----- Original Message -----
From: "Donley" <donley...@comcast.net>
To: <k4...@mindspring.com>; "drakelist"
<Drakelist@zerobeat.net>
Sent: Wednesday, July 13, 2011 7:29 PM
Subject: Re: [Drakelist] TR-4C Bias Adjustment
Garey , Richard I, et al
Since I am the student here, I can only be Richard II.
I measured the resistance of R45, in circuit with the
relay pulled. I'm not sure that made any difference. It
measured 2.4 ohms on a Fluke 179.
Since it is difficult to adjust for MAX or MIN on a DMM, I
changed to a Simpson 260 (I know, maybe not too accurate).
Then set SIDEBAND X, MODE X-CW cranked in some XMTR gain
and tuned PLATE for MAX on the PLATE CURRENT meter
(because tuning for MIN didn't work). Max reading on the
PLATE CURRENT meter again occurs about mid-range and
falls off on both sides. The PLATE METER read 200 mA and
the voltage on R45 was 0.46 VDC. Since R45 measured 2.4
ohms from before, the calculated current is about 192 mA.
Close to the 200 mA on the PLATE METER.
Then I tried tuning PLATE for MIN voltage on R45 with the
Simpson. I could not tune to MIN, there was a MAX value
about mid-range and it falls off on either side of MAX.
The voltage on R45 was very close to 0.46 VDC, the same as
above. Again, the calculated current was about 192 mA.
Plate current or RF output?
I checked the slide switch on the LOAD control. I took
resistance measurements in each position and it seems to
be working. I added a little DeOxit.
I looked at the circuitry for RF output on the relay board
( I have most of Garey's Drake CD's). If the problem is in
there, it is going to be a real pain to get to. I hope
someone comes up with the "Golden Screwdriver" solution.
When I was working, our solutions were "Silver Bullets".
Of course, that was before the price of silver
skyrocketed.
Richard II
I guess that makes me Richard the Lionheart. The
Simpson 260 is fine, these measurements do not need
laboratory accuracy and I think continuous reading
instruments are much better for this kind of measurement.
There has to be some way of determining what the plate
current is really doing. Since the Simpson has curent ranges
it could be put in line with the plate supply line to the
tubes. I think at the outside part of the feedthrough cap. I
may have a downloaded schematic for this thing but have not
looked at it so I don't know what the resistor designations
are. At any rate, if you can measure the actual _plate_
current, rather than cathode current, as I think Garrey
suggested, it would tell immediately if the meter is telling
the truth. If it is there is something very strange going
on. The symptoms are of the final tube oscillating. This is
common on the high bands due to poor neutralization but
should be rare at low frequencies because the coupling
through the tube interelectrode capacitances will be lower.
Nonetheless oscillation can take place. If you have access
to a high frequency oscilloscope, or even better a spectrum
analyser, you can see what is going out the antenna port.
Operate the TX into the dummy load and use a sampling loop
of a couple of turns to pick up some RF from the cage.
Also, check up on the dummy load you are using if you
have not already. Most of these loads are direct coupled to
the connector so a resistance reading will show if the
resistance of the load is way off or if its shorted or open.
If its not really a resistor any more it could explain the
problem. Also, do the tube plates show any color, if they do
you probably have oscillation. Once it cured you will have
to replace the tubes. I am grasping at straws here because
transmitters just don't behave this way. Something is being
missed. If the thing really is doing what the panel meter
says I would also look at the coils and condensers of the
final tuning circuit to make sure they are still doing what
they are supposed to do. Begin with a _very_ thorough visual
inspection. Disconnect the power supply while doing this. At
the least the voltages in the final can give you a very
nasty zap and at worst put you in the graveyard. Don't make
the mistake of thinking that because this is relatively low
power ham gear that it can't fry you as effectively as that
thing at Sing Sing.
Lets FIX this thing!
--
Richard Knoppow
Los Angeles
WB6KBL
dickb...@ix.netcom.com
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