Hello everyone!

I have a problem with an old DC motor drive... The drive takes a +/-10V signal and it rotates the motor shaft in one direction if it is + and the other if it is - . And it controls the speed depending on voltage...
Anyway I think i stressed the drive too much and now a transistor is shorted.
When looking at the schematic of the drive I got confused...
I know that the thyristors are used for direction. And transistors are used for PWM modulation (since the drive is powered by an DC current). And the thyristors have to shut off...
But why are there transistors on the positive bus and transistor on the negative bus.
In my case T14 failed and now the drive goes into an alarm (i think overcurrent). If i reset the drive the motor vibrates volently and it goes into the alarm again..
Can some one please explain me why are there transistors on the upper bus and on the lower? I can't get the transistors anymore (BUX22) or if i find them they are too expensive.
I'm looking into possibilities of swaping the NPNs with IGBTs. But i would have to adapt the controlling circuit.

 

First picture is the power stage, sencond is the transistor drive electronic
The third is the whole drive in a block diagram and some scope traces

 

Looks very similar to the old Westamp drives, I have used Motorola MJ11032 as replacements on these amps.
Max.

 

Thank you for your reply!
The transistor you specified has too low voltage... These are rated 250V. I already looked at similar transistors and MJ10023 is the closest one...
Fo you maybe know what are they used for? Why are they on the high side and the low side? And not only on the low side?

Thank you!