[Kenwood] improved ALC transverter out TS-850

Tue Jul 6 06:31:01 EDT 2004

I have a TS-850 used as IF for transverters to 50-144-432 and 1296MHz.
The transverter out is used with the modification "IMPROVED IMD performance
for the TS-850 transverter output", by Dave WA3U later modified by PE1OGF.
The circuit you can see here: http://home.iae.nl/users/pe1ogf/ts-8501.htm
This circuit is a must for control of transverter drive level. Without it
there is no ALC action and high risk for overdrive and horrendous splatter.
However there is a couple of problems. The mod suggested by PE1OGF is a
must, or the ALC will be too slow. The detector input works best with a
small cap.
Next problem is transverter output voltage that is 30-40% higher with SSB
modulation than with CW or whistling in SSB. This was first discovered by
measurements made on my rig by Leif SM5BSZ at the Nordic V/U/SHF meeting. I
measured the output from a 144MHz W2GN Arcos type PA with a Bird signal
sampler and found flat topping with SSB at full power. There was little grid
current on the meter, but that is no excuse for continuing with flat
topping. The problem seems to be in the "ALC" circuit by WA3U. After some
reseach in diagrams I decided that this circuit should replace the circuit
that is missing using transverter output. That is the envelope detector on
the filter board. The circut should not try to be a ALC circuit because that
is already in the TS-850. Looking at the diagram, the 0.01nF capacitor C3
in the feedback loop gives a time constant that will reduce gain within the
modulation frequency range. This imply that there is less VSF signal and
less ALC action with SSB modulation than tuning CW. CW is DC output from
the circuit. That could be the reason why SSB output flat topped.
My modification to the WA3U circuit is to replace the 10nF C3 capacitor with
a 1nF capacitor. Then VSF will have correct feedback of envelope voltage and
there is almost no overdrive and flat topping in SSB. OK there is a little
overdrive with a lot of mic gain and ALC, but my signal is much better off
frequency at full power.
Any comments before modification is sent to www.mods.dk ?
Could not find WA3U's email for his comments.
73 de LA6MV
Helge