Interesting explosion inside 2018-RM Laser power supply:

The UV Laser in our mass-spectrometer department had a strange failu

blown supply fuses, and a huge splotch of golden mirror coating on the
PCB connected to all the large power components. There was a direct
short across the main power line on the DC side. The point of contact
was explosively vaporised, coating everything around it with metallic
copper.

The failure apparently came from a MOV device which had been crushed
between two circuit boards, where a big screw with DC main power had
pushed through the epoxy coating on the MOV. This placed a short
on the output of the main 3-phase bridge rectifier, which blew out one
of its diodes before the 50-amp cartridge fuses blew.

I found that Spectra Physics sells a repair kit associated with such
failures: the bridge module plus cables and shorter screws. Also, the

50-amp cartridge fuses can be had from Mouser. (and if Spectra Physics

should discontinue their kit, the three-phase bridge is similar to a
known part available elsewhere):

Bridge Rectifier Upgrade Kit, 4801-1108UPG, Spectra Physicsbr
Bridge Rectifier, 3-phase, EH100 Microsemi.com
http://www.microsemi.com/datasheets/COE-69.PDF
50 amp cartridge fuse 504-NON-50 mouser.com

After replacing the bridge, the laser still didn't work: it ran at
full power (50 amps,) and the current-adjust pot on the remote console
box had no effect. Occasionally it went into overcurrent shutdown.
I traced this to a shorted zener. Besides killing the bridge module,
the transient had shorted out CR-23 on the logic/driver board, a 1N4735

6.2V zener associated with the transistor driving the base pin on one
of the big darlingtons in series with the main 50amps.

One last note. I had one of the big darlingtons removed from the
water-cooling heat sink and noticed that the heat sink has large holes
allowing direct water contact with the bottom plates of the transistor
bricks. If you should try removing one of the bricks while the cooling

pump is running, you might get a big wet suprise!



((((((((((((((((((((((( ( ( (o) ) ) )))))))))))))))))))))))
William J. Beaty Research Engineer
UW Chem Dept, Bagley Hall RM74
Box 351700, Seattle, WA 98195-1700
ph206-543-6195 http//staff.washington.edu/wbeaty/