Once a quarter I volunteer at our local repair cafe for repairing (in my case) mostly electric appliances. (Note: our repair cafe only takes place once a quarter, so that is a 100 % volunteering rate.) Partaking in a repair cafe is great fun as it provides one with a societally acceptable excuse to open up various other people’s electric devices and take a look at their internals.
Since I also love speaking about my repair activities I was asked to take a look at someone’s Sangean PR-D7 radio. It can be powered by both an internal battery and an external DC power supply; I was told that it no longer worked when powering it solely with the external power supply. The first order of business was measuring the voltage provided by the external DC power supply, which proved to provide the required 12 V DC. However, when plugging in the connector, the radio showed no sign of life. Therefore it became time to open it up.
There’s a small PCB located on the inside of the radio’s rear case. One can see that both battery box and power receptacle are feeding energy into it, with the final power being supplied to the radio via the dual black/white cable.
Probing various points on the PCB quickly allowed me to determine that no power was fed from the power receptacle towards the rest of the power circuit. Removing the screws mounting the power PCB to the rear case allowed me to take a look at the bottom side of the power PCB. Doing so quickly revealed the culprit: the PCB had cracked and various copper traces had been visibly interrupted. There was no electrical connection, as was confirmed by probing with a multimeter.
I started fixing the board by bridging the open connections with insulated wires: red for the positive power supply, yellow for one of the speaker cables (speaker +).
I could now successfully power up the radio, but no sound was to be heard. Looking around further, it turned out that the ground plane, to which speaker – was attached, was not connected to the overall ground, as the ground plane copper had also been ruptured. Adding another black cable re-established a proper ground connection.
Now I was able to both power the radio and hear actual sound. In my estimation, the radio would have been silent even when running on battery power, as the speaker circuit was equally interrupted. In hindsight, I should have verified that by trying to power the radio via batteries before starting with the full disassembly.
In order to stabilize the repair and add an additional layer of insulation I generously applied hot glue to the area of the repair.
After complete assembly and a thorough test I was happy that the radio was now fully working again.
The likely root cause for the fault seems to be that the radio fell down while the external power supply was plugged in. Since there was no strain relief for the power receptacle, the force of the plug being pulled against the receptacle was directly forwarded to the PCB, cracking it. A better way to design this would be to ensure that the receptacle is mechanically linked to the case, rather than being mounted solely on the PCB. This was the approach taken for the headphone connector mounted directly above it.