This charger worked fine for 30 years. When it stopped working a couple of years ago,
I pried into its interior to see what was wrong. It turns out that the failure was purely
mechanical. I must have dropped the unit, which dislodged the copper PCB (printed-circuit
board) from its isolated mounting and allowed it to short-circuit the case. In
addition, the movement of the heat sink caused a pin to break off the output transistor
that was mounted on it. I repaired the unit by pushing the heat sink back onto the insulated boss
and re-soldering the wire to what remained of the transistor pin. It seemed that the charger was
still broken until I remembered that it was designed to have no output until it was hooked to a
battery. Once I biased up the control board with a few volts from a dead motorcycle battery, the
charger worked fine, including the adjustable output voltage on the control board. An important
factor in the longevity of the unit is that the designers did not use electrolytic capacitors.
1. The design uses puck-style
diodes similar to
those in automotive alternators.
solder them directly to
the copper heat sink. The
two wires to the diodes
are the ends of a center-tapped
2. A thermal circuit
in case the output
The breaker is
wired into the
of the output.
3. The mounting of the
control PCB allows an
to extend from
the back of the case.
The board regulates
the output voltage and
prevents output unless
an attached battery is
4. The output pass transistor
has two thick wires soldered
to each of its pins. The
wire from the control PCB
caused stress, which broke
off the transistor’s base pin.
5. A thin copper sheet serves as a heat sink. A square hole in the bottom snaps over a plastic mounting boss in the outer case. The heat sink operates at 15V, the raw unregulated rectified output of the transformer.
6. The charger’s main transformer
features primary and
secondary windings on separate
bobbins, similar to the
that medical equipment uses.
The case includes a ground
wire in accordance with UL