This is a beautiful instrument that suffered from intermittent lockups. There are a number of possible causes for this, but it's easiest to start with mechanical problems... by that I mean loose molex connections between boards, cracked solder around voltage regulators and filter caps, anything around heat sinks and all solder around electromechanical devices like the power switch, etc. Cracks in circuit board are also prime suspects, as are socketed IC's where pins can work loose over time.
Start by using the highly advanced
technique of poking around with a stick, (a non conductive poking
device) while the unit is turned on, to see if you can induce the lock
up. Poke around all those areas I mentioned. That's what worked here...
see the highlighted portion of the photo... some IC's needed to be
reseated and re tensioned. I found them by poking on them with the
eraser of a pencil.
If that hadn't worked, I would have
removed boards, starting with the power board, and then the
microprocessor boards, and looked for cracked solder, especially around
the heavy caps and the heat-sunk components.
visual inspection completely failed, I would have also checked the
voltage rails with a scope to make sure the device was getting clean
Sometimes thermal issues can be induced by
blowing some hot air (not too hot!) around various areas of circuit
boards. Seriously, don't use too much heat, but often you can find a
faulty component or other thermal issue by blowing some hot air on
various areas to induce a lockup.
If you have a
politician handy, you can get some hot air that way... if not you could
use a hair dryer or a hot air soldering station with the heat turned