[Elecraft] Remote Antenna Switches and K2 Questions

[Dauer, Edward]

Two questions about the K2 and remote antenna switches – particularly, the Ameritron RCS-4.

The K2 I finished building three months ago is finally on the air and, so far, working reliably, with great thanks to Don Wilhelm for his knowledge, skill, and patience.

It is now on its fourth set of PA transistors, plus a raft of other components in the PA circuits.  The story until now had been the same – the rig works fine on Don’s bench; he returns it to me where it works fine with a dummy load on my bench at my city QTH where real antennas are forbidden.  I then take it to my operating QTH where my antennas are, hook it up, and within 20 minutes or less a loud pop and a puff of smoke precede the darkness.  PA transistors blown away again.  Three times, all the same.  Replaced power supplies twice, added cascades of surge suppression on the AC mains, no dice.  Sent the rig back to Don for another round of radiology and surgery.  Thrice.

This time, on a hunch, I removed one thing that differed between the operating QTH and the test bench in the city – an Ameritron RCS-4 remote antenna switch.  With that out of the circuit, so far the rig seems happy.  No pop; no smoke; no darkness; no tears; no round trip back to North Carolina.

Don suspects the problem has been some mismarked component in the base K2 which, though it looked right, was of the wrong value and caused an imbalance between the two matched PA transistor circuits.  On the rig’s last trip to NC Don replaced virtually everything in those circuits.  And now it works OK, so far.

I wondered, though, if the RCS-4 could also be a culprit.  That switch inserts into the coax feed line the AC and DC voltages that energize its remote relays out where the antennas are.  According to the schematic there is no way DC voltage could appear at the input side of the switch console.  But with a DMM reading the Ameritron’s RF input side, I see some.  There are DC transients as high as 5 volts, negative and positive, occurring at odd times and of very short duration.  They have never seemed to affect the K3/KPA/KAT, nor the KX3/KXPA when these rigs are connected to the same RCS-4.

So, two questions:  First, is it possible that these weird transients from the control box could have been the cause of the PA failures?  Has anyone else ever experienced anything like this?  Second, rather than test that hypothesis by hooking the RCS-4 up to the K2 again, a test that could be expensive, I may just look for another remote switching system.  Any suggestions about a replacement?  If I need to use a separate DC voltage cable I’ll have to wait until the spring thaw in order to bury it – but maybe the DC voltage insertion system just isn’t a good idea?

Tnx for any advice anyone can offer,

Ted, KN1CBR