[Kenwood] TS-120S Additional questions

[Clif]

One more time....

Replace D2, D5, and D8 on the RF unit.

Clif Holland, KA5IPF
AVVid
Authorized Kenwood and Icom Amateur Service
www.avvid.com
1-800-214-5779


----- Original Message -----
From: "Dennis Berry" <dennisberry@worldnet.att.net>
To: <kenwood@mailman.qth.net>
Sent: Monday, March 25, 2002 11:15 PM
Subject: [Kenwood] TS-120S Additional questions


> 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
>
> - - -
>
> Your moderator for this list is:
> Larry Wilson KE1HZ kenwood-owner@mailman.qth.net
>
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