[Elecraft] K3S KPA3A No Power over 12W

[Michael Carter]

Hi Mike,

I'm glad you were able to test receiver sensitivity loss again when the KPA3A is engaged. If you have an XG3 or any reasonable signal generator with output level around -73 dBm (S9 equivalent), you should be able to confirm sensitivity on each band.  Your tests indicate that diodes D2 and D3 in the T/R switch on the KPA3A module are working properly and that the 7R voltage switching is OK. That doesn't resolve any possible 7T voltage switching issue as D5 and D8 depend on that circuitry to be switched on.

As to running the KPA3A separated from the K3s base, I have not attempted to do it.  Looking at the several voltages that need to be supplied from the base K3s as well as the high current 13.8V DC to the PA stage itself, it would be tedious to make up the requisite cables.  It would appear that you could separate the KPAIO3A board from the PA board so that rig front panel controls would still be operative, and make up only a cable for the J68A and J68B connections to P68A and P68B, respectively.  I wouldn't drive the PA with RF, but just use the DC supplies to check the PA board voltages.

Before doing any of that, I would pull the KPA3A, leave the KPAIO3A in place (or not), and check the integrity of the gate terminals on PA finals Q7 and Q8.  If the gate oxide is intact, you should measure perhaps 250 - 400 ohms to DC negative bus, and that presumes that the Q11 Darlington turns on.  Any resistance reading much less than that value on either gate indicates a gate-channel short.

73,
Mike, K8CN
P.S. The hold-down L-bracket is not essential unless you travel a lot with your K3s.  It secures the KPAIO3A and KPA3A in the RF board sockets so that the modules do not become dislodged by vibration or sudden mechanical shock (think airline baggage handlers tossing your K3s onto the conveyor belt).