I was recently helping a friend troubleshoot a Tek PS280 that he had bought on e-bay. In the process I found a fault which is design related and may be more common.
The symptoms were that the adjustable positive supply had no output, and the negative supply had regulation issues. Upon opening the supply, I found one of fuses (F102) from the transformer secondary was blown. As this feeds the output pass transistors directly, I figured there were many possibilities of root cause and troubleshooting could take some time. But a physical examination quickly uncovered the cause.
The main regulator board was loose near the top of the instruments. There were also the broken pieces of the thin nylon standoffs used to support it at the bottom of the instrument case. I could see with the standoffs broken, it was possible for the trace side of the circuit board to touch the upper support bracket, shorting something to ground. Closer inspection of the bracket showed a slight burn mark, with a corresponding burn mark on the circuit board.
Examining the broken brackets shows they are inadequate to support this board. The ones used are the thin ¡°snap in¡± type which do not use separate screws. The bottom of the board is retained in a large slotted channel that it slides into, which serves the purpose. The board contains several relays, large capacitors and other heavy components. When exposed to shock such normal shipping handling, the thin standoffs which the board is mounted with can and do break. As the unit has no other obvious dents or damage, I would suspect that the drop was not excessive. I would also guess that the vendor who Tek OEMed this instrument from in Taiwan never did a standard shipping drop test, or they would have found this design flaw.
I replaced the standoffs with more robust 1/4¡± diameter, 3/8¡± long nylon standoffs which have a 4-40 tread to accept a screw. Nylon is necessary because some of the mounting holes pass through board traces. McMaster Carr part number 96110A560 is a perfect fit. The board and bracket have mounting for 4 of these spacers along the top, but it appears that only 3 of the thinner ones were actually installed in this unit.
I replaced the fuse and powered up the supply and found it totally functional. I did not check the circuit to see what had shorted, but in this case it looks like something before many semiconductors, as the 5 A fuse protected everything from additional damage.
If any readers have either the PS280 or PS283, I would suggest you removed the cover and check to see if the main regulator board in your unit is securely mounted. If not, I would recommend using the more robust standoffs to replace the inadequate ones in the original instrument.
- Steve
