OSI 600D Superboard Build

[rubber_jonnie]

So, the LS75s & LS125s arrived and I can now get to the BASIC prompt!!

IMG20240322133627.jpg

It's not all calm seas yet though, because I still have a bad LS138 (Maybe more than one) at U20, U22 & U23. These are fairly critical for many functions, and I've managed to get so far by swapping them around.

For instance, check out the amount of free ram, it looks a bit off.

Hopefully the new ones will arrive soon and that'll hopefully leave me with a working machine :)

Progress nonetheless :)

Once I get this build done I can start on the 610 board, which is a RAM expansion double decker and adds another 6850, a 6821 PIA and allows for FDD connections too.

Fingers crossed I can have the OSI 600 running as expected fairly soon :)

[stephen_usher]

Where did you get those previous chips? I need to avoid that supplier. :)

[rubber_jonnie]

Where did you get those previous chips? I need to avoid that supplier. :)

I'd have to check back but they were from eBay. I ran all the new ones through the TL866 and they all passed fine.

[stephen_usher]

Well, the 74LS00 I removed from the Kaypro would technically have passed the tester's test as the voltages it was outputting were above the TTL level (as long as you didn't change them too quickly) but it wouldn't work properly in circuit. I've had several marginal chips like that come out of systems.

[PhilC]

I've found when running the tests on a TL866 that I've had to test them 5 or so times to make them fail or pass.

[rubber_jonnie]

Well, the 74LS00 I removed from the Kaypro would technically have passed the tester's test as the voltages it was outputting were above the TTL level (as long as you didn't change them too quickly) but it wouldn't work properly in circuit. I've had several marginal chips like that come out of systems.

What tester are you using?

The TL886 Pro I have isn't just testing voltages, it's testing the logic too, and it's the logic that is seeming to fail, but I agree it's not perfect, and marginal chips do sometimes slip through. I had an 74LS00 I put in an Atari 1050 that passed all the tests, but wouldn't work in the drive.

@PhilC I've not had that problem, but when I find a faulty one I do test repeatedly.

I'm considering a Retro Chip Tester Pro sometime in the future, and I really need to get my supply of LS logic IC's better sorted out as it's hard to find anything quickly.

And before you both ask, yes I do take anti-static precautions.

[stephen_usher]

Yeah, TL866... But that's just looking for logic levels, possibly even CMOS, which is lower than TTL, and it runs the truth table once probably at a lowish speed.

The 74LS00 in question would have passed this as the output was above TTL levels (just) if it wasn't constantly run (the drivers probably couldn't sustain the voltage) and the truth table would have passed.

Anti-static precautions... never stay still and hence never static. ;-)

[rubber_jonnie]

OK, so I got the LS138s today and this is the result:

IMG20240325113846.jpg

So the amount of memory is correct now, and I have the full 8K installed. It's stable and will run basic programs, though only small at the moment. The keyboard appears to work normally BTW.

I still have this odd video flickering though, like a key is stuck down (Not visible in this picture), and if I remove U5, an LS125 that forms part of the keyboard circuit, the fuzzy video stops. Also, if I hold down certain keys, the flickering stops.

This IC looks after 4 columns on the keyboard, so I first need to make sure there are no solder bridges around the socket, and I need to try and figure out where the issue is at.

On a positive note, I have video, the machine boots to basic and I can see what looks to be a full complement of RAM, so one I figure out this keyboard issue, then I think it'll be a runner. I'll need to figure out a better display than the 5 inch B&W TV I'm using though!

[stephen_usher]

:dualthumbup:

[PhilC]

Well done. Complete nightmare with those logic chips.