is multilayer board and after that i was sure that board is finished.
I bought re-amiga 1200 and now i'm part where i need solder chip memory to board, board post with diagroms nicely so far.