N_Cook wrote:
Those white ceramic cased ones about 3 watt in the form of a rectangular
block 15x5x12mm . No cracks in the ceramic but white powder laying around
some of them.
eg 6 power transistors in "parallel", one has a lot of powder around it on
the pcb, one has some around it and the other 4 are clean. No fan air
directed in that area to blow anything away. Both have the powder deposited
not along the long sides but at the end away from any possible heat radiated
from the transistor/ heat sink, perhaps just coincidence. Can anything be
inferred from these deposits/ associated transistor?

I've seen failed o/c ones of this type with a crack in the casing, does it
mean the powdery ones have been overheated and will fail imminently or an
indicator that the associated transistor is taking too much of its share of
the current?



The cased ones are just essentially wire wound resistors that are potted
in a ceramic material. From age and heat, the potting compound has
broken down and is turning to dust. This in itself does not mean
the resistor is bad. measure it. if it measures in the ballpark
its still usable. If you feel bad about it, replace it.

Now, i have seen them in audio power amps that look burnt and have the
lettering burnt off or disfigured. Usually when this happens, the
outputs are shorted and the emitter resistors are open and shot.


bob