[Kenwood] TS-120S Additional questions

[Dennis Berry]

First, thanks to all who respond to my question on the TS-120S concerning
the carrier adjust control and power out.  I wanted to post what I have
found to date to see if there are more ideas.

The problem is that the rig outputs max power in both ssb and cw, all bands,
and the carrier control is almost ineffective in controlling the power out.
I say almost because if I turn the knob rapidly to the left (decrease the
carrier), I see the power dip down, but then climbs right back to max
output.

Here is what I have confirmed to myself.  (To those without a schematic,
this may not make much sense, sorry)

1.  The GEN output of the AF GEN unit appears to vary as the carrier knob
does.  So I do believe this is telling me that the carrier pot is working
properly (I also measure the pot directly and it varies from 0-9v over the
range) and that Q12 and the preceding diodes appear to be working properly.
The manual seemed to indicate that there was a feedback circuit as part of
the protection circuit that goes into Q12, but my schematic did not validate
that.  In fact, it appears the PRO (protection) pin on the interconnect is
actually ground, applying a fixed bias to the gate.  Can/has anyone
confirmed that also?

2.  I applied a negative voltage to the external ALC input, and was able to
vary the output using this method.  I tried this to a) see if the
ALC/Protection circuit in the filter unit was doing something strange (which
I don't think it is) and b) to see if the signal from the IF unit forward
'appeared' to be working OK.  I think this quick test helped me answer that
the RF path in general was working from the IF unit on.

3.  Where I seem to be focusing on is in the actual IF unit.  The ALC test
above applied voltage to a second gate in a mosfet.  The GEN output from the
AF GEN unit goes thru a diode switch on the IF unit, thru the SSB or CW
filter, and then thru chain Q1, Q2, and Q3.  This is where the signals got a
little strange acting and I don't know how to interpret what happened.  For
those that may have a manual, or have been there, this is what I saw.  If I
put a scope probe (or DMM) at the drain of Q1, the circuit would behave
properly, ie the carrier control worked.  As soon as the lead was removed,
it was back to it's old self.  It appeared as if the loading of the probe
had a definite affect.  If I went to the gate of Q2 (output of Q1 to input
of Q2), it had no affect and the radio was still ill behaved.  I could see
that decreasing the carrier would decrease the signal, but then it would
climb right back up.  All of the G1, G2, D and S voltages looked OK on the
devices.  It did not appear any were shorted or open from following the
signals with a scope and voltmeter readings.

The ALC voltage during this time was about 2.1 volts (positive) which
appeared to be correct.  This voltage was going to the second control gate
of the amplifier chain and appeared normal.  If this should be a negative
voltage, could someone let me know that too.

I have not worked with dual mosfets to know what the additional loading may
do to one.  Would a defective one behave like that?  Any other ideas from
someone who has tackled this problem before?  As I said, I got many good
ideas.  I have cleaned ground connections, relays appear to be working, etc.

Thanks to all who provide tips,

    Dennis, NU8S