Mastervolt Battery Charger MASS 24/75
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This very expensive 24
volt 75 amp battery charger came from a boat in Turkey where
it had failed after a mains supply surge. I repaired it and it
went back to Turkey. According to the owner it didn't work so
it came back to my workshop for further investigation. It seems
that the owner had checked his mains supply with a cheap multimeter
with which he wasn't familiar. Looking at the switch he decided
to use the 5 Amp setting to test the incoming AC mains voltage
and as best as I can tell put the probes on live and ground.
The results were quite spectacular, tripping not only his supply
but that of the entire boatyard. I repaired the meter which had
suffered a broken 5A fuse. I did notice however that the only
current ranges were DC so there were two measuring errors.
The original fault which I'd
repaired was a blown varistor whose demise had been signalled
by that soot below. What I hadn't spotted was that area to the
right which is not quite as obvious.
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Here's a view of the top
under the aluminium extrusion which slides off. On the right
are the 14 screws which are removed to detach the chassis from
the outer case. |
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Bottom left are the solder
connections to the new varistor (marked S20 K275) plus a couple
of capacitors.
The equipment has a 16A mains
fuse which had blown. It's a dodgy business selecting fuses and
suitable varistors and in this case the latter had split open
and liberally coated the adjacent capacitors. I removed and cleaned
these then refitted them after testing to ensure both were good.
A new varistor, a new 16A fuse plus a new fuse holder (the original
was too sooty to re-use), reassembly and the charger was returned.
I'd noticed there was a resistance
of 172 ohms across the mains input and, but from the characteristics
of the charger I'd assumed this was the primary of a small transformer
for Standby/ON.
No reason to suspect the thing
wouldn't work so I was a bit surprised to hear it wasn't working.
On its return I looked again
at that 172 ohm across the live and neutral....
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I needed to confirm the
presence of the transformer but I was confused. Several wirewound
components that looked like transformers were present but these
turned out to be a pair of switching transformers and three chokes.
One transformer appeared to provide power for the equipment itself
and the second provided power for charging 24 volt batteries..
no sign of a small mains transformer.
The implementation of the design
is very crowded and lots of parts inaccessible so, still looking
for the elusive transformer, I checked with my clever Chinese
meter thinking it would say 100mH or something similar, but no...
it said it was a 5 volt zener diode... very odd. I found a small
junk box mains transformer and measured its primary.. again my
clever Chinese meter declared it was a zener diode. Not so clever
but as the results were the same.. slightly promising?
Not wishing to blow things up
(that boatyard tripping worried me) I connected a mains transformer
having a primary resistance of 180 ohms in series with the charger.
The result was 6 volts AC across the charger and 240 vots across
the test transformer. So.. nothing proven and instead I tried
tracing the circuit tracks and found the mains went directly
to a bridge rectifier (below.. those 4 in-line pins left of centre).
This had 162 ohms across the mains input (less because ...in
series with one leg of the rectifier was a 10 ohm surge limiter).
The rectifier appeared to check out OK with all four diodes registering
about half a volt on my diode tester. Maybe the bridge rectifier
has a leak? But no.. after isolating one pin the leak was at
the tracks not the diodes. I noticed a couple of solder blobs
that carried mains live so peered inside the mess of heatsinks
and spotted a second varistor.... and a soot coated capacitor.
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By shifting the equipment
around I soon spotted a sooty crack in the varistor and removing
it of course it was clearly the culprit, measuring 162 ohms.
I fitted a new S20 K275 device
plus the freshly cleaned and tested capacitor. Now the leak has
gone, but surely a leaky varistor and a sooty capacitor wouldn't
prevent the charger from working. It must have been serviceable
when I got it back? I connected a mains supply and it seemed
to work OK.
I dug out a 12 volt lead acid
battery that had a highish resistance plus a hefty rheostat and
connected these plus my multimeter set to DC Amps and switched
on. The rheostat allowed me to ramp up the charging current to
over 10 Amps so it seems all is well.
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This is a tiny plug-in
control panel which is fitted after the outer case and cover
are in place.
Plugging in the charger provides
power to the main circuitry whereupon a battery can be hooked
up and that switch turned on.
A relay governs the battery
charger switching supply.
The red LED comes on then the
charger determines all is well, turns off the red LED and illuminates
the orange LEDs in accordance with its findings.....
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My guess is the boats
mains supply fuse has failed. This probably blew when that surge
occurred. |
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