[Hallicrafters] Close Call With a SX-100

[Bill Stewart]

I recently built a wooden rack to house a SX-100 and several transmitters. I rigged up the control wiring so I could use a common Dow type (actually an Advance relay) T/R relay. First tests of the SX-100 and a Viking 2 combo thru the switching worked fine. I went back to dress up the wires and to add an elbow coax connector to the common connector on the end of the T/R relay and then sat down to enjoy my great switching circuit. Upon keying the Viking 2 I noticed the loading was way off and no output on the swr meter....also noticed the SX-100 was now dead as a hammer on 7 Mc (where I was loaded up)...other ranges were ok. On scratching my head and undoing some of my coax wiring I found that when I had screwed on the elbow, I had twisted the center contact lever on the T/R relay causing it to short across the transmit and receive connectors thus putting about 120 watts of RF into the SX-100. I found the ant. coil for the 7 MC range to be open and saw on some wires close by the coil the tell-tale black mark of a flash-over. I was lucky in that the burn-out occurred about 1/4 inch from the connection and was able to solder the burnt wires back together. The rcvr is back to normal now...whew!!.

I hope my experience will help others to be careful in dow-key type installations. I'm considering replacing it with an open frame type relay. This also brings up a question of antenna input protection. What type of back-to-back diodes should be used at the antenna input of a receiver? Also, could a very small amperage fuse be used in series with the antenna input(such as used in a DMM). Thank goodness it wasn't a modern SS rcvr...no telling what would have fried.....
Thanks for any comments....73...Bill K4JYS