[Kenwood] TS-850S 15M No ALC & Distortion

[Ken Winterling]

Good evening,

I have an interesting problem for you TS-850S repair experts regarding a
problem on 15M (21mHz).  I will describe it in a moment but first a bit of
background.  I have two TS-850SAT on the bench that belong to a friend of
mine, a good bench tech.  He has tried for a while to resolve the problem,
identical in both rigs, and finally gave them to me to get a "second
opinion" and a fresh approach.  I, too, have been unable to correct the
problem in either of his rigs.  He also owns a third TS-850SAT that has the
same problem but he is using it in his shack and I have not tested it.

The serial numbers of his three radios range from early to late production
and have both the old and new DDS chips.  All three have the internal
antenna tuner.  We also have a few friends who own a TS-850SAT.  All of them
have serial numbers in various ranges and contain old and new DDS chips as
well as internal antenna tuners.   My friend has duplicated his 15M test on
three of their rigs and all but one exhibit the same problem.

Now for the problem and test set up.  All of the rigs, with the one
exception noted above, work properly EXCEPT on 15M.  On that band, unlike
the other bands, if you turn up the mic gain you do not get any ALC
indication on the internal meter and the output power continues to climb.
The transmitted signal becomes quite distorted and broad as monitored on
a separate receiver.

The test equipment set up consists of a 2 tone generator connected to the
front panel mic jack, an oscilloscope monitoring the transmitted RF signal,
and a dummy load connected to the transmitter.   Injecting the 2 tone signal
while the rig is set to 15M and increasing the mic gain produces a test
pattern on the oscilloscope that flat tops and broadens as the mic gain is
turned to maximum.  There is no ALC indication on the rig's meter and the RF
output continues to climb.  On the other bands a point is reached where the
power output does not increase even if the mic gain is advanced to maximum
and an ALC level is indicated on the rig's meter.  This is an indication
that the ALC is working.  The same test has been repeated on all of the
other rigs with the same result with the one notable exception mentioned
above.

 The above behavior leads us to suspect that the ALC is not functioning on
15M but we can not determine the cause.  We do not believe the vintage of
the DDS chips plays a part in this since rigs with both old and new versions
are affected in a like manner.  My friend made an interesting observation
regarding power output.  He noted that if the "all power down" switch on the
RF unit is set to the low power position, approximately 50W, all of the
rigs operate normally on 15M with good ALC action on all bands.  This switch
is common to all bands so we do not see how this should affect operation of
the ALC on just 15M.

We need your help.  If the problem was confined to one rig we would keep
digging, parts swapping, or take the "shotgun" approach.  However, five (5)
rigs have been observed exhibiting the same behavior so we are considering
there may be a design issue.  If you have the proper test gear we would
appreciate it if you would repeat our test on your rig and report your
findings.  If you don't have test gear you can still check ALC action and
power output on 15M compared to 80M or 40M.  Listen to your 15M signal if
you have another receiver.  Report your findings.

Thanks to all for your assistance,

Ken, WA2LBI