I'd say the switching FETs have failed. Correctly operating, there is
no measurable attenuation in the receive line when in OPER or STBY modes.
As to T/R switching, make sure the amp is in receive and not in
transmit when your radio is in receive. Any of the items in the amp
display preceded with a * indicates the amp is in transmit mode. If
so, something is pulling the KEY line to ground.
73
Bob, K4TAX
Message: 1
Date: Sun, 18 Apr 2021 17:36:48 -0400
From:<
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To:<
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Subject: [Elecraft] KPA500 Question
Message-ID:<001001d7349a$f0a01570$d1e04050$@gmail.com>
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I just purchased a used KPA500 (SN 0456) from a close friend. The amp was
just back to the factory for several repairs and was thoroughly tested. The
report reads, "KPA500 #0456 meets or exceeds all factory specifications." My
friend unboxed the amp and made several QSOs to be sure all issues were
fixed. He then personally delivered the amp to my QTH.
Over the weekend I got the amp added to my operating position and was quite
surprised to find that input signals are attenuated significantly when the
amp is switched from STBY to OPER. No, this is NOT the issue with having the
aux cable and the PA key line connected at the same time. This is with NO PA
key, and NO aux cable connected. The amp has no connection but RF input and
output. It is independent of band. This is with a matched antenna, band
switched manually with the front panel buttons. When I engage OPER the input
signal is attenuated by ~20 dB.
On the TX side, the amp is fine. On most bands I get full rated output into
a dummy load with approx. 20 Watts of drive from the K3.
I noticed on the service report, one of the tests was, "Tested receive
attenuation and IMD." I'm curious about "receive attenuation." This would
seem to be related to my problem. Looking at the block diagram, I surmise
something in the TR switch is malfunctioning.
Before I bug an already-overloaded Elecraft support, I am open to
suggestions. Am I missing something obvious?
TIA es 73,
Jim - WS6X
P.S. Yes, I have read the manual, and scoured the archives. The only thing I
can find regarding input attenuation in the OPER mode involves incorrect
cabling.
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