HP LaserJet III, 50 service error, fuser´s thermoprotector always getting blown

[Joachim Wunder]

I have a HP LaserJet III which immediately throws a 50 service error
after powering up. A first diagnosis showed a blown thermoprotector
within the fuser. Thermistor and bulb were still absolutely OK. After
replacing the fuser against a brand new one, the thermoprotector of
the new fuser got blown as well. Thermistor and bulb of this fuser are
still absolutely OK.

I suspect the AC power supply to be the culprit one. As I currently
don´t have any new AC power supply at hand, could anyone please point
me into the right direction of which component(s) of the AC power
supply PCBs may be defective and are to be exchanged? If it matters,
the AC power supply unit is a 220/240V one.

Thanks in advance for your help, Achim.

 

[ato_zee]

If it matters,
the AC power supply unit is a 220/240V one.

In the early LaserJets the fuser lamp is controlled
by a Triac opto-coupler solid state relay. These
can fail, so the lamp doesn't turn off, overheats,
and relies on the bi-metal protection cut out.