Receive signal drops when KPA500 is selected to “Operate”.

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I am operating the Flex 6400 with the Elecraft KPA500 and KAT500. With the KPA500 in “Standby” the receive signal strength is normal; however, when the KPA500 is selected to “Operate” the receive signal disappears as if the antenna is disconnected by switching the amplifier to Operate. The cabling appears to be correct. The KPA500 and KAT500 seamlessly track band changes and automatic antenna selections properly as the bands are changed in the Flex 6400. What is the cause of this problem? And what corrective action is necessary? Thank you! Scott
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Scott W. Lowe

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Posted 3 months ago

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Clay N9IO

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Sounding like a relay issue somewhere in the KAT500.
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Hi Scott, I just had this exact problem here in the KPA500 and my and FLEX6600M 
The circuit in the KPA500 on the "TR Switch board" does NOT use relays for the Receive patch but uses PIN diodes @ D7, D8 and D9 and are "S1M" part number and D10 is a 1N5408. 
I put my AMP on my Test bench and checked the diodes with my Huntron tracker and found NO bad Diodes 
BUT what controls these diodes is R7 and R6 a 51 OHM resisters, R7 had gone bad (Check resistance with power off) between TP4 and Cathode junction of D8 and D9, this should approx. 60 Ohms. 

The resistor and D7,D8,D9 and R7 are surface mount 

If your not into circuit  board level component repair I would send this in for the repair but with 45 years of experience this was easy to fix for me at least. 

If you want to visit about this some more my email is  my


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Gary Sheridan

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I have had a similar issue and I resolved it by shutting the amp off then powering it back up.After a "reset" it operated normal. Sometimes when a high SWR occurs it faults and just needs a reset. 
Let us know how you make out. 
Gary N1RNJ
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Pat - WH6HI

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I had a similar incident that caused harm to the TX/RX circuits.  In my case the cause of failure was my keying line not switching, causing excessive RF on the RX side of the switch.  There is a selection in the KPA500 settings that should keep it from happening.  I can’t find it right now. So can’t give a proper reference.  The self protection in the amp cuts in, and you can get away with the high power condition a few times, but be warned that it will eventually fail.  I am using the KPA500 as a spare amp now, as I upgraded to 2K-FA.