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VFD motor drive PSU. Questions on components

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fastline

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We had a drive fail the other day. Drive did NOT kill any high power components, the entire logic board lost power so no display, no error codes, no nothing, just dead. All fuses checked good, DC buss is solid, etc.

If anyone here is familiar with VFDs, this has the high power components at the bottom to include the main DC buss rectifiers, power caps, transistors, and brake transistor. The next board in the stack is the PSU board which I believe creates the low level logic and switching power and delivers that to the logic board for output and to operate the transistors.

Issue is all is close to dead. Seems all the 24V terminals and one 24V fan are all getting 5.5V, not 24V. Other voltage terminals are totally dead. This seems a pretty simple board because the main DC buss power is scraped off for the PSU at 330VDC. I tested all electrolytics with ESR meter and all are well under 1ohm. No obvious shorts in transistors, nothing visually concerning, etc.

I tested all diodes in circuit for obvious concerns and all show some level of voltage drop. resistors all show resistance and are not shorted or open. One area of concern for me is there is a multi tap PCB transformer that has "an odor". Maybe not roasty toasty but certainly an odor and it will emit odor when circuit is lit up. I tested all pins on it and all conduct on both sides except one which seems to go to earth ground and it does not conduct to anything.

I have a repair manual for the drive but no PCB layout. I am going to work on that. I am curious where you guys might start with something like this? Start pulling known to fail components for independent bench testing? In circuit tests? Obviously the level of power in the drive makes this VERY dangerous to do lots of bench testing at full voltage. I don't have a bench PSU to make 330VDC either.

Having tested all the caps in the circuit with the ESR meter, I am not yet sure I trust the thing but it has spotted a couple things on other projects that proved valuable. From that work, I determined that the ESR value will likely be MUCH higher than a typical cap and will be obvious. I pretty much ignore anything under 1ohm as good depending on capacity.

Because of the immediate failure, no smoke, no burns, no blown fuses, etc, I cannot help but think this is VERY repairable.
 
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