HPAK (HP Agilent now Keysight) 3562A: power supply repair

Somehow, all 3562As in the Greater New York City Area seem to fail these days… This one arrived completely dead. Didn’t take a long time to find the issue – a defective A18 power supply assembly.

This power supply design can only be recommended to anyone interested in electronics. It is build with multiple controls, overcurrent, overvoltage protection, and build quality-robustness is not far from being able to power life support devices, or moon landing vehicles from from. Still, this one had failed.

A quick look at the board revealed two blown resistors. Framed red in this snipplet of the schematic.
3562a pwr supply schematic

However, these are the gate drive resistors – there must be a reason why they failed, and this reason was quickly found in the MOSFETs, Q400, Q401, HP part number 1855-0473. Looking around, no real equivalent found in the HP cross reference table. However, an IRF450 (500 V, 12 A, 150 W) appears suitable, and 4 pieces (2 spare, just in case) were easily sourced.

The resistors, R404 and R410, are of more concern. CMF60-64 type. These are 0.5 W resistor fuses (fusible resistors). In contrast to other resistors that can easily start a fire and develop a short when overloaded, these go open circuit, and are flameproof.
Unfortunately, a value of 3k9 seems impossible to source in any reasonable quantity – but 3k3 should work perfectly fine. Digikey offers the Vishay NFR25H series, also 0.5 Watts.

Spare parts
3562a pwr supply irf450 3k3 fusible

Note – why did HP use a fusible resistor? Why not replace with a regular resistor? Please, never even consider it, unless you it’s a client that doesn’t pay the bill… it’s a serious risk of fire, and it is not just a bodge but an unacceptable safety hazard.

That’s why… the red trace is 400 VDC at multiple Amps, and imagine the Q40x have a gate-drain short…
3562a pwr supply schematic detail

Didn’t take long to replace the IRF450s and the two resistors – and, to everyone’s full satisfaction, the 3562A powers up, no issue. Not sure what caused the defect in the first place – maybe some overvoltage in the power line? We might never find out.

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