Ron(UK) wrote in message
...
N Cook wrote:
Blows mains fuses. Ashdown supplied replacement fuses to the owner,
better
than FOAD I suppose. Survives long enough for the meter lamp to come on
half
a second. Nothing found obviously suspect in ps or pa probing around
cold
and inspection. Not looked at prea yet. Will power up on variac tomorrow
with lower rated fuses and prea disconnected initially. Bad internal AC
supply fuse holder contacts , arcing burn marks on the fuse barrels.
Both
are 5 amp rating but neither blown or showing stress on the fusewire.
The
mains primary of the torroid measures 6 ohm which seems reasonable for
300W
amp and no other filter items . Anyone ever found bad ac rail fuse
contacts
leading to mains fuse rupture ? perhaps inductive back?/forward? emf
kicks,
surely not.

I`ve just this minute opened up a little Sessionmaster combo amp, both
LT fuses open. Customer said there was a flash when he plugged a lead
into one of the rear sockets (he couldn`t remember which) to his little
mixing desk.

The internal clip type fuse holders are so flimsy that one of the leaves
(leafs?) snapped off when I pulled the fuse, another snapped off when I
gave it a squeeze to see if was really as brittle as I thought it was!

The ac voltages are all over the place, cause seems to be that the
centretaps of the two separate mains tranny primaries arent making
decent contact - the tranny is remotely located in the bottom of the
cabinet and they used a spare terminal on a multiway connector to join
the two centretaps. the crimp is onto the sleeving and not the wire core.

I`m gonna have to strip out the entire pcb to replace two crappy
fuseholders!

Ron(UK)



Much the same problem as replacing 1/4 inch jack sockets where the spring
action is only going to weaken again. I'm thinking a twist of nichrome wire
around the yokes at each end of each fuse holder, at position between
fuse-cap and holder-base is in order, to beef up the spring action. Nichrome
as less likely to fracture at the twist than copper wire.


--
Diverse Devices, Southampton, England
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