For others that may have one of these in for repair here's my experience;
Under normal playing conditions the output is dead. However, if the piezo knob is held down and the guitar powered up it will continue to play in piezo mode so long as the knob is held down. The preamp then behaves normally but as soon as the knob is released the guitar goes silent. Monitoring the voltages, It briefly operates normally for about 1 second or so and during this time the voltages read good at both ends of each respective wire from the PSU. During this time it plays and is loud and clear.
The voltages are;
However, all of the outputs except BATT collapse immediately after this and the guitar goes dead. I've monitored the red/white wire (OFF) and this remains at 0v throughout as seen with my DMM. However, I contacted Ernie Ball and they said the OFF line should remain low during normal operation but when the guitar is unplugged there's a 50ms pulse on this line to shut down the power supply. It's a complicated unit - the schematic shows 14 connections to the PSU.
I've discovered by scoping the OFF line that the PSU is getting a shut down signal from the processor board about 1 second after the guitar powers up. As a test, If I disconnect the red/white OFF connection the guitar powers up fine and stays that way. However, whilst the rotary controls operate as expected and the guitar can be switched between piezo/mag+piezo/mag, the magnetics are stuck in the neck position and the pickup selector is inoperative. I'm thinking that the processor is detecting an abnormal condition with the switching and is shutting off the power because of this. However, the replies from EB dried up so I couldn't get further with the guitar. There's a set of programming pads on the processor board but this is only used during manufacture. The processor board looks like it controls power management, self-test and switching functions. The pickup selector doesn't directly select the pickups but its position causes the processor board to switch.
The components in this guitar are:
1 A power supply that lives behind the battery box. Not accessible for maintenance due to being tethered by the short connection leads.
2. The piezo preamp board
3. Piezo select mono/stereo switching board
4. Main processor board.
In the end I couldn't progress with this and it's gone back to the customer unrepaired. There's insufficient technical information and parts are expensive. According to EB/MM "Our engineers have designed a more robust Majesty preamp that we are using in production this year. The only difference is that the mono/stereo mode switch is now a button accessible through the back control cavity instead of pushing and holding the piezo volume knob down for 10-15 seconds. The new preamp kit is (part #M061000) available for order through the channels of our distributor."
That's just the preamp and back panel for $380 fitted price from the distributor. To give some idea of the complexity of these guitars, they say; " For the 2019 Majesty, our engineers drilled into every aspect of the model’s electronics, trimming the fat while adding elements we knew would make performance easier for modern players. We’ve eliminated over 50 point-to-point wiring connections....... Players can now change between mono/stereo using a reliable mechanical switch – while we’ve even replaced the high-speed digital controllers with faster, more efficient logic circuits."
I hate admitting defeat.