Kenwood TH-F7E power supply troubleshooting

Question

I have a Kenwood TH-F7E handheld transceiver, bought second-hand a bit less than a year ago. Until about 3 weeks ago, it worked perfectly, then I put it in a drawer and didn't touch it until yesterday, when I noticed that it didn't switch on on battery power, and the charge light didn't come on when plugged in. Here's what I've found out so far:

1. I measured the voltage between the battery terminals as 0 volts, so my first thought was the battery had simply gone bad. The battery is a PB-42L Li-Ion pack.

2. However, there are also some signs of a problem with the transceiver. When the mains adapter is plugged in, the receiver seems to work normally, but the device heats up a lot after a couple of minutes. This happens even if I remove the battery pack.

3. With the mains adapter plugged it, I can transmit with extra-low (0.5 W) or low (2 W) power. In the high-power setting (5 W), the transceiver restarts as soon as I hit PTT.

4. I tried connecting a 7 V power supply directly to the battery terminals of the transceiver and noticed that it draws 900 mA even while turned off. I immediately disconnected the power again, without any testing, when I noticed that.

I've found the service manual and was planning to open it up to figure out what exactly is producing the excess heat and make some measurements, but I was hoping to get some advice on how to troubleshoot this before I start digging around in the transceiver.

Answer 1

I suspect you have a plastic brick. Unless you have the skills at working on surface mount pcb, like a steady hand and good magnification lens or video microscope, needle probes (easy to make) and suitable soldering equipment repair of these tiny radios is quite difficult but not impossible.

I prefer an old fashioned VOM like a Simpson 260 when troubleshooting for shorts and opens as there is no lag in a digital display count updating. The meter needle instantly moves or it doesnt. Also you are not looking for precise measurements at this point of troubleshooting. So follow the schematic and probe accordingly following the circuit path from the external charger and battery inputs, of course with all power off. You dont want to fry your meter. Since the radio draws high current with it turned off it should simplify locationing the short. It has to be before the power switch. Which means the charging circuit / jack/ battery circuit paths. And always double check the the setup you used to determine your initial findings of current draw under the various input and battery connection scenarios. In other words, start over. And document each step as you go. It often helps to develop a fault tree when before troubleshooting complex problems. It will keep you on track in a orderly and sequential manner of troubleshooting, so that your not guessing and randomly probing around, although many problems have been solved by random probing.