Make mine melamine

Brad Thompson, Contributing Technical Editor brad@tmworld.com -- Test & Measurement World, 12/1/2008 2:00:00 AM

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Long-time readers of this column may recall that I serve as a volunteer at WinCycle.org, a nonprofit computer-recycling center. When possible, we refurbish PCs for reuse by other nonprofit organizations and to provide operating revenue. One recent arrival from a local business contained a shipment of several hundred HP Compaq d530SFF desktop PCs, some of which exhibit an interesting and spectacular failure mode.

Applying AC power by plugging in the power cord produces a muted “pop” and a flash of light, followed by the release of a small puff of smoke—all originating from the PC’s power-supply subassembly. Removing the power supply’s cover reveals several severely damaged components. When the fault occurs, a soldered-in AC line fuse doesn’t always open.

In an effort to learn more about the failure mode and label codes that might reveal affected manufacturing lots, WinCycle’s CTO contacted Compaq’s (now Hewlett-Packard’s) online technical support service. Repeated and escalated message exchanges garnered no information, except for denial that the problem exists.

The power supply’s construction comprises discrete through-hole components mounted on single-sided printed-circuit boards. A sheet-metal enclosure protects the assembly, and a small fan provides cooling (and smoke) dispersal. The power supply’s label bears several safety-agency approval and recognition symbols.

The power supply’s Chinese manufacturer applied generous amounts of an eggshell-colored and porous sealant to stabilize components mounted on the circuit board. The sealant darkens with exposure to heat and time. It also becomes conductive, creating multiple sneak paths among components. Inserting an ohmmeter’s prods in random globs of sealant shows resistances in the 1-to-10-MΩ range. That’s enough to inject small amounts of AC into the primary-side switching components and cause uncontrolled circuit activity.

We’ve reverse-engineered portions of the power-supply circuitry and determined that Revision B (or higher) power supplies contain a white sealant that’s presumably nonconductive. To date, these power supplies haven’t exhibited the failure mode.

What’s in the conductive sealant? Given recent news reports about contaminated Chinese medicine and milk, my guess is melamine. What’s yours?

Kaboom!

Brute force sometimes trumps subtle measurements. Without access to the manufacturer’s original test criteria, we can’t plan a test program for the Compaq d530SFF’s model PDP124P power supplies. Instead, we’re going to blow them up.

We’ll assemble a simple timer and relay circuit to deliver AC to a group of supplies for a hundred or so on-off cycles. Power supplies that fail will get scrapped, and survivors will get disassembled and inspected for their (presumably) nonconductive sealant. We’ll record the survivors’ production-lot numbers and submit them to extended life tests.

Salvaging an unexploded power supply requires disassembly, removal of the tenacious conductive sealant, and replacement of potentially overstressed components. We don’t have enough volunteers to perform repairs, nor does component cost and demand for these older PCs render repairs economical.

Would tests ferret out defective supplies? Probably not—the supply’s input-filter components already draw leakage current from the AC line. Also, the destructive failure mode may manifest itself only at full line voltage. Will an exploding power supply cause a fire hazard? Probably not—sheet metal encloses the explodable components. Besides, a technician will oversee the festivities. For good luck, we’ll keep a fire extinguisher handy.

No one can test “quality” into a marginal product. Tests can weed out the immediate failures, and short of a magic wand, that’s the best we can hope for.

This YouTube video clip from an unknown source shows a Compaq d530’s power-supply failure:

www.youtube.com/watch?v=nHPf6jHdEFg

This Web site offers instructions for removing, repairing, and replacing Compaq’s PDP124P power supply:

webdevsys.com

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Interesting article. When I was working in our Customer Service dept, we would frequently come across a similiar issue with the CRT modules from one of our spectrum analyzer product lines. Because the material is porous it would absorb any moisture in the air. The failure results were the same. Generally there wasn't any damage to the PCB material itself and we would replace the bad components along with removing the rest of the compound before applying power again to the boards.

Mark Elzinga - 2008-22-12 13:17:00 EST

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