"N_Cook" wrote in message
...
bob urz wrote in message
...
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


Unfortunately my m-ohm-meter is 10V pk-pk. Using Fluke DVM and "time
division ratioing" of last digit I cannot see any ohms difference between
them, so all about 0.3R +/-0.05R presumably.
Of course if they were vitreous Re there would be no tell-tale dust and
also
the other 6 on the other channel that are inverted so dust would drop into
case
I last saw this amp a year ago and placed some 4mm diameter irreversible
thermochromic labels on the output transistors and the 2 associated with
the
dusty Re are a bluer shade of cyan , so having been hotter. This time a
preamp problem so
placed new dots on the devices, tightened all bolts and will leave at
that.
A band that thinks its a planet-bound Disaster Area
and has survived a year of intensive use with no power amp problem so if
it
ain't broke, don't poke



if it
ain't broke, don't poke

I like it!

Mark Z.