Fukuda FRE207 - LASER Level Repair *SOLVED*

[FactorySmoke]

I bought a rotary laser level that is faulty.
Original fault was that the laser would turn on immediately when the battery is connected and operate normally, but after pushing the power button the LCD would dim and the backlight would flicker, but never power off.
During my diagnosing I decided to power the laser level direct from my digital bench power supply to see what the current draw was at 6V.
Unfortunately I found out 6V was more than a component on the main PCB could handle (even though the laser level also runs off 4x 1.5V C cells in series??).
A SOT23/SOT753 5-leg component with the markings 9A9B let out the factory smoke.
I am having a hard time finding out exactly what it is and what it does, but I believe it may be a LM3671 DC converter (or equivalent). If it is, I suspect it's the fixed voltage type, but I'm not sure that it isn't variable.
The component burnt out between where the switching FET might be on a LM3671 (Pin 1 and 5).

What is the burnt component?
Answer: SOT23-5, 4V LDO regulator. Possibly 'Belling' brand.

For bonus points, what might cause the original fault of not powering off?
Answer: The 'N2 and N3' regulators were leaking voltage through, even with no voltage to the enable pin.

Note:
The burnt component was in 'N3' position on PCB (lower left quarter of PCB). I took 'N2' from next to the beeper (upper right quarter) and put it in 'N3' to see what would happen.
N1, N2, and N3 are all the same part and have the same circuitry and components in immediate vicinity.
AFAIK the lasers run on 3-5V (connectors on lower left).
The line colours in the picture (except for red and black) have no significance other than to make them more visible.

I have contacted the laser level manufacturer about getting a replacement PCB (if I have no other option) but they haven't responded.

[fzabkar]

XC6113, 4.0V voltage detector? (I don't think so)

https://product.torexsemi.com/files/status/xc6106.pdf

[fenugrec]

I believe it may be a LM3671 DC converter (or equivalent)

Um... I don't see inductors around N1/N2/N3 if indeed they're the same part ? Components on the back side ?
That one connected to the PIC mcu , probably an Enable pin ? Could be just plain LDO regs, though I don't know why they'd have 3 identical regulators

[FactorySmoke]

Thanks for the suggestion.
But I can't see how it could be an XC6101 since on my laser PCB that would have 5-6v going to the reset pin (Pin1) and a I have measured a constant 4.9V out to the laser diode connector, where the XC6101 has Vin (pin 5).

[FactorySmoke]

I believe it may be a LM3671 DC converter (or equivalent)

I don't see inductors around N1/N2/N3 if indeed they're the same part ? Components on the back side ?

That one connected to the PIC mcu , probably an Enable pin ?

I agree about the inductors. I noticed that the recommended circuit was slightly different too. But, I have looked at maybe 10 different SOT-23-5 IC's and the voltage regulators are the only components that are very close to fitting the description.
Sorry, I forgot to mention there are no components on the back of the PCB. Only vias and copper traces.

Yes. I'm fairly sure there is a signal from the MCU to turn on the regulator.

I have a big development that I will post soon.

[FactorySmoke]

So, the latest update is:
After I took the IC from N2 and replaced the burnt IC in N3, I assumed that the MCU was no longer properly connected the the rest of the circuit and the board was giving no signs of life when connecting the battery (previously lasers and all components would immediately operate when battery connected).
I decided to start fault finding the original problem I had (would not shut down with power button). After tracing the power button back through the connector (long white connector) to the PCB I could see it went to VD2 transistor and then on to Q1. Then to the MCU. I tested all components related and they are good.
With that logic I realised, it must work now. I pushed the power button and everything powered up and functioned perfectly....except the small motor that rotates the laser turret/reflectors. So, (the now vacant) N2 must be the source of power to that motor.

A few hours later I also got a response from Fukuda. They can supply a new board for 20USD (wish I knew that before sinking a crazy amount of time into this, but that's how it goes).
I asked them what the 9A9B is, so fingers crossed they give me more info.
I will buy the new board, but also finish fixing the faulty PCB. I'm not giving up now, when I'm so close to all the answers.

Something else of note is that the used laser was sent to me with a 24v power supply!! I have to assume the clueless last owner plugged it in at some point. That may explain why the power button stopped working and why the N3 component burnt out during bench testing. Either way N3 appears to have been damaged before it was sent to me.

Will update when I have more....

[FactorySmoke]

Fukuda have been useless. They say they can sell a new PCB for $20, but 110USD is the lowest shipping cost they will offer and refuse to use any more economical option!
Fakuda 'support' guy also doesn't know anything about the components. "That is the R&D department, and I can't get that info". What BS!
Now they just ignore my emails and don't bother responding.

I figured out that the 9A9B is a fixed 4v SOT23-5 LDO voltage regulator (possibly Belling brand) with the same pin out as a MIC5219 (LG36) or a SPX3819.
The 9A9B regulator in N3 probably burnt because it was damaged internally from overvoltage from the incorrect power supply.
N2 was also damaged internally (since N2 and N3 have the same power source) before it's relocation to N3. The regulator from N2 wasn't regulating the voltage properly and was also active at all times, even without voltage to the enable pin.
That explains why Laser 2 was lit with the MCU powered off.
I replaced all voltage regulators (N1, N2, N3) with new 3.3v SPX3819 LDO's (MIC5219-3.6V, LG36 would also be a good choice).
Any regulated voltage between 3-4v is fine, as all the IC's and lasers operate in that range. However 3.3v is the optimum operating voltage for the lasers.

Even though unnecessary, I also replaced the 'KEC' components in QX1 and J9 with new SOT89-3L CJA03N10 mosfets.

The new regulators fixed all faults. Now I have a fully functional laser level.