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.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

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)

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
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

So much for so called "Top Cheers" transformers.
Initially could take variac to 50V and then run away , now cannot even put
the minimum of 5V on the primary , with nothing connected to the
secondaries.

So just coincidence of bad fuse holders on the 48V ac lines or directly
leading to failure ? Could repeated breaking of secondary current induce
very high voltage in the primary leading to shorted turns?
Nothing visibly wrong through the tape wrap or smoke trails from inside. DC
measurement on primary now about 1 ohm , unpowered.

Replacement would probably have to be 2 transformers 48-0-48, 3 amp plus
15-0-15 0.2 amp or so


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

Replacement transformer available from Ashdown £49 trade/£66 , saves putting
in 2 generic , plenty of space to do so if was necessary.

I intend (time dependent) unwinding the original to explore the fault. Other
than using a long and thin shuttle/bobin to wind the wire onto , any tips?.
Hopefully the central epoxy mounting filler is heavily padded with mineral
filler , so easy to mill out.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

I don't know anything about transformer winding but it looks as though it is
bad winding lay-up.

The first winding was the low current 15-0-15V secondary. This was wound
with doubled up .45mm wire in one go so reducing the number of shuttle
passes to wind on. The 2 wires brought together to form the "0". Thought to
myself, perhaps that is ok for low voltage winding as only 50V peak or so
maximum for 2 thicknesses of lacquer to sustain.

Then the 3 amp 48-0-48 wound as 2 separate layers of 1.06mm wire, not run as
paired winding, fair enough. Anyone know why 3 runs of loosely packed turns
per layer when one run of tighter turns per layer could have been done, so
avoiding overlaps?

Get down to the unruptured thermal fuse, tails of the primary and the .7mm
wire primary and nothing obviously wrong in the way of hot spots. But
blow-me-down a return to the paired up winding and ends joined to give
120-0-120V for 240 UK use. But of course now 2 thicknesses of lacquer having
to sustain a normal maximum differential of about 350 volts. Not wound off
yet but somewhere in the bulk of windings must be bridge/s.

Thermal fuse marked
17 AM
033A5
M4 AB
if anyone knows what that means as temperature.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/