Hi there,
I own a Leadwell VC25 milling machine which has a similar DC supply for it's spindle. The servo drives each have their own supplies. It is a Meldas M50 system (1995).
Usually these things have a SCR (Silicone controlled rectifier) bridge to produce a variable DC bus. The SCR's are the big (often white) power modules mounted on the heat sink with the aluminum bars screwed directly on to them. You have to look at it more as a power regulator than a voltage regulator. The capacitor bank served as a buffer to even out peaks in power demand. When the load draws current out of the bank, it's voltage will drop. The circuit senses this and sends more current to the bank and load by opening up the SCR's at a wider angle. a certain power but you have to look at it more as a power regulation than actual voltage regulation. When the load brakes, it sends back current to the buffer and the voltage will rise. The circuit also senses this and will in it's turn "waste" power in a braking resistor or send it back on the net.
If a capacitor has failed, I would replace them all. It is kinda costly but it will make sure the supply will last you many more years. Also replace the smaller caps on the switch mode supply that supplies the control circuit. It is likely located on the board opposite of the power bank. It can even be a multi stage supply with a buck converter first to take the edge of the 800VDC folowed by the actual supply. No real need to change all capacitors on the board but concentrate on the ones that experience a higher frequency ripple.
Also be careful please! The capacitor buffer may hold many hundreds of joules of energy, enough to start several harts but to stop yours instantly. It is not the voltage that kills you, it is the potential energy
The buffer needs several minutes to bleed out. Check that the P-N terminals are below 30VDC before handeling. I use an external bleeding resistor of 1k 500W and when I am at approximately 0V, I short out the terminals. Why? Because the voltage in the buffer can ramp up again after you take away the bleeding resistor, especially in systems without internal bleeding resistors.
Nice fix by the way.