TS-940S 

           RECIPROCAL MIXING NOISE


In early March I talked to someone at International Radio
regarding the reciprocal mixing noise problem with the
Kenwood TS-940S.  I had been in the process of installing the
fix described in your newsletter (late 1986 issue) and found
that it had already been installed on my TS-940S (S/N
7100269).  The factory installation had one problem, the
resistors used for R120/R129 were color coded for 900 ohm
(close enough to the 1K in the newsletter), but in actuality
measured 465 ohms.  Apparently, Kenwood had gotten a bad
batch of resistors from some supplier and had not dicovered
the problem.

At the time you indicated that was the first report you had
received of the resistor value problem and recommended that I
contact Kenwood. I called them and was told that they had not
heard of the problem before.  They also stated that a new fix
for the reciprocal mixing noise problem had been developed
and was described in a Service Bulletin dated March 2, l987.
I requested a copy of the bulletin and have attached a copy
of it to this letter for your information.  (See Issue No.
76, Pg. 30 and 31 for Kenwood Service Bulletin No. 917 and
schematics pertaining to this subject.)

Since receiving the bulletin from Kenwood, I have installed
it on my TS-940S and found it to make a very significant
improvement in weak signal handling in the presence of nearby
strong signals.  I would recommend highly that anyone
experiencing receprocal mixing problems install the new fix.
It should be noted that some of the newer TS-940S have the
fix installed.  I was preparing to install the fix on a
friends's TS-940 which had a serial number 100 lower than my
own and found that the fix had been factory installed.
Apparently, more than one manufacturing site is used and
serial numbers are given to each in blocks.  Consequently, it
is possible for higher serial numbers to be produced at one
location without the fix, while another site any have cut in
the fix but is using numbers from a lower block.

The quickest way to verify whether the fix has been installed
is to check R120 and R129 on the PLL Unit (X50-2020-00).  If
these two resistors are 3.3 ohms in value, the fix is already
installed.  Do not depend on the on the absence of C176, C180
or C181 as an indication, as earlier attempts (factory or
field) to correct the mixing noise problem may have removed
these same capacitors.  The instructions in the bulletin
state that when making the modifications to the RF Unit (X44-
1660-00), it is easiest to move C132/C133 to the foil side of
the board.  As the component side of the section of the RF
Unit containing these two capacitors has been filled with
wax, it is definitely not easier.  The factory installation
of the fix left C132/C133 on the component side and
insatalled the R154/C193 and R155/C194 series RC networks on
the foil side.  This is definitely easier.  As a side note,
the instalation of the fix took me about 2 hours.  Both the
PLL Unit and RF Unit modifications must be completed before
the transceiver is usable.  If you install just the PLL Unit
modifications and then try the receiver, CW signals will
sould like raw AC.  Also, to make life simple, do not remove
each of the boards above the PLL Unit individually.  The easy
way to gain access to the PLL Unit is to remove the top two
screws (one on each side) holding the front panel and loosen
the bottom tow screws.  This allows the front panel to be
tilted forqard.  The speaker assembly and all the boards
above the PLL unit may then be removed as a unit by removing
only 4 screws and tilting this unit towards the front of the
TS-940S.  No cables need be removed from the boards above the
PLL Unit.

I hope the above information is helpful to you in dealing
with the reciprocal mixing noise problem.  (Thanks, Rich
Maher, WZ4Z, 1117 NW 7th St., Boynton Beach, FL 33435)
      RELAYED   BY...N0KGX...GENE