Tuesday, 12 November 2013

Wireless Charging on a Star S7189 Mobile Phone

Warning: while soldering be careful of solder fumes, wear eye protection and don't burn yourself.  Note that charging batteries can be dangerous especially Li-Ion but the work detailed here means we are effectively generating our own internal 5V supply in the phone and connecting this directly to the USB power connections as they come into the phone. It is the equivalent of connecting a micro USB cable to the phone from a 5V USB charger, only its wireless. You have been warned and I am not responsible for your actions.

S7189 Display

This S7189 quad-core Android phone from Star was pretty cheap around £100 and it worked well.  I found the front facing camera was not the best, but for the price I can live with it. Screen was also pretty good with a reasonable viewing angle. It was also pretty slick running any Apps I downloaded from Play with no signs of lagging.

Palm Internal Charging Circuit
I wanted to add wireless charging to the S7189 so I didn't have to keep plugging the charger into the micro USB connector. I noticed on HackaDay that some people had added wireless charging to their mobile phone , this is what I wanted. So I ordered a wireless Palm case back from Ebay for £3.

Palm Back Cover

I then using scissors cut out out the area of plastic bounded by the Plastic label with Palm on it. This was to allow one to be able to peel off the label more easily from inside the case. I then stuck the label on some plastic film (after removing the circuit board, coil and metal discs) to preserve its stickiness.  

Circuit Cut Out of Plastic Cover
The coil and circuit board were removed from the old Palm cover by soaking in a plastic tub of Methylated Spirits for a few hours to soften the double-side sticky tape holding circuit and coil onto the plastic cover.

Sticky Cover Preserved

Stuck to Clear Film

S7189 Back with Cover Removed

The S7189 internal back cover is removed (quite a few small black screws) exposing the Circuit board. I plugged in a Micro-USB adapter to find out where the center conductor was connected to on the Power PCB at the lower part of the phone using a multimeter set to continuity tone check. Ground was the metal tabs of the Micro-USB connector. I used Kapton tape to insulate the sticky copper foil I was using to route out power to the top of the back cover.

Ground Connection

I then soldered flexible tinned insulated wire between the two Copper tabs next to battery, taking care not to overheat the plastic.

Routed Copper Tape

Palm Charge Circuit Added to S7189 Back Case
The removed Palm charge circuit was stuck to the middle of the S7189 back cover using its original sticky cover. Of the two small square gold connectors to the left of the Kapton tape the Lower one is the +5V the upper one is 0V or Ground. To be sure check the output yourself with a multi-meter to ensure you have the correct polarity before connecting it to your valuable phone.

Phone Placed on Charger

Phone Now Charging

The S7189 is now able to be charged either by using the Micro-USB or wirelessly via the Palm Touchstone charger. Note: don't use both at the same time or there could be overcharging problems with subsequent overheating and fire from the battery and circuit boards.

Pure Evoke 1 Crackly Burbling Audio

Warning: while this radio is low voltage around 12V dc no mains voltage appears in the actual radio, so it is relatively safe to work on. The wall-wart power supply itself however is mains voltage ( in my case it's 230V AC ) so this needs to be treated with respect. Also while soldering be careful of solder fumes, wear eye protection and don't burn yourself. You have been warned, I am not responsible for your actions.

I have had this Pure Evoke 1 DAB radio for about 7 years but it started having problems. The audio became crackly and it also had an associated burbling noise. The signal quality displayed for the radio station I listen to normally which previously was 97 was now wavering between 40 up to about 70.
The first thing I tested was the most accessible thing the wall-wart power supply. This power supply seemed to be ok, measuring around 17.4 volts dc on my Fluke multimeter.

Evoke 1 Amplifier Board
I opened the case of the Evoke (remove all the screws on the back of the case) this is where I noticed that the antenna f connector was loose. I tightened the f connector using an adjustable spanner and pliers. Thinking it was a poor signal because of the loose connector I switched it on again but the audio problem was still present. I then proceeded to measure the regulator voltages on the main audio board. Most voltages appeared ok, however I noticed that the 3.3v supply which supplies the RF board was not solid but was wavering round quite a bit. I examined the electrolytic capacitors on the board but found no evidence of them being swollen due to localised heating from the various heat-sinks on the board.
So I started removing the capacitors on the amplifier board one-by-one to test them on my Peak capacitance and equivalent series resistance meter. All capacitors measured well within 10% of their marked values and low ESR apart from one capacitor, C7 a 100uF 16V electrolytic which is located very close to a regulator heatsink that runs very warm. Its value measured low and its ESR was higher than it should be so it appears that it is effectively being dried out over time.

Faulty Capacitor C7 on the tester
 I replaced this capacitor with one of equivalent value but its voltage rating was higher at 25V this is not a problem in this application.

When I soldered the new capacitor into the board I bent the new capacitor body away from the offending 3.3 V regulator heat-sink to prevent the new capacitor being dried out with the heat.
Capacitor bent away from heat-sink
The signal quality returned to 100 after the repair, I soak tested the radio to ensure that it was repaired.

Job done. Thanks for reading and good luck if this is what has happened to your Pure Evoke 1.