[BC] New SX 5 fun

[Mike McCarthy]

Here's another head scratcher for our SX-5. After we blew a C-1 
doorknob...TX needed some fine L1-C1 match adjustment.

We just went through the output network with a network analyzer and made 
sure it was dead nuts on per the OEM data.  It is...all stages. Should be 
22-j5.  Within 0.2 ohm R and 0.5 ohms X.  Harris says this is fine and 
confirmed the R-X data with several other TX's on the same or adjacent 
channels.  Common point at the connector is 50-J0 (never mind the rotation 
for the moment) and 15Khz bandwidth is <1.3:1. Dummy load is 47-J0 cold and 
49.8-J0 when warm and better than 1.05:1 at 30 Khz...as it should 
be.....   So we KNOW the loads are fine.

Operating characteristics when the TX is "cold":
TX is still not able to make full power and efficiency is something less 
than 70% into either load. Tops out the adjustment ranges at 3KW with 110V 
and 55A with or without audio. The B+ is 270V at all power levels, common 
point meter coincides with the TX TPO. All modules are active, fuses for 
everything are fine, detailed meters on the display are nominal for 3KW 
(except for the RF power parmaters) and there is sufficient drive on all PA 
modules (30V).   FWIW, this was the operating character of the TX before 
the doorknob blew...and it played this way for years....right or wrong.

FWIW, the OEM data has only one set of readings...rated TPO and nothing for 
the nominal TPO despite them listing the power settings showing the 
licensed TPO.

Now...As the TX warms, operating conditions change...operating at 2.7KW.

Metered PA-I with no audio eventually goes to 55A regardless of power 
level. Power and voltage remain the same as does the common point 
meter.  All other displayed meter indications are also nominal and within 
expected ranges at all power levels.

With modulation applied, the PA-I meter resembles a gain reduction meter 
"indicating" negative carrier shift down to the PA-I current one would 
expect for that power given the above "cold" conditions.  Power, PA-E and 
the common point meter all show continued concurrent operation including 
expected positive peaks with modulation at all power settings.  The lower 
the TPO, the deeper the swings in PA-I meter indications always returning 
to the 55A point at dead carrier.

At shut down, the following is observed:

The LARGE coil (I want to say L-2) is VERY warm to hot in the active 
turns.  I understand the coil is big for a reason...lots of current 
flowing.  But the temperature I'm feeling is warmer by some amount than 
normal.  The various capacitors are all cold or only mildly warm.
The "TUNING" coil is also VERY warm.  It's about 5 turns from the 
top...which is where Harris advises the coil should be when tuned properly.
PA-4 tends to run about 5 deg. C warmer than the others at about 44 deg. C 
at 2.5 KW. Always has.....

Harris says the PA-I meter might be impacted by a current sense module on 
the power control board.  Any one else ever have this occur? Bigger 
question is simply...why won't the box make rated power?

It sounds fine on the air, passes NRSC and there is nothing apparent beyond 
the mask which would indicate a flying TX. Has anyone seen the box go into 
parasitic oscillation?  If so, where to start and what to look for?

MM