Crystal for 540B Display board

[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]

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]

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]

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]

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]

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]

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]

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]

Thanks Gary. When you find the time your notes will not go unappreciated :-}.

[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 (1.35 MiB) Viewed 6727 times

OSI-12.09xtal.JPG (1.35 MiB) Viewed 6727 times

In case you want the BASIC program that made this display:

fillcolor.bas

OSI BASIC program
540B Color display test

(233 Bytes) Downloaded 573 times

Cheers,
-Mark