Today’s teardown target is a DLO Power Bug model MH-01DU. This was a relatively expensive portable charger, that lasted a few years before getting into Kuzyatech lab. The complaint was that it made arching sounds, smelled hot and was not charging.

Top view

It's a pretty convenient design, with folding power plug

Not seeing any obvious screws, and not feeling any under the label, we try to pull the case apart to no avail. Then the label comes off, and of course the screws are right there:

Only after bending the plastic did I realize two screws were hiding under the label- too close to corners to detect by touch

Lets’s open the case:

ST Viper12A power supply. No smoke or fire signs

What a surprise- everything looks clean and intact, with no signs of damage.ZD1 and R2 pins are touching but they are on the same net. The design is based on an ST Viper12A controller, and even has a feedback opto. The board is single sided, with routed channels to separate HV parts. First impression is that it’s not a bad looking design. Without going into full reverse engineering mode, here is a rough schematic.

"DaveCAD" schematic

Nothing really exciting here, though notice their use of a Zener as a feedback trigger element. The usual 431 must’ve been too expensive, or not needed. The status is displayed by two LEDs- the Green one is always on as long as power is present and the Red one is on whenever output voltage dips under load. Together they create the orange “Charging” signal. At idle, the output is around 5.4V, dipping to about 5V under load. Not quite sure why they needed the second diode on the output-some type of a backfeed protection? Also, note the voltages at the output pins. The dividers are not shown, but they basically generate 2.9 and 2.1V needed by certain devices. Flipping the board over, we finally notice something slightly out of order:

Wait, I think I see something funky.

The AC plug blades swivel  to store, and when in use are touching springloaded contacts. Those are held by some plastic pins and a single bracket that appears bent. Bet all those insertions into outlets wiggled them enough to bend the plastic.

Yep, the plastic hold-down is bent

After carefully straightening the piece and putting everything back together, the blades make good contacts and no arching seems to be present. This is a fairly temporary fix, as things will eventually get wobbly again. But for now the charger works. What a silly mechanical problem though.