A2000 help please

[Steve_w]

Hi,

I have an A2000 rev 4.3 that had battery damage and some bad rework carried out by a previous person in the CPU area. I have removed effected components, cleaned up the corrosion, applied UV mask and replaced effected components/sockets.

I tested every CPU and ROM pin for shorts and have not found any. I have confirmed continuity for /AS, LDS, UDS, R/W and DTACK between CPU and Gary, ROMEN between Gary and ROM, A18-23 between Gary and CPU.

With Diagrom in the ROM socket, when on power on I get a black screen. I have checked the following signals
RST - Low then High
HLT - High
/AS - High then Low (some activity then low using KS ROM)
UDS - High then some activity then Low
LDS - High then some activity then Low
R/W - High then Low
DTACK - High then some activity then High
E - 710 Khz square wave signal
BERR - High
ROMEN - High - some activity - High
OVL - High

I swapped the CIA's over but that made no difference, I think this may be a address bur or data bus issue as it there is some activity for about a second then nothing.

Any help or guidance would be much appreciated.

[Hellraiseruk]

Sounds similar to this thread...

viewtopic.php?f=84&t=7067&hilit=A2000

JD

[Steve_w]

Yes I read this but not quite the same, I only get activity for about a second compared to about15 seconds.

[Steve_w]

I'm wondering, could incorrect jumper settings cause this issue?

[Maximilian]

Are you using the original PSU?

Did you test the CPU and the ROM?

[Steve_w]

Good question, I'll test the CPU and ROM, yes I'm using original CPU.

[Maximilian]

I said PSU (powersupply)
The original generates a tick signal that most replacements do not.

[Steve_w]

Sorry mistyped, my mistake.

I have the original PSU.

I just tested the CPU and Diagrom in a working board, they both work fine.

[Steve_w]

Sounds similar to this thread...

viewtopic.php?f=84&t=7067&hilit=A2000

JD

Yes very similar on reading again until it gets to the post

"Could you check Pin 19 on U105 and U603 and pin 1 on U106. They should always be high at this point. If they're low then they could be allowing the 245s or 373's to be erroneously outputting data".

The next post the OP said the board is suddenly recognising diagrom.

I've replaced U603 and u604 as there was some signs of corrosion. I'll check pin 19 on u105 tomorrow.

Also the board I'm fixing does not have the Buster chip.

[Maximilian]

Not having the buster in the socket does not hinder the startup of the machine.
It is a shame that PCB explorer does not have a A2000 schema.
(Maybe the A500 one can help a little if it is a rev 6 A2000?)