> Also our above mentioned power supply which operates our 2
> Meter and 440 Repeaters and a low power link started humming
> yesterday. A trip to the tower showed that the two large
> wires coming from the Pass Transistors to the post on top of
> the regulator board and into the 1000 Uf Electrolytic got so
> hot it melted the insulation an inch back on the wires,
> burned an area the size of a quarter on the fiter side of the
> regulator board, turned the terminal black on the Cap. and
> cracked the plastic on the cap. It never blew the fuse and a
> check of the voltage showed it regulating under load and
> hardly a trace of AC on the 13 volt output. The MOV or eight
> amp AC fuse never blew. All the equipment hooked to the
> supply took off and worked well on another supply. Anyone
> have a guess as to what caused this obvious surge ontop of the cap?
> I am going to replace the Cap. and one resistor on the
> regular board which is discolored and hope for the best. Any
> advise appreciated.
>
> Thanks in advance JIM KA2AJH Wellsville, N.Y.
I've seen this happen a number of times to RM-50's and RS-50's, most
recently to an RS-50M that's one of my "bench" supplies. That connection
(where the high-current wires connect to the top of the filter cap with the
PC board sandwiched inbetween) leaves something to be desired. Eventually
it becomes a point of high resistance, either due to the screws/lockwasher
no longer being tight due to vibration or through thermal cycling, or the
copper foil oxidizes a bit, or similar causes. Once the resistance goes up
even a little, the heat caused by I2R at that point only worsens the
problem, and ultimately it becomes a thermal runaway kind of a situation,
yielding the results that you saw.
Bottom line - there probably wasn't any surge that set this off, it was a
function of design and age.
--- Jeff WN3A
