I have a MB33 (which is the same as the Freebass I think).
Better use DC adapter, since there is only a single diode as polarity protection and NO rectifier.
Not good to use AC power, the whole circuit oscillates with 50Hz since only one halfwave is rectified thru the diode.
Could possibly kill the polarised electrolytic capacitors. I don't know the microcontroller (8051 derivate) exactly
but I think it is a bit more sensitive for overvoltage and short circuits than a PIC for example.
Maybe that's the reason why the autotune could not get a stable tune (if the ground potential oscillates)
Some ideas:
- the 2 voltage regulators mounted on cooling plate. (7812 and 7805)
measure the voltage on the right pin if you look onto it (7812=12V, 7805=5V)
The MB33 needs 5Volts for logic (microcontroller,EPROM...) and 12Volt for the analog part (filter etc).
- that big capacitor next to the power connector.
The EPROM is UV sensitive, content will be erased if radiated with strong UV light. Better cover that
glas window with some tape. But while the case is not opened no UV light will come inside.
I don't think the content of your EPROM is erased or altered.
Update:
- in the MB33 a 100nF cap is soldered across the autotune button
- the input power can go up to 35V, the voltage regulators hold that - but get warm/hot
- optocuppler CNY17 (6 pin IC next to the MIDI connector) can be demaged, holding the
serial data stream (MIDI data) to the microcontroller on a high level
which results in a midi command 0xff which is the reset commando (System Reset (Sys Realtime))