Our minimill is suffering from bus undervoltage errors unless it has been on power for a long time, was an hour but its getting worse, yesterday it took 6 hours.
Its a 2007 machine with a 20HP drive.
I have had the drive in bits and found some thermal damage to the driver board on the area that the 4 wire wound 20k resistors for the LED are located, it has been hot enough to discolour the board and solder around them.
I found in another post a helpfull circuit diagram for another similar hass vfd with very usefull info (hass closed loop vector drive .pdf) , and am begining to suspect that the isolation amplifier (HCPL7800A) located besides the droppers has been cooked by the dropper resistors over heating, this device has according to the HP data sheet a max operating temperature of 80c.
The iso amp is an opto coupled device and as such is temperature limited by the leds which if heated excessively (like about 90c) will permanently loose emmision and the gain/linearity of the device will be severely compromised. I think this is a likely candidate for the fault.
The machine has been left on overnight and is up and running so it has been temporarily put back into service.
I shall not be touching it untill the current work tasks are finished.
It is my intention to have a look at reverse engineering the high voltage detection circuitry to get a better grip of that circuit segments operation.
My plan is to then replace any components in that part that may have been cooked beyond sensible limits, and to shift the dropper resistors off the PCB to hopefully prevent a re-occurence of the failure.
Does any body have any sugestions or comments about this?
regards
malcolm