Yamaha DX7 power issues

This is an original mark one DX7 from about 1984. It began to spontaneously reboot itself, and at times not boot past the initial screen where the LEDS show "88". The problem was traced to a connector on the PSU where there was a cracked solder joint from thermal stress. This pin (pin 8 on header C1) connects a little circuit on the PSU (a zener diode and two transistors, hanging off the 5 volt line) with a similar circuit on the main board (marked on the schematic as "Initial Clear") that controls the CPU reset. A 10uF electrolytic cap charges up from zero to 5 volts over about 2 seconds, which then allows the CPU to operate normally - if this doesn't happen, the DX7 won't boot. The cap tested fine. The headers are very close to the heatsinks for the voltage regulators, and the board was dark from heat effects. Many of the solder joints on this board were cracked. Surprisingly, none of the large electrolytic capacitors on the PSU board were out of spec, as checked with an ESR meter. This unit has been in almost daily use since purchased new in the eighties!

A few months earlier, I did have to replace an X2 mains capacitor on the mains input board, that bypasses the power switch, which had failed as a short. Symptom: power switch had no effect, the unit was always on if the mains was connected.

One thing I have noticed with this recurring problem of cracked solder joints in old units - if the problem is more or less frequent when the unit is in a different orientation to usual, be suspicious of a cracked joint. In the case of this DX, it would boot more often lying flat, but if propped at an angle, it wouldn't boot at all. The effect of gravity on the surrounding hardware can open or close the faulty joint.