On 30/07/2013 15:12, dave wrote:
On 07/30/2013 07:00 AM, N_Cook wrote:
Both 6.3amp fuses blown, 2 diodes of the bridge rectifier s/c. Internal
failure of this bridge, dodgey looking splodge logo, presumably Fronter
logo and 8amp rating, marked
AC BP8-04
or something else to change as well?
Main caps look ok and ESR (for what its worth in such cases) ok
The 2 failed diodes are
-ve to AC
+ve to other AC
Suspiciously 2 solder points look like crazed PbF solder but all is SnPb,
melted and reformed? the 2 points are at the -ve terminal and the AC
terminal of the other failed diode. Output devices DVM-D cold test as
expected. No heated traces or even obvious gaseous eruptions from the
bridge or anything out of the normal. No reported preceeding noises
prior to failure.

DVM-D cold test? Amps can become radio transmitters and fry themselves
without any audible indication, other than maybe some "rizzing" on the
violins. Are the output devices shorted?

No , output devices, cold test , seem fine.
Looking at the Fronter datasheet, the bridge has to be derated to only 3
amps at 40 deg C, which seems surprisingly low temp. This amp was all
day sitting outdoors in the cloudless sun and failed after 1 hour of use
in a set. So 40 deg C was quite likely exceeded, so probably the reason
for failure.
As constricted in space , without going off board, I see a heatsink
being bolted to the replacement same rating bridge, or wiring off board
and bolting to the chassis