A Variable Frequency Drive (VFD) was received for service with a complaint of dim display and inability to control motor speed. The customer reported that although the unit powered on, the display was barely visible and the speed adjustment function was not responding properly.
First inspection of the machine, I found corroded PCB's covered in dust.
Very important. Before start the repair, you must make sure that the device is repairable. time is money. don't invest your time on irreparable things.
How to test repair ability,
Check if it has a microcontroller,FPGA or any programmable device. Download the datasheet. diode test on all pins. Check supply rail impedance. Power up the IC, 1V below the rated supply voltage from bench power supply and check drawn current. Check the ic temperature.
If diode test fail on some pins. isolate them from other connections and test again. if you have the datasheet, check for internal connections. if diode test fail and you found internal short circuit to ground. you have to replace the microcontroller. Find the firmware and re-program a new chip will be impossible because most manufacturers do not publish their firmware. therefore repair will be impossible.
Supply rail must have at least 20ohm. if supply rail impedance is too low, check for parallel resistances. cut PCB + path to ic and test again. if ic continuously shows low impedance, it may have shorted to the ground. therefore repair may be impossible.
If ic gets heat after power up and heat rises continuously, replace is the option. Replacing is impossible if you don't have resources.
if ic draws less current and heat is normal, power it up at the lowest rated voltage as described in the datasheet. measure crystal oscillation using an oscilloscope.
If crystal runs. good point. move into other parts. MOSFET,Op-Amp, gate drivers,etc... because these are replaceable. you have some certainty that replacing such part will solve the problem.
I was able to repair the VFD. problem was the corrosion of PCB motherboard caused by some liquid or something. Some SMD resistors had dry joints with the PCB. i have cleaned the PCB and replaced all corroded parts. finally it came back to work.
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