OBERHEIM SYNTH

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.