Tecsun S-2000 World Receiver Won't Receive On External Antenna

 The Tecsun S-2000 World Receiver is a radio that has AM, FM and SSB listening capabilities on the LW, MW, SW, VHF and AIR bands.

This radio was bought second hand online and at first glance was in working order using the external aerial whilst scanning the FM broadcast band. However, the owner stated that the radio was silent on the short wave bands using an external aerial. This can be plugged into the side of the radio.

Usually a long wire is hung up outside and attached to the short wave input, then the mechanical switch is moved to select the external aerial instead of the internal one. The short wave listener can then enjoy distant stations, especially at night.

In this case the owner stated that not even hissing could be heard after switching to the external aerial. The radio simply went dead. 

The user also stated that the display also did not show the swich over between the internal and external antennas, despite the switch having been selected.

So I first tried to simulate the fault on my test bed, and connected the external jack to my radio set tester and injected a normal 7.000 MHz signal at -113 dBm. This is normally more than enough to hear a good steady test tone and sure enough, there was the test tone loud and clear. Bang on frequency. Other bands were tested, all worked perfectly!  SSB, AM, FM. Everything worked perfectly. So where was the problem the owner was having? How come the radio worked just fine for me but would not work for him? Curious....

We chatted and talked about what was different about my setup compared to his. We tried my outdoor longwire aerial instead of the test set. However, that worked just fine too.

Then we talked about my dim bulb, current limiting power supply (read more about it in this article) which also has a variac to limit the voltage. I use this often when testing older tube radios. It just happend to be set to 110V as I noted that his power supply for the radio can do 110 and 220V. So we decided to up the voltage from 110V to 220V and immediately his problem was reproduceable!

The problem was due to the power supply and did not involve the radio at all. By increasing the mains voltage, this must have added pressure to the electrolytic capacitors in the power supply making the power supply a little bit too noisey for the radio's internal aerial switching electronics to work correctly. So the power supply was opened.

 The caps were removed and tested for leakage.

Replacement caps were installed and the radio retured back to life again, as good as new.