​
14. Other Repairs, Modifications and Teardowns
​
INSTRUMENT REPAIRS & MODIFICATIONS (IN ALPHANUMERIC ORDER)
​
# Action Status Problem Manufacturer Model Function
1. Repair Fixed Stopped working Hangsun HOC700 Oral Irrigator
​​
1. ​Hangsun HOC700 Oral Irrigator (water flosser)
​​
Problem: Stopped working
Xmas 2023, a relation gave us a new Hangsun water flosser she recommended, that we had earlier enquired about. It worked fine for 9 months and then started doing strange things. First it started coming on by itelf in the middle of the night, then it ignored selected speed settings and eventually after about a month, the on/off push button no longer lit its blue LED or started the motor, yet the function select push button below it lit the 3 speed selection LEDs in sequence as usual although they too went off after about 15 seconds.
A search in the 'Net revealed the following repair of an outwardly similar model with a different manufacturer and slightly different pcb, the fault here apparently being down to the chip in the Li-ion battery needing to be reset:
​https://www.drive2.ru/b/637944618968687117/
​However this could not be our problem as the function select button still did something, albeit useless. ​The only solution was to open it up (hover over each photo below for its explanation):​
Inside I found water drops on the pcb and visible signs of corrosion. We had never immersed it, so this must be down to bad sealing.
The 3.7V 1400mAh Li-ion battery seemed fine and was fully charged, sitting at 4.0V.
Below, the underside showing the pump outlet and, split open, the motor and battery in the centre.
I removed the pcb, cleaned off the corrosion, left it to dry overnight in a warm airing cupboard and tried it again the next day but it made no difference.
The pcb is also marked Rentech and a quick search revealed they are the OEM, not Hangsun. Rentech had actually paid attention to the possiblity of leaky seals bcause the pcb has a conformal coat, but it wasn't very well applied and there were big gaps. I scraped it off the tops of the ICs and two SOT-23s near the motor take-off in the hope of identifying them.
​
Below left, the SOIC-16 marked Rentech FS012 is the µC.
​
Centre, the SOTs are numbered 2300 which identifes them as BM2300 Trench NMOSFETs: 20Vgs, 5.2A/20Apk, 1.5Vgth, Ron 40mΩ. Very nice.
​
Right, the SO-8 IC near the battery terminals is a TP4056 Li-ion charger IC. Despite the huge amount of corrosion on it initially, it evidently still worked.
​I now began tracing the pcb tracks from the uC, identifying port pins for the switches, the motor drive and out of curiosity, the LEDs.
​
µC pin 5 leads to series resistor R4 1kΩ and pulldown R10 100kΩ on the NMOSFET gates. The MOSFETs are wired in parallel, I assume to halve Ron.
To their right are a couple of big SMD 50mΩ sense resistors which I assume are for current feedback as they connect in parallel to the MOSFET source and into a resistor routed in the direction of the µC.
​​
The top ON/OFF switch S1 goes to µC pin 2 with R19 10kΩ pullup.
The lower FUNCTION SELECT switch S2 goes to µC pin 3 with R20 10kΩ pullup.
µC pin 1 is V+ and pin 16 is 0V.
The LEDs are driven off µC pins 7,12,13 with 100Ω current limiting resistors R15, R7, R6.
I didn't trace any more circuitry, mainly because the tiny SMDs are covered in conformal coat and at that point I still hadn't got around to buying some nice sharp probes, although afterwards this finally prompted me to order some.
​​​Hooking up a 4V PSU and pressing the buttons, NMOSFET drive µC pin 5 was always ~0.4V.
Uncertain if there was a µC O/P fault or a short on it, I pulled out series R4, injected 2V and with a 5V incandescent lamp in place of the motor (you can only get the pcb out by unsoldering it from the motor) and confirmed the NMOSFETS worked.
Examining S1, I got 0.4V nominal until I pressed it when it dropped to 0V. S2 went from 4V to 0V when depressed. There are 2 vias on the pcb tracks leading to S1 and I drilled them both out but it still remained at 0.4V. Unable to debug further I assumed the switch was faulty. Since S1 was now disconnected from its µC pin 1, next I added a 10kΩ pullup and tried pulling it low, but it still didn't work, indicating a problem with µC pin 1 too.
I rarely chuck stuff in the bin but that's what I did next.​
​​​The next morning in my bath it occurred to me all I had to do was feed S2 into an inverter to drive the NMOSFETs in place of the µC, and I retrieved the bits from the bin.
I found a 74HC1G02 single NOR gate in a SOT-353 but my first choice 10kΩ was too big; the nearest fit was a 2k1Ω.
I unsoldered the µC using low temperature ChipQik solder and cleaned the pads with braid.
Above right: since V+ came into the µC at pin 1 and S2 came in at pin 3, I soldered it between these pins, adding a pullup 10KΩ SMD between them, a wire to 0V on pin 16 and a wire to a 1kΩ SMD soldered to pin 5 leading to the NMOSFET series R4 I had pulled out and now shorted with a solder blob (poor practice but fine for this: it was hard enough soldering my bigger SMD Rs in place but far more difficult putting the tiny R4 back without disturbing surrounding SMDs).
I checked it worked with the lamp and sprayed on more conformal coating.
​
I cleaned dirt off all assemblies when I dismantled it. I now removed mould growing inside the water tank, and mould in the water pickup tube with a pipe cleaner: something to watch out for in the future.
​​
I also added grease around the various O-rings; it won't last forever but should improve what was there. I re-attached the pcb to the frame and soldered the motor back in. The nozzle release button is a little fiddly but it's only a matter of pushing it through its recess in the top before re-assembly.
​
The flosser is now working again, albeit stuck on its full power setting which is a little fierce (I may replace the gate with a regulator) and there are no LEDs on now, but that's 4mA less from the battery.​
New text box
µ Ω ± ° ⌠⌡ ∫ │ ─ √ φ θ Θ ∂ δ ζ ξ ς λ ψ ω τ µ Ω âˆ† Δ ∑ ∏ π Ξ â—‹ ≠ ³ ² ±