Prophet 10

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.

If the visual inspection completely failed, I would have also checked the voltage rails with a scope to make sure the device was getting clean power.

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 down.

The integrated circuits in the highlighted area had worked loose. They needed to be carefully removed, the pins were cleaned with solvent and re tensioned. Notice that they are right above the Power Supply area... they were constantly being heated up. This surely contributed to their coming loose.