[Elecraft] KPA500 Issue

[kd4iz at frawg.org]

Jim, All,

James Bennett<w6jhb at mac.com> Wrote:

> However, what I?m seeing is that when the amp is in OPER mode, my receive signal strength drops way, way off to the point I have to go full AF volume to hear signals. If I put the amp in STBY, signal levels go back to normal. I also notice that the amp fan doesn?t take too long to crank up to high speed, and the PA temp is sitting around 67 C.

I occasionally use an IC-705 to drive either a KXPA100, a KPA500/KAT500 pair, or both in cascade. I have a keyline splitter (the one for the KX3 to the KXPA) that allows me to cascade the KXPA100 to the KPA500. 

After I accidentally popped ~100W into the input of the KPA500 while experimenting with the switching arrangement, my system suddenly had the identical problem Jim describes, a much attenuated signal on receive and normal transmit. It was R9. Taking the amp apart was much more involved than replacing the resistor. I found R9 split in two and the two halves standing (yes, really) on the solder pads. (photo available on request).

The very kind folk at the mothership sent me a replacement for R9, a surface mount 50 ohm resistor, and it was an easy repair. I have everything set up with switching so I don't do it again. 

Now, when I put the IC-705 into the "cascade" mix, I set the rig output power to 5% which results in a 25-30W output from the KXPA100. 30W will drive the KPA500 to full power. I also use a sequential check list every time I bring the IC-705 into the shack to avoid the uncomfortable "oops" moment again. 

Note: make sure that the keyline that goes to the IC-705 has a 480 ohm resistor in series with the keyline jack tip when using a KXPA100... without it you can't use the IC-705 voice memories or the "DV TX Message" data in DR/DV mode without having the rig lock up in a transmit key up loop. The series resistor seems to have no effect on the ability of the 705 to key the KPA500. Please ask me offline if you have any additional questions about this setup. It has been stable and predictable ever since the repair was done 2 years back.

Jack Spitznagel – KD4IZ
kd4iz at arrl.net