Repairing a failed Tecsun S-8800 radio due to damaged charging board

In June, my Tecsun S-8800 which was purchased in Feb 2020 stopped working totally. It had started 2 months earlier with FM reception being weak and I could only hear FM stations on external antenna (not the telescopic antenna) – receptions on other bands were not affected. A few days later, I noticed that the radio drained the batteries very fast. I assumed that the 18650 batteries had died after 3 years and replaced them with new ones. The S-8800 then worked for two more weeks when it totally stopped working. Nothing was shown on the display when I pressed the power button, whether USB was connected, even with freshly charged 18650 batteries and the battery LEDs were not lit. Interestingly, If I inserted two fully charged 18650 into the radios, after around 6 hours, one battery would be totally depleted while the other would remain full. I tried to reset the radio but it didn’t help.

After some further tests with a multimeter I observed that the middle terminals of the top and the bottom 18650 battery compartments appeared to be connected. This was strange, because from what I understand from the manual and various online sources, the two batteries are independently charged and monitored, not charged in series or connected in series. Resistance between the terminals of the bottom battery compartment was 4.49MOhm (essentially open) whereas the same resistance for the top compartment was 0.025Ohm (essentially shorted). Any battery inserted into the top compartment will be drained after a few hours – the bottom compartment is not affected. This leads to the suspicion that there was an internal short somewhere in the circuitry for the top battery compartment.

I contacted the seller and received an immediate reply saying that they would contact the engineering department and get back to me. Meanwhile I disassembled the radio and immediately realized that the middle terminals for the 2 battery compartments are soldered together. This means that while the two batteries may be independently monitored, they are probably charged in series:

I then isolated the charging board near the USB port and immediately noticed a few burned components:

At least two of the SOT-23-3 transistors (4Q5 and 4Q7) had been burned. The markings on the chip had been darkened so I was not able to tell the exact part number to get a replacement. Two nearby SOT-23-3 transistors had X1AV markings, possibly for AO3401A, a MOSFET commonly used for 18650 charging/monitoring. The radio still worked if power is supplied directly to the motherboard, bypassing the charging circuit, however I preferred to find out exactly the parts that had been burned and replace them, to achieve full functionality.

This is the original position of the affected board:

I contacted the seller again and was told that the two affected parts are a FET SOT-23 CE2300 and a FET SOT-23 LP3401LT1G. The same person also shared with me a partial photo of the PCB component positioning:

With this information, I purchased the replacement transistors and reworked the board. However, after replacing 4Q5 and 4Q7, I still couldn’t get it to work. The replaced transistor would become very hot once power was applied, indicating that there could be shorts on other parts of the board. According to the seller, a replacement board cost 16 USD (shipping included), so I decided to purchase and test it out.

Two weeks later the replacement board arrived. It looked new, however a close-up inspection revealed signs of rework in the area of 4Q5 and 4Q7. Perhaps this is a common failure mode for this board which had been previously discovered during the testing process at the factory. Whatever it was, I decided to connect the board and my radio instantly came back to life:

USB charging now works properly and FM reception is also normal. The cost of 16USD for the replacement board is definitely worth it. This is the repaired radio, partially reassembled, with the front knobs missing:

The radio once fully reassembled:

Lacking a full schematics, the caused of the failure of the 4Q5 and 4Q7 can only be guessed. Perhaps it was due to voltage spikes from the less-than-perfect USB charger, or perhaps it was due to other factors. Whatever it is, I am happy that my S-8800 is now working and I can now once again enjoy shortwave radio, while it still lasts.