[ICOM] 7546 Pro II

[Adam Farson]

Hi Tony,

Heat-sensing thermistor R50 only controls the cooling fan (IC-756Pro2
service manual, section 3-2-10, p. 3-6). The fact that the VSWR rose sharply
suggests that the RF signal path between the LPF output and the antenna
socket "went open" as a result of the heat. Perhaps the dummy load developed
an open circuit as it heated up during the test. Another possibility Is a
relay or a coax jumper in the transmitter output signal path. Was the ATU
engaged during the test? If it was, you could re-try the test with the ATU
off.

I would suggest that you either repeat the test with a different 50-ohm
termination, or place a reflectometer in the line between the radio and the
dummy load you previously tested. This will enable you to determine whether
the load "opens up". If the fault recurs and is not due to the load, you
will need to troubleshoot the RF output path inside the radio. 

It is quite normal for the PA to run hot after transmitting continuously at
100W full-carrier output (e.g. RTTY or FM). I noticed this in the course of
2-tone transmitter IMD testing on my Pro2 and amplifier.

Hope this helps.

Cheers for now, 73,
Adam VA7OJ/AB4OJ



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