Electronics > Repair

Repair of Ryobi BCL14181H Li-Ion charger

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GraemeG:
Before I ordered parts I wanted to be fairly confident that there were no more failed components on the board. The most likely candidates are components connected to or close to the failed parts that have already been found.

Checking around Q2 it was easy enough to meter R11, R12 and ZD2 in circuit. I didn't check the zener in detail but it is still acting as a diode so it is probably still a functional zener. I used a simple multimeter diode test to check it forward and reverse. The multimeter is not a high enough voltage for the zener to conduct so it shows as open circuit in the reverse direction just like a normal diode.

I was able to use 2 multimeters in their diode test modes to check the FET Q4. I attached both meter negative (black) leads to the BAT- terminal which is connected to the Source of Q4. The positive (red) lead of one meter was then connected to the Drain or to R12 or the empty Gate terminal of Q2. Note that Q2 is still removed from the board. This meter should show no conduction/connection through Q4. Then the positive (red) lead of the second meter is touched to the Gate of Q4 and the first meter should show Q4 is now conducting. Removing the meter from the Gate should show Q4 open circuit again. One catch I have found is that one of my meters applies 1.5V in diode test mode while the other applies 2.5V. The 1.5V meter is not always high enough to drive the gate of a FET so I use this one across Drain-Source and the 2.5V meter on the gate.

I used a similar approach around Q1, metering the various resistors and diodes mostly showed reasonable values. One exception is D1. The low value resistors R3 22 ohms and R4 150 ohms make it difficult to test this one in circuit. It appeared short circuited so I removed it only to find it is working just fine. That's when I realised R3 and R4 were causing the problem. I used the 2 multimeters again to confirm the opto isolator PC1 was functional. Again I had to use the higher voltage meter to drive the diode and the lower voltage unit worked fine on the transistor side.

My last concern was with IC1. This IC drives the gate of Q1 and if the IC is dead and the output driven high it would simply turn Q1 hard on and probably destroy it again. IC1 needs at least 13-14V minimum to start up and operate. To give some level of protection I set my bench supply to 18V and connected it through a 2K7 resistor (2700 ohms but not critical) to EC2, the 10uF electrolytic capacitor that supplies Vcc to IC1. In the attached screen capture from the oscilloscope IC1_Vcc+output_to_Q1_gate.jpg you can see the green Vcc trace increases to about 14V then the IC starts up and outputs a square wave for about 50mS then shuts down. This is normal protection behaviour of the IC. Q1 is out of the circuit so the transformer is not driven and the IC has not detected any feedback on pin 2 from the opto isolator. We can confirm that in the screen capture IC1_Vcc+pin2_Comp_Feedback.jpg. The pin is sitting at 5V from the internal bias supply. The final screen capture IC1_output_to_Q1_gate.jpg is a closer view of the output pulse train that will drive Q1. The output looks good and is running at about 65.8kHz which is pretty close to the nominal 68kHz that a 100K R1 resistor on pin3 should produce.

So it all looks good and now all I need are the replacement parts. I will have to be patient, I think the bits are still a couple of weeks away...

Greybeard:
If anybody needs schematic, see attachment.
I found it in the www some time ago, when I repaired my charger.

Defective parts of my charger:
D3 or D9 (shorted)
Power MOSFET Q2 (shorted, case cracked)

GraemeG:
Thanks very much Greybeard! I had not seen that circuit before. There are slight differences with my unit, there are a few differences in component numbers and I also have a 20K resistor from G to S of Q1 on mine. But mostly it seems to match up so far.
I'm still waiting for parts and have a quiet day today so I plan to try powering up the 5V regulator and see what happens on that side of the circuit. I expect the controller should show some sign of life.

GraemeG:
To see if I could test any more of the circuit I supplied 8V from my bench power supply to the input of IC4 the 5V regulator. As expected it supplied 5V on the output and the red LED turned on. This is the correct indication for a charger with no battery connected so it's a good sign!

Everything else seemed to be effectively shut down. All the outputs of the quad op amp IC2 were low but I used a 100K resistor from the 5V rail and carefully probed each of the op amp + inputs. This was enough to drive the output high except for IC2C. On pin 10 there is a 1K resistor to ground so I grabbed a handy 4K7 resistor in place of the 100K to toggle the output. Now I know all four op amps are still working.

I was able to use the same method to check Q3, Q6 and Q9 although they had all previously measured ok in circuit. For bipolar transistors as long as I can measure 2 diode junctions (base-emitter and base-collector) in circuit using a multimeter then it's likely the transistor is still ok. But switching them on and off while powered up in circuit pretty much guarantees they are still functional.

So I think that is it for the testing until the replacement parts arrive.

GraemeG:
I am still waiting on the MBR20200CT diodes but the NDF04N60ZH and IRFR5305 (Edit: corrected this part number) MOSFETs have arrived.

It turns out that the original NDF04N60 has not failed!

I discovered recently during another repair https://www.eevblog.com/forum/repair/repair-of-hp-0957-2304-power-pack-for-hp-photosmart-7510-all-in-one/msg4615453/#msg4615453 that the method I use to test MOSFETs with my multimeter is not reliable. It seems that some power MOSFETs, in particular high voltage devices, need more than the 2.5V that my multimeter can provide to turn on the gate. I now add a 1.5V battery in series with the test leads of my multimeter or use the very handy Multifunction Tester to verify these devices (see photo). There are a whole range of these devices available on the internet for just a few dollars, all based on an "open source" project, and they are really very effective.

So the only components that now appear to have failed are the D3/D9 diode and the Q2 9565 MOSFET. Others have speculated that Q2 needs extra cooling or is under spec'ed, possibly because the common failure mode is to find a hole blown in this device! But looking at my circuit board I don't see any sign of long term heat around Q2 but it is definitely visible around D3/D9. In the photo of the back of the board in the original post by WaynesWorld and in the photo I have attached to this post you can see the brown discolouring around D3/D9 but there is no sign of this around Q2.

My theory is that D3/D9 goes short circuit and then dramatically kills Q2 which explosively blows open.

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