EC motor with modbus, inject DC

[MartinFO]

Hi
I have multiple EC fan motors that operate at 400V AC. The IGBT is fried and powering the EC via 400V just trips the relay. The fan have Modbus communication which can be used to readout a log file, which is important to find the root cause. I suspect that I could somehow power the low voltage logic of the PCB by "injecting power" somewhere and establish modbus communication. It is difficult, since the main PCB is covered with potting compound.
I have tried the obvious of applying power to the 24V output on the small add-on PCB. It works somewhat, since the status LED (on the small PCB) starts flashing with the error code "internal communication error". I would assume there is a diode somewhere that prevent the main board being powered. I have also tried powering where the small PCB is connected to the main PCB, but I get the same result as via the 24V output. Is there is better place to try? The board is already fried, so it is not a problem to break components to gain access.

[croaker]

Try to dismantle the board and remove the compound by heating it (with a hair dryer or in an oven).

[MartinFO]

Good suggestion, but I have tried that. The compound is impossible to remove. I have tried alot of different methods.
I have thinking about breaking one of the capacitors off and power the exposed leads?

[langwadt]

what on the JST connector?

[croaker]

You can try to remove the compound chemically. I use this paint remover, it also removes epoxy compounds.

[Jeroen3]

Polyurethane potting is not easily chemically removable. You'd need dangerous solvents like tetrahydrofuran or dichloromethane (the stuff @croaker mentioned).
Careful, only use this with respirator or in fume hood with gloves.

[MartinFO]

I could try to remove the compound with more aggressive solvents, like suggested, with proper PPE. However, I need to read-out the data from multiple of these EC fans in a place where is not an option to remove the compound. It could serve as a method to trace to PCB to find a good place to apply power with the compound in place.
About the JST, it is a connection to three hall sensors (for position).   

[langwadt]

About the JST, it is a connection to three hall sensors (for position).

then there's probbly also a 3-5V supply in that

[MartinFO]

JST has a 3.3V supply, however it was not enough to power the Modbus. I expect I need to supply power close to the conversion from 400V AC to something DC. Since 24V is the highest DC voltage I would assume the low voltage part of the board has a 24V "supply". 

[Jeroen3]

It appears you also have a 10-pin connector probably for firmware updates.
Can you pull out a capacitor after the swtichmode transformer and power it there?
Can you power it through the encoder/hall sensor connector?

[aeg]

Is it possible to destructively remove the IGBT and then power the motor normally?

[MartinFO]

I have removed the large capacitor, within the yellow circle on the image and applied min. 70V DC (9V batteries in series). The status led starts flashing, though not as bright is under normal operation. The fan has 24V and 10V output, but I do not get any reading on the 24V and the 10V fluctuates (with the led) between 3V and 5V. The 5V increase with the voltage applied to the capacitor. At 170V I got smoke, but still not 10V at the output. I have more, so not a huge problem.