Tag Archives: fusible resistor

Clothes dryer repair: AKO 742336 control unit type EDR0692XAX

Repairs of household appliances aren’t generally my field of expertise, but in some cases, it can be a very rewarding activity. Some particular models of clothes dryers (“AEG”, “Whirlpool”) common in Europe use a control board, and this is not designed up to standard. The design flaw, it renders the machine inoperative in the very sense, you can’t switch it on, and it doesn’t show anything on the display. Typically, this occurs after roughly 3 years of service….

This is the control unit:
dryer control

dryer control board

dryer control board detail

edr0692xax board

After a quick inspection, it is fully clear that the defect rests with the control board, model EDR0692XAX. Easy fix, just get a spare! Well, easy, but outrageously expensive.

ako 742336 edr0692xax

The defect, it is related to a DC switch mode regulator converting mains voltage down to some useful voltage to run the controller. This is a step-down converter, NOT mains insulated!

lmk304 application

Two dead parts were found, the LNK304 switch mode converter circuit, and a 47 Ohms resistor.

dryer control lnk304gn 47r resistor

The LNK304GN is easily found, and replaced. The resistor: WATCH OUT! This is a fusible resistor, NEVER replace with just a common resistor – it will potentially set your house on fire, when the next switch mode failure happens.
The 47 Ohm resistor, it’s a 3 Watt resistor, and the ULW3 series, available from all major suppliers, should be a good replacement.

ulw3 fusible resistor

This is the failure charcteristics of the ULW3 resistors – it will go open circuit if overloaded, without release of any flames or particles. It a hybrid, partly fuse, partly resistor.

ulw3 fusing characteristics

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.