Tuesday, January 29, 2013

DC into the antenna socket: Very bad idea.

Never apply DC voltage into a receiver antenna input. Never.

It should be DC blocked but this is not always the case.

Last weekend I decided to install permanently at the window one PA0RDT Mini Whip Antenna I build almost two years ago. Everything went nicely, except I double checked the DC power injector was mounted in reverse direction and anyway I connected it to the transceiver. Yes, I did it. And yes, I powered up the system. Yes, I'm stupid.

In half a second a column of gray smoke went out my IC-746 transceiver, and that is not funny.

After disconnecting everything, I tested the transceiver and it was deaf both in HF and 50 MHz.

Checking the transceiver's schematics, I found both SO-239 HF/50 MHz connectors go to a switching relay, then to the TX-RX relay and then the RX signal is feed to the RF unit, where a pair of antiparallel clamp diodes protect the receiver's input (D1 and D2). After these diodes, there is a DC blocking capacitor (C2) .

So it was clear the 13.8 volts burned one of there clamp diodes, more precisely, D2. Interesting to see the input attenuator (R1 and R2) is also connected directly to the antenna. If the attenuator is turned on, R1 and R2 can also be damaged by DC at the antenna, but this was not my case.

Ham radio equipment manufacturers have the tendency to place the more prone to fail components in the less accessible areas inside the equipment. And this time things will be not different. D1 and D2 are located at the bottom side of the RF unit, so you need to carefully remove some coaxials and a pair of flat ribbon cables. After some minutes removing things there was the faulty / burned / semi-disintegrated diode:

Fortunately the 1SS302 diode is very common in ham radio equipment. Yaesu, Kenwood and Icom use it in several transceivers and receivers so it was easy to find a pair of replacement diodes. Once I removed the burned diode and replaced it with a new one, the transceiver worked again. As I had the RF unit removed from the transceiver, I took the opportunity to do also this modification to try to reduce the frequency drift when the transmitter's fan is turned on.

1 comment:

  1. Qué les hubiera costado colocar otro condensador antes de los diodos. Interesante labor de cirugía y ya puesto, hiciste la otra modificación.