[Kenwood] Ref: TS-850S Driver Bias Problems

[Larry, K4WLS]

I wish "self made" engineers would stop ruining good Kenwood
rigs by tinkering with the driver and final components in order
to reduce ALC to get a few more watts out on SSB, or change
the final bias and screen voltage to get a few more watts out on
CW. Doubling the output power of a rig only results in a signal
increase of 1 S Unit on the receive end anyway (under the most
ideal conditions).

Check schematic or board layout in service manual to determine
the correct value and wattage rating of the darkened resistor. It
has been damaged by heat, and the resistance is probably changing
with heat and/or current. This is prob why the driver bias voltage
is fluctuating. Replace the 100 K Ohm resistor with one of the
correct value and wattage rating as per schematic/service manual.

Before replacing the 100 K Ohm resistor, it might be a good idea
to make sure this resistor was not changed by a Tech Field
Bulletin. Someone on the list who has had a TS-850S for a
good while can verify the correct value of this resistor.

After doing this, remove the 12BY7A Driver tube, turn on the
power and feel both resistors for a few mins to make sure they
are not getting excessively hot. If OK, replace Driver tube, and
do the same thing. If OK, check the driver bias and see if it OK.
Turn on Fil switch to finals, set Drive Control fully CCW.
Key in CW and adjust the Final Bias for the correct idling
current.

With covers still off, connect rig to dummy load, Mode to CW,
and 20M. Tune the rig with low power, and slowly increase
Drive control to around 180 mA of plate (cathode) current.
Re-check Plate Dip. Keep your nose close to the Driver board,
spot check how warm the resistors get while sending V's (be
careful and do not get a jolt). When not spot checking the
heat of the resistors, keep your fingers crossed that someone
did not change the value of these two resistors in an attempt
to correct a problem.

GL Pal - You Might Need It,   Larry   K4WLS