LC III - Will it ever run?

[PhilC]

Remember a lot of the CPU pins are also on the expansion socket.they will be a lot easier to access.

Indeed. However, possibly a symptom of the problems, the address lines on the expansion connector don't seem to be connected directly to the CPU, or at least they don't tone out.

There was something unusual about that on the LC2,;some didn't ho yo the connector but went elsewhere, I can't remember without looking at the schematic it's got something to do with ram and why the max ram was limited.

[stephen_usher]

Well, I've found a short on D24, 1 ohm, to Vcc. However, it's not a bridge on the CPU as far as I can see at the moment. D12's leg has come off the pad so I'm going to have to resolder that.

The short on D24 would definitely break things and stop the boot.

[stephen_usher]

Well...

Doing resistance measurements the D24 short has to be around the pin of the CPU. It's 0.9 ohms there and 1.0 by the time you get to the FPU socket or the PDS socket. There is definitely no bridging between the legs so it's either underneath the CPU, where it just goes to a via to the other side of the board or it's the CPU itself which is faulty.

The only other slight possibility is underneath the FPU socket where nasty goo has somehow bridged between Vcc and D24, but I'm not sure Vcc is anywhere near that trace as it goes under there.

Feels more and more like a bad CPU as the Vcc leg is right next to D24.

IMG_3632.jpeg

[stephen_usher]

OK. Definitely bad CPU as the resistance is lowest as the legs enter the package. I don't have a hook tiny enough to lift the leg to test further.

Seeing as the other CPU I took off is in a far worse state I'd never get that on the board.

I think that's it then.

[stephen_usher]

Oh! Running a knife between the legs at the package seems to have cleared it!!!

[stephen_usher]

All the address and other lines I can check from the PDS socket are fine.

I may try cleaning the ROM sockets and pins next, just in case.

P.S. ordered some 40 pin sockets from Farnell. (RS only had them in batches of 50!) Should be 42 pin but the ROMs are only 40 anyway.

[atari030]

I have one of these. I was bitterly disappointed there is no demo community like the Atari or Amiga. I did the LCIII+ overclock mod on mine which is pretty easy. The Falcon/TT030 still wipe the floor with it, but hey its a nice looking box and it goes boing.... :-)

[stephen_usher]

Apparently it’s about the same speed as the IIci. It’s called LC for a reason (Low Cost)

[rubber_jonnie]

Oh! Running a knife between the legs at the package seems to have cleared it!!!

Assuming then there may have been a tiny solder whisker or similar. What is it doing now then? Still no boot?

[stephen_usher]

Still just reads a few words from ROM and then bus errors, suggesting that either the address the ROMs are seeing is wrong or the CPU is seeing incorrect data and is jumping to an invalid address when trying to start execution.