Call me a dummy cause I'm certainly stumped... Sorry to hear my suggestion didn't pinpoint the issue. Anything can be fixed just some problems take longer to solve than others. If all else fails, try swaptronics. Get a good OB-Xa to use as a test bed and swap suspected faulty components into it, a technique that works at the component level as well as the assembly (PCB) level. When the normally functional instrument exhibits the same problem, you'll know you've isolated the component. Sometimes the component you think is the problem isn't really but the result of a problem further upstream (ie., the power supply). I've made some strange discoveries doing swaptronics and on more than one occassion just found it to be something as simple as a bad connector. Just trying to offer some advice...

--- In oberheim@yahoogroups.com , Martin Ator <cyllall@...> wrote:
>
> What is the function of the LM311 in the circuit
> (If I had a teleporter I could pop round with my Boardmaster 4000)
>
>
>
>
> ________________________________
> From: Doug <dmeek@...>
> To: oberheim@yahoogroups.com
> Sent: Tuesday, 4 December 2012, 19:11
> Subject: [oberheim] Re: OB-Xa Diagnostic EPROM needed
>
>
> Â  
> 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.
>