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Ashdown MAG 300

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

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N Cook
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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

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N Cook

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

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N Cook

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

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Reply to
N Cook

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