These are my notes from attempting to revive it.
This would not have been possible without the Laser FAQ, the Holography Forum, the Holography Wiki, Rotorwave, Wikipedia and countless pages Google found.
Quantel lasers are wonderfully modular, and remarkably compatible between individual lasers and revisions. My laser currently has parts from 1982, 1986, and 2001.
A minimal laser will have a control unit (CU 401), a power supply (PU 420 or PU 430), a capacitor bank (CB 430), a cooling unit (CG 503), and then the laser head.
The control unit does all the interlocks (which it calls security), generates the fire signal, and can control up to 4 power supplies. There are individual enable switches on the PC board inside it for the power supplies. If the security light is illuminated, it means the interlocks are closed and the laser will fire.
The power supply generates the high voltage, and a 24 volt interlock/fire signal. The fire signal is a short pulse on the otherwise 24 VDC 24 volt line. Each power supply has a timing delay, it will fire its capacitor banks between 100 and 600 microseconds after the fire signal from the control unit.
A PU 430 power supply is larger than a PU 420, and will drive two capacitor banks. A PU 420 will drive one.
The capacitor bank takes the high voltage and 24 volts from its PU, contains the capacitor and pulse forming inductor, and generates the trigger pulse for the flash lamps. They are series-triggered. A CB 430 capacitor bank uses the lamp for the switch. A CB 430S has a simmer supply, and a very large SCR to switch the pulse. A capacitor bank has an interlock to make sure it is plugged into a lamp assembly, but it just disables the capacitor bank, and is not connected to the control unit's interlock. (So you can [more or less] safely unplug a lamp assembly and just let the cable loll.)
The connection between the capacitor bank and the lamp/rod assembly is a custom cable about 3/4 of an inch in diameter, with a custom connector on the lamp/rod end. The cable is not intended to be removable from the capacitor bank, it will have to be de-soldered. The connector is potted, and similarly unremovable.
The cooling unit is mostly a water to water heat exchanger. It contains about five gallons of deionized water in a tank with a coil through it for tap water. A thermostat will switch the tap water flow on and off as the temperature hits the high and low set points. There is a second temperature sensor which will open the CU's safety interlock if the laser overheats. There is a fill cap on the top, if you pull it a couple of inches out of the rack, and a drain plug on the right side. There are valves on the back which can be closed to disconnect the laser head without loosing the coolant. Finally there is a circulating pump, a filter pot with a deionizing filter, and a flow sensor.
A more complicated laser will have a single control unit, with multiple power supplies, each with one or two capacitor banks.