Electronics > Repair

Canon binoculars IS - power supply fault

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jamarjan:
Good day to everyone!
I am trying to solve a fault in the stabilisation system power supply of Canon IS binoculars.
Battery power supply is based on the MB8300 regulator.
After inspection under the microscope, I found a damaged (corroded) via on the Vcc supply line just before the regulator IC.
After repairing the track and restoring power, unfortunately the circuit still does not work.

There is a sawtooth waveform on the OSC pin, PWM appears momentarily on the OUT pin, after which the circuit detects a short circuit on the output and shuts down the output.
A short circuit is detected even if the output path is disconnected (0 ohm resistor soldered out).
There is no short to ground on the output and all surrounding components have the correct values.
Does this mean that the circuit could be internally faulty or should I look for the fault elsewhere?

Link to MB3800 datasheet:
https://html.alldatasheet.com/html-pdf/902186/CYPRESS/MB3800/891/3/MB3800.html

magic:
You say MB8400 and then post MB3800 datasheets. Is it a typo? Are these controllers equivalent?

How do you know that it "detects output short circuit"?

Does anything happen on the output node?

jamarjan:
[small update]
I think im not getting a feedback signal on pin -in
Could this mean that Q108 is defective?
Multimeter measurements show no short circuit.
Does anyone have an idea what this component (PD 5 F4) could be?
Unfortunately with the help of https://smd.yooneed.one/ i am not able to identify it.

jamarjan:
of course, MB8400 was a mistake  |O (will edit)

regarding short cirquit protection - take a look at the SPC pin waveform
the first rise is from soft start than it should settle low but instead it rises again and the OUT pin is pulled low

before shutting down the OUT pin square waveform looks ok (second pic)

at start up there is about 40mA current draw, then it drops to about 8 mA

magic:
I would verify the feedback resistor going from output to IN-.

Then follow the signal path with a scope: the OUT pin, switching transistor collector, both sides of the diode, the IN- pin.


If I understand sections 10.3 and 7 correctly, the chip enters short circuit protection when it thinks that output voltage is too low for too long.
This may be bad transistor, bad diode, bad inductor, bad output capacitor, bad feedback resistor. Or bad chip, but hopefully not.

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