The crystal was Ok. Measured 2.456 MHz at pin 6 Z80
(divided by 2 by IC A116). CPU reset circuit is OK
A Christmas tree ornament sounds like a good idea.

Doug

1b. Re: OB-Xa Diagnostic EPROM needed
Posted by: "wesleyknick" wesknick@... wesleyknick
Date: Mon Dec 3, 2012 4:43 pm ((PST))

Doug, I've been following the email traffic on this issue rather
sporadically however now that you've described in detail again I can
certainly say I have a funny feeling the crystal on the upper control
board is bad (next to the memory protect switch). If the quartz crystal
oscillator next to the processor IC chip is bad, the processor will
process nothing as it sets the processor speed which if I recall
correctly is 4.9152 MHz on the OB-Xa. The same problem can also arise
if it doesn't set the correct processor speed either. I own two OB-Xa's
at the moment and one of the two exhibited exactly the same symptoms you
are describing - some of the LEDs will illuminate when the unit is
powered on but will not respond to the buttons, and the modulation panel
will still seem to function normally. Power on, off and back on again
will sometimes change the configuration of LEDs that light up but the
unit will still seem frozen and not respond to the buttons. I had the
very almost identical problem a didn't realize the crystal was bad unit
I removed the upper and lower control boards and heard something
rattling next to the memory protect switch. The quartz crystal inside
the housing had physically broken off an was rattling around. Sounded
like a miniature cow bell. Swapped it out and the OB-Xa's worked great
ever since. Took me a while to figure the crystal was the problem and I
got so frustrated I wanted to just give it up and hang it from my
Christmas tree as it was about the only thing I could think of to do
with the pretty LEDs that would light up on the top it and do nothing
else. Try changing the crystal and see if that does anything... I
recently repaired a Matrix 6 with this similar issue.