MAZAK QT 10N Spindle controller problem

Hi, I am kind of new at CNC (worked with it 15 years ago)
I bought a Mazak QT10N and the seller told me that sometimes it blows a transistor (QM150DY-H) in the spindle controller Mitsubishi FR-SE.
When I got it it already has a blown transistor so I ordered 2 sets of 6 new from Malaysia (ebay).

So I checked the spindle motor with a megger (isolation tester 500v) that could not see anything wrong.
Then I changed all 6 transistors to the new ones and powered it up, worked perfect for a few min whith some starts and stops then I got the same alarm (204) and the spindle controller has a blown transistor again.

Anybody having a good idea where to look for any problem?

The power supply at the top of the "hinge" has lots of output voltages.
There may be one or more missing. Most of these output voltages are used to provide the turn on voltage pulses for the large transistors, that you are having trouble with.
With missing, or low voltages, the power transistors may not be operated properly, leading to failures.

There are 2 fans in the back of the FR-SE. The power leads for them go to a terminal block that also powers the power supply (the 3 wires from the back of the P.S.). The fan module is held in with 2 screws.
There should be a screen panel that you can look in (on the end of the electrical enclosure) and see if the fans are running

Normally only one transistor is shorted E to C. Only the shorted transistor needs to be changed.
It may be possible that the snubber components, or base drive wires, have not been put in the proper places, or have been damaged.
There is a lot of wiring and parts lying around if all are changed at the same time.
This is very suspect if they have replaced many transistors in the past.

One needs to know how to use a multimeter to locate the bad transistor.
Mitsubishi has procedures for locating the bad transistor, and for checking the many power supply voltages.

Never use a megger on a motor that is still connected to the spindle controller.

Bill

Machine is now up and running
I replaced the 6 of the 12 DK434 that controls spindle acceleration (6 top most) in my FR-SE and put in 6 new of the large transistors (QM150DY-H) and now it works perfect