Crystal for 540B Display board

gekaufman
Posts: 72
Joined: Mon Nov 23, 2015 12:57 pm

Crystal for 540B Display board

Post by gekaufman »

Trying to build up one of the 540B replica boards (B&W) and can't locate a crystal.
Schematic calls for 11.79 Mhz, actual on my old board was 11.79648 Mhz.
Alternative for colr is 12.08 Mhz.

Anyone have a spare, or a source? Is 12mhz close enough?

- Gary
dave
Site Admin
Posts: 710
Joined: Tue Sep 09, 2008 5:24 am

Re: Crystal for 540B Display board

Post by dave »

12 MHz should be OK. NTSC hardware tends to be pretty forgiving of a bit of deviation. Look for fundamental mode rather than parallel mode crystals. Digikey has some crystals that are close. You may consider:

11.98 MHz

12.000 MHz

For color, you could try: 12.096 MHz

Dave
gekaufman
Posts: 72
Joined: Mon Nov 23, 2015 12:57 pm

Re: Crystal for 540B Display board

Post by gekaufman »

Dave -

Thanks, I found a 12mhz xtal and installed it - seems to work fine. I constructed it for B&W only, and re-jumpered it for a 2716 Character Generator.

My board came up and gave me a nice "random" display, but I still have some troubleshooting.

If I install a CPU board (502) I don't even get the random display.

Interestingly it also won't get a display if I substitute anything other than a 74LS04 for U6J. A 7404 or 74F04 will prevent any display.

I'm guessing that something in the bus interface or clock divider is the problem, but am not sure where to start looking. All IC's are socketed and I've tested each IC prior to install in a small Chinese IC tester. Can't test the 74123's and 8T26's however.

Any suggestions are welcome!

- Gary
dave
Site Admin
Posts: 710
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Re: Crystal for 540B Display board

Post by dave »

Hi Gary,

I think I'd first start out by looking at Jeff's thread. There are some layout errors (missing traces, etc.) on the original 540B are there on the reproduction, so your blue wire fixes will be faithful to the original :-) I plan to fix these in the Gerbers at some point, but haven't done so yet.

Once you have applied those fixes, you may want to revisit the U6H issue, even if the board seems to be working. It's better not to leave "interesting" behavior unexplored. . .

Cheers,

Dave
gekaufman
Posts: 72
Joined: Mon Nov 23, 2015 12:57 pm

Re: Crystal for 540B Display board

Post by gekaufman »

Sounds like the fix to my problem. I'll report back.

I can also type up the jumpers needed to run the board in B&W mode. At least for me there is little interest in the color features, and I saved a few IC's.

I now have the replacement keyboard built and tested - and purchased a stripped out C4P case. Well on the way to a home built C4P :)

- Gary
gekaufman
Posts: 72
Joined: Mon Nov 23, 2015 12:57 pm

Re: Crystal for 540B Display board

Post by gekaufman »

In follow up, I had the jumpers correct (I copied from my original C2-4P display board) but missed the trace to cut. The U6H issue is now cured and plugging in a CPU board no longer locks up the board.

Now to troubleshoot the 502 board...

- Gary
pbirkel
Posts: 32
Joined: Mon Feb 27, 2017 8:06 am

Re: Crystal for 540B Display board

Post by pbirkel »

Gary: You earlier stated "I can also type up the jumpers needed to run the board in B&W mode." Please do so :-}? I'm working on my BOM and trying to determine what I need, or not.

Also, what are you using at the display-end; a converter or some sort or else ...

Finally, did you get the B&W mode working yet?

Thank you,
paul
gekaufman
Posts: 72
Joined: Mon Nov 23, 2015 12:57 pm

Re: Crystal for 540B Display board

Post by gekaufman »

Paul -

I'll try and dig it out and document if I can find some time!

I'm just using an old 9" composite security type monitor that I purchased surplus.

- Gary
pbirkel
Posts: 32
Joined: Mon Feb 27, 2017 8:06 am

Re: Crystal for 540B Display board

Post by pbirkel »

Thanks Gary. When you find the time your notes will not go unappreciated :-}.
Mark
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Joined: Tue Sep 16, 2008 6:04 am
Location: Madison, WI
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Re: Crystal for 540B Display board

Post by Mark »

As a test I swapped in a 12.09MHz crystal for an original OSI 12.083915MHZ crystal on an original OSI 540B with no appreciable difference in display. The slight differences in the pictures are due to my ancient digital camera trying to adjust color balance and autofocus. The display is a "modern" digital LCD TV which works fine with OSI color output on or off. Generally I use a 9" B&W security monitor for the display.

OSI-12.083915xtal.JPG
OSI-12.083915xtal.JPG (1.35 MiB) Viewed 5167 times
OSI-12.09xtal.JPG
OSI-12.09xtal.JPG (1.35 MiB) Viewed 5167 times
In case you want the BASIC program that made this display:
fillcolor.bas
OSI BASIC program
540B Color display test
(233 Bytes) Downloaded 381 times
Cheers,
-Mark
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