My OSI 610 build - Expansion for the OSI 600

[HigashiJun]

Good luck.

Hope you'll find out soon what the issue is.

[rubber_jonnie]

Good luck.

Hope you'll find out soon what the issue is.

Me too.

Given I know there's a problem with the BR/W signal I'll focus on that for now.

Given that the signal is good from the buffer if I lift the leg, I need to find out what's pulling it down and I know whatever it is is on the 610 board.

I've already looked pretty hard for shorts but I need to look harder.

[rubber_jonnie]

OK, so the BR/W issue appears to be caused by the PIA, the signal works just fine with the PIA removed, however that brings along another issue.

The service manual BoM shows a 6821, but the 610 schematics show a 6820.

Now these ICs have identical pinouts and I assume either should work, but what's recommended? OSI have stated 2 different parts.

PIA aside, the RAM is still not seen, however I have another odd issue, in that U6 pin 4 seems to have no output. Pins 5 & 6 have input and it looks correct, and they are similar to the other gates that have output on the same IC just fine. I tried with the leg lifted and still nothing.

Because it has an input to U7, the output from U7 is wrong, but at least it's progress I suppose.

[rubber_jonnie]

Talking with somebody over on OSIWEB, it seems they are using 6821's, so I've got a 6821 on order, and I'm kind of thinking why my 6821 might be duff, as it seems I didn't buy one during the build.

Wracking my brains I remembered that my Dragon 32 also uses 6821's and it had a bad one I replaced. Did I throw the old one? I can't remember but that might be why it's bad.

I did lift pin 4 of U7 and it made no difference at all, 2 inputs, no output...

[rubber_jonnie]

OK, so hold my beer...

The replacement 6821 made no difference much to my annoyance, so I started looking again for shorts.

And after much peering I found a short!!! It was connecting pin 7 (BA2) and Pin 12 (ID2) together for every other row of RAM. I cleaned it up.

IMG20250608180306.jpg

However when I powered it on nothing changed, I still had 7423 bytes free, but checking through J2 to look at the signals, BR/W was very low, so I powered off and pulled the 6821.

On power on I got this:

IMG20250608182626.jpg

So I added some more RAM and now have this:

IMG20250608183109.jpg

So this means I can order some more RAM, as it's pretty likely that the RAM part of the board is now working.

Next up I need to figure out why installing the PIA is pulling the BR/W down so that the signal is off and it won't see the RAM

I've tried 2 PIAs and the result is the same no matter what, so the whole area is going to get a good close up look.

Progress at last!!!

[stephen_usher]

Yay! More RAM! :-)

[rubber_jonnie]

Yay! More RAM! :-)

And more on order 😁

Gotta fix the 6821 issue next.

[HigashiJun]

Good progress !

Well done.

[rubber_jonnie]

Good progress !

Well done.

😁😁😁

[rubber_jonnie]

OK, so the 6821 issue was a dumb mistake, me being misled by the silk screen on the PCB.

My PCB has some limited silk screening on it, and the left hand end of the 6821 position (When viewed from the front) shows the notch:

IMG20250609105319.jpg

And that's the wrong end for the notch as it turns out. I went back to the service manual and found this image:

Screenshot 2025-06-09 at 10.54.27.png

It clearly shows the 6821 180 degrees rotated from the way I had it.


I flipped it round and see this now:

IMG20250609105347.jpg

This is BR/W at J2 (Yellow trace) and pin 21 of the 6821 - it's all fine now, plus the memory is visible at boot. Very happy, now I can start to look at getting the FDD set up.

I'll need to work through the final setup stps now.