Fixing my Kenwood TH-D74e
I accidentally left my Kenwood radio on charge and it broke. When turning it on without the external power connected, it would show the Kenwood logo and then reboot and start again. With the power connected, it would boot fine, but not be able to transmit as that's locked out in firmware. I emailed Kenwood asking if they offer repair, but they did not reply.
After a bit of searching the web, I found one reference on a mailing list which suggested it was the charge controller. The problem description matched what I was seeing, and they said they sent theirs for repair and the repair person swapped out an NCP1871. So I took it apart to find where the chip is. Unfortunately you have to remove the chassis from the case, then the LCD screen, the top board, and then finally the bottom board. Removing the bottom board is a tight squeeze and I was slightly concerned that I'd break the circuit board. It came out eventually though.
The NCP1871 is a QFN20 package and it has no legs. Even if it did have legs, it's so densely packed that getting a soldering iron in there to hand solder it would be a challenge, so I opted to use hot air and plenty of flux. The shielded part made it quite difficult, and getting the hot air gun and tweezers under the microscope at the same time was a bit of a challenge. A microscope is basically essential for this - or at least it was for me, I could barely see the pins without it.
Anyway, here are some photos of the process, and the radio working. The first photo shows the footprint after removing the broken chip. The next is of a very fluxy replaced chip, and finally a partially assembled radio which has successfully booted from battery.
I forgot to take a photo after cleaning with isopropyl alcohol, but it looked much nicer once cleaned up.
Update <2025-03-23 Sun 17:42>: A couple of days after the repair, I tried to charge the radio and noticed wouldn't charge! The radio is obviously working more than it was before the repair (above), but there's still another issue. I decided that either I'd shorted something soldering on the new charge control chip, or there's another fault somewhere. This morning I decided to replace the chip again to be on the safe side (it was much easier this time, I'm gradually getting better at small rework stuff), but the radio still would not charge.
Checking the SW (switching) pin for activity was promising - the controller should be (and was!) driving the FETs which drive the inductor to generate the appropriate voltage. This heads off to another stage of switching, through a 0.1 ohm current sense resistor, which measured fine (thanks to Sam for providing the four wire measurement probes!). After a bit more probing, I discovered, I think, the problem, which is that one of the FETs appears to be a dead short. The smoothed output from that inductor should probably be around 8-12v, but is only 2.4v, which is basically just the smoothed switching pulses.
I'm guessing what may have happened is: the charge controller died (for some reason), which then lead to the FET also dying. I'm going to order a new one, hopefully that's the issue. Another update soon…
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Comments and feedback are welcome by email (aaron@nospam-aaronsplace.co.uk).