Avatar Controls Smart Plug - Into the Belly of the Beast

Having convinced myself that the smart plug was not working, it was time for a look inside.

But before that, an external inspection found no obvious charring or melted plastic that might have implied some sort of catastrophic failure.

Avatar Controls AWP14H 10 A UK Smart Plug

On the back of the device were manufacturer and model details and some basic electrical performance data (220-240 V input voltage and 2.3 kW / 10 A resistive load). There was also an IEC 60417 - 5016 symbol for a fuse described as a T10AL240V - which is a T[timed(slow blow)]10A[10 amps]L[low overcurrent rating]240V[mains voltage]. So somewhere inside is a 10 A fuse, which appeared to be a reasonable place to start.

The initial hurdle, then, was to open the case. A job easier said than done, as it is designed not to come apart for obvious reasons. Bigclivedotcom has published a video showing a fix for a similar plug in which he says that the cases are glued or possibly ultrasonically welded [timestamp 12:19]. Some vigorous poking with a blade and jeweller's screwdriver didn't make much progress. But a second YouTuber came up trumps; a bit of asymmetric squeezing from a quick-grip to generate a bit of radial stress resulted in a couple of cracking noises. After which the plug parted quite easily revealing the circuitry for inspection.

Avatar Controls AWP14H Smart Plug - Internal Layout

Again, no obviously burnt out components, but one of the electrolytic caps (arrowed) was suspiciously domed compared to the other two. If you watch Bigclivedotcom's video and another from the Buy it Fix it channel it appears that these caps are particularly susceptible to failing.

While it was open I had a look at some of the other components. From what I could glean I think the plug is organised as follows;

1. The T10AL250V fuse is in a board mounted package with a fusible resistor and a zinc oxide varistor for circuit protection and a single diode to rectify the AC supply into a half-wave DC supply.
2. The mains voltage is then converted to a rough 5 V DC supply by a small buck converter and a couple more electrolytic caps.
3. Our questionable 470 μF 16 V electrolytic cap smooths the output of the converter further.
4. Then a 3.3V voltage regulator supplies the control board on a riser.
5. This is a Beken BK7321 system-on-chip.
6. Finally this controls a 10 A relay that switches the plug.

The screen print on the PCB gives the board's details as HYS-01-127_V1.2 2020-07-17, meaing that the maximum age of the plug is 5 years.

I can check the fused components once the board is free from the plug base, but it looks like the low voltage cap is likely going to be the culprit. So out with the soldering iron it is then.

Most of the kit I already had; soldering iron, solder and soldering bit cleaner; and clamps to hold it steady. What I didn't have (or couldn't find) was any desoldering braid, flux or IPA. So these required a bit of shopping.

The IPA (isopropyl alcohol) for cleaning the board after soldering/desoldering came from Amazon for the low, low price of £6.59. I did have some of this before, but the advent of Covid-19 and the great IPA shortage of 2020 emptied my stocks. Obviously I don't want to be sloshing IPA out from a 1 L bottle onto the workpiece, so I poured a bit into an old screw-cap perfume bottle one of the tykes had discarded. Amazon had no SDS available but the supplier, Trade Chem, were quickly able to send one by e-mail (a sign of a good company). The only issue was the cap liner coming apart but that is what you get for cheap-as-chips.

Next was flux and desoldering wick. I hadn't used an electrical soldering flux before but as this job would rely on desoldering I decided not to try and bodge it. So had a look at a couple of videos on flux (What is this flux stuff anyway? and Which one should I buy?) and decided that the whole topic was too niche to get into for a small job. In the end I bought what I felt was one of the more general purpose no-clean fluxes, MG Chemicals 8341 No Clean Flux Paste, at £7.58 on Amazon. An SDS was sourced from CPC Farnell. Along with a hilarious instruction video showing exactly the wrong way to attach a Luer lock needle.

Needlestick injury incoming

Hint: Use the needle cover to twist the needle onto the syringe, then take the cover off. That way if you slip whilst putting the needle on you don’t end up injecting yourself with rosin (yes I know it is a blunt needle but you can stick yourself with it just the same).

Onto desoldering wick and at this point I was running out of steam so I just bought some MG Chemicals Super Wick No-Clean Fine Desoldering Braid, 2.5 mm x 1.5 m 426-NS for £3.22 from CPC Farnell. Good, bad or indifferent it will have to do.

Now I had the tools and materials to hand it was onto desoldering the board from the 3-pin socket assembly. This wasn't quite as straight forward as I had hoped. As the board's main input pins were connected back to the plug socket pins they sucked heat away really fast so even with the soldering iron up at 400 ^oC and added fresh 60:40 solder it was a bit of a struggle. The wick worked quite well but I couldn't easily part the board from the base and when it did come away it took the tag off the neutral pin. There was also some naughtiness with the 10 A fuse.

Avatar Controls Smart Plug - Board dismounted (left), mains tag issue (top right) and unintended fuse damage (bottom left).

Fortunately the fuse was still OK, just a bit melted, as was the fusible resistor next to it meaning that a blown fuse was not the issue. But now I had to repair the tag before I could get back to the original repair. The tag that had come astray looked like it hadn't been punched on properly as I didn't find any pieces of brass that had come away. I did try to punch it back in but it didn't work, so in the end I flooded it with solder. This might not be a great fix but, as regluing the case will make the plug weaker as well, I am not planning on moving it around a lot. So it will have to do.

The final job was to remove the suspect capacitor, which went without issue.

Failed electrolytic capacitor

As well as the slight doming on the top, the bottom was also deformed. When I brought out the multimeter to check its value I found that its max range was 40 μF meaning the replacement cap I had bought would be over range; however measuring the old cap gave a value of 12 μF which is quite a long way from the 470 μF that it should have been. Definitely the prime suspect.

The faulty cap was from a Chinese manufacturer, Dongguan Zhuo Wang Electronics Technology. The full extent of the specs that I was able to obtain was what was written on the outside of the component's packaging. A search for an identical replacement or a similar 470 μF 16 V cap in the same size format yielded nothing so I plumped for the closest that I could find which was a Panasonic EEUFR1C471. These FR- series caps from Panasonic have low ESR (electrical series resistance) and are designed for long service lives with high ripple currents. So I replaced the duff cap with a new one and resoldered the board back onto the base.

Repair Avatar Controls Smart Plug

Once reassembled and plugged in a blue LED came to life and the socket could be switched on and off at the power button. The plug was also visible on Bluetooth as a Tuya LE device.

So it seems to be functional again. The next step is to deal with the firmware…

P.S.

I did contact both Farnell and MG Chemicals to let them know about the H&S flaw in their video, both of which replied that they "would pass it on to the relevant Dept." But it is still up. So if you have stuck yourself, they have already been warned.