[ICOM] Icom 7800 Problem ?

[Adam Farson]

Hi Harry,

I assume you are referring to the IC-7800 output devices. There are many
possible causes; a parasitic oscillation in the PA, RF feedback into the
PSU, an over-voltage fault in the PSU or severe mis-termination of the PA. A
defective production run of MOSFET output devices could also be responsible.

The PA can be badly mis-terminated when using an external ATU - especially
if the internal autotuner is on whilst tuning the external tuner. This can
result in complex load impedances at certain setting, which wil reflect high
reactances into the LPF and/or PA. This can develop dangerously high RF
voltages in the autotuner, LPF and/or PA, resulting in component failure. In
some cases, the reflectometer at the PA output will not report the mismatch.

To my knowledge, there has been a relatively small incidence of these
failures. I am confident that Icom are taking this failure mode very
seriously; it is the sort of thing that one does not expect in a transceiver
at this price level.

Cheers for now, 73,
Adam VA7OJ/AB4OJ


-----Original Message-----
From: icom-bounces at mailman.qth.net [mailto:icom-bounces at mailman.qth.net] On
Behalf Of we1x at comcast.net
Sent: Thursday, May 03, 2007 04:08
To: ICOM Reflector
Subject: RE: [ICOM] Icom 7800 Problem ?

Adam,

Any insight as to what may be causing the finals to fail?

Harry




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