I bought a defective calibrator capable of +/- 1200V and 1000V 1kHz. Output current 10mA
The HV part was faulty, wrong HW transformer so I had to special make a 2x1270VAC 70VA transformer, not super cheap

To make it even more funny, the HV amplifier was also broken and potted in epoxi, grrrr
But luckily I got the service manual from a friendly forum member, so I did a clone of the HV amplifier.
In the end I could repair the original potted version, they did not pot secondary side of PCB, there were some almost invisible cracked solder joints.
Anyway, I ordered 100pcs transistors MJE3439 from Mouser rather cheap and build the clone, worked perfectly!
Output voltage is controlled with current to the optocoupler, on my prototype:
0mA= -1479V
1,13mA = -1000V
1,71mA = -500V
2,25mA = 0V
2,79mA = 500V
3,32mA = 1000V
4,05mA = 1573V
I have Gerber files if you want, or PCB.