HIGH VOLTAGE SHORT IN CARY 5000 SPECTROMETER


TWICE! Our UV/VIS/NIR spectrometer lost its -1KV high volt source, and the cause both times turned out to be an internal short on the PMT-control PCB.. Both times this fault was easily found with a DVM: an unexpected leakage path, well below 50Kohms, seen between the 1KV source and ground. The shorted HV trace and the groundplane are both internal layers in the 4-layer circuit board. Verify: when the 4-pin molex cable connecting the main PCB to the PMT-control PCB was removed, the -1KV at the main board reappeared again (actually measured as -980VDC when working correctly.)

The PCB with the actual problem is positioned vertically all the way to the right of the instrument. The -1KV switching supply is on the main board covering the left rear of the instrument. The HV cable is the cable with the 4-pin Molex connector, with one pin having a heavy red HV conductor.

The PCB design unwisely routes a 1000vdc power supply trace across an internal ground plane. Given enough years, the HV slowly eats through the paper-thin epoxy-glass layer and arcs to ground. Perhaps the same fault will occur with many other units?

The cure is to route the small 1KV supply trace across the PCB using a separate wire. But first you must use a Dremel tool to slice away the carbonized paths connected between the groundplane and the original 1KV trace that's found on an internal PCB layer. The resistance to ground for the HV signal should be around 3.4Meg, not 12K ohms! We had one carbonized short appear at the spot where that trace led away from the 4-pin Molex. Later another carbon path arose at the via hole near the end of the chain of white H11D1 optoisolators. While looking for the internal short I also drilled out the connection to the hot end of the orange disk capacitor, so used white RTV silicone so that cap would still be anchored mechanically by more than just its one remaining lead.

SEE: http://staff.washington.edu/wbeaty/cary1.jpg

The white arrow in the JPG above shows the added wiring. Another wire appears on the circuit side of the PCB between the orange capacitor and the 4-pin molex seen just above the white arrow.

The earlier failure also fried a couple of SMT power resistors in the switching supply on the instrument main PCB. I replaced these with 3watt through-hole versions.