Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Thomas -
OK. I'm not sure you did what I asked. Let me try again...
I believe I understood you to say that the "20M thing" happens in
_both_ transmitters when the "good" tubes are installed. What I want
to try is with two of the "good" tubes and one of the "bad" tubes, what
happens. If the problem is still there, then leave the two good tubes
and put in a _different_ one of the bad tubes. If the problem still
exists, again leave the two good tubes and put in the third of the bad
tubes. What we're trying to determine is if just ONE of the bad tubes
is really bad, or if the situation is that two good tubes with any one
of the bad tubes results in the oscillation or whatever we're seeing.
You also need to check the neutralization with each tube change. It
doesn't have to be perfect, but you should be able to get close enough
on 20M pretty easily.
The voltages you are seeing are certainly in the ballpark. The HV will
go to 700 VDC with no load, and should be around 650V at full load of
~350 mA.
There's also the possibility that one of the "good" tubes is actually
the problem, but I can't think what it might be! :-) Do you have one
other "good" 6JB6 that you could rotate through the "good" trio?
I thought Gary might have a clue, but I agree that it's unlikely that
both transceivers have a bad capacitor resulting in the same symptoms
with one set of tubes and not the other.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B & C-Line Service Supplement CDs
<www.k4oah.com>
Thomas Wright wrote:
"Thomas Wright" <[EMAIL PROTECTED]> made an utterance to the
drakelist gang
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Garey:
The saga continues. This is getting weird. Back to the "unable to
control the transmit, and unable to dip the plate current on 20
meters" thing. Using your advice I used two new tubes and one of the
oscillating tubes on both radios (TR-4 and a TR-4C). The same symptom
is there using two new tubes and one of the oscillating tubes. The
only way I can get this thing to act normally is if I include one
older tube that is only producing about 50% output. Any combination
of three strong tubes in either radio causes the problem. Now, the
only other thing in common is the power supply I just recapped and
redioded. No load bias voltage is approx -45-85, low volt is 257, and
high volt is a little over 700. At the PA feedthrough under the
chassis when idling the voltage was also around 720. Could too high a
voltage on the high section cause these tubes to oscillate on higher
frequencies? I can't believe that I've got two transceivers that
suddenly caught the same bug. WAY too much of a coincidence. Tonight
I plan to open the PS and check everything I did, but if you have any
thoughts I'd sure like to hear them.
And to Gary P...thanks for the comments. I tried replacing the driver
tube with a new one, and measured the capacitance of the capacitors
you mentioned, and they were good. The new tube had no change.
Thanks guys.
----- Original Message ----- From: "Garey Barrell" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Sunday, December 02, 2007 1:15 PM
Subject: Re: [drakelist] AC3 Bias
Garey Barrell <[EMAIL PROTECTED]> made an utterance to the
drakelist gang
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Tom -
It sounds like you have an oscillation in the final caused by a
defect in one of the tubes. It's a little puzzling why it only shows
on 20M, but probably has to do with the neutralization circuitry and
the defective tube. The fact that the plate current won't dip
implies that the frequency of the oscillation is outside the range of
the pi-net when set for 20M. You might be able to find it with an
external general coverage receiver, but it's really not that important.
I would expect one of the defective trio of tubes to show some color
in the plate, since that one is probably drawing the excess plate
current. If not, what I would do is take one of the tubes in the
"good" trio, and swap it with one of the tubes in the "bad" trio. If
the oscillation is still evident, replace the tube removed from the
"bad" trio and try another one until you find which one is the cause.
If the two tubes in the "bad" trio are otherwise in good shape, you
can probably get a good new one and it will be a close enough match.
You can check for "match" by putting in one tube at a time and noting
the plate current at idle. It should be about 33 mA for one tube.
More accurate would be to measure the voltage across the 15 ohm
cathode resistor of the socket that the one tube is plugged into.
Swap in each tube into the same socket and note the plate current of
each _without adjusting the bias voltage_ and select the three
that are closest in current. Hopefully they will be within a couple
- three mA of each other. This is not a "real" matching, but will be
close enough for all practical purposes.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B & C-Line Service Supplement CDs
<www.k4oah.com>
Thomas Wright wrote:
"Thomas Wright" <[EMAIL PROTECTED]> made an utterance to the
drakelist gang
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John:
Well, I tried swapping finals. In fact I swapped with my TR-4
finals, which are weak. The finals that were in the TR-4C that was
acting up do the same thing in the TR-4, and vice versa. When in
the TR-4 the tubes produce very good outputs on all bands except 20,
just like they do in the TR-4C. Obviously the problem is the
Sylvania 6JB6's, or at least one of them. 20 meters simply won't
dip on either radio when it has those tubes and output stays very
high even when reducing xmit gain to full CCW. Can someone explain
that to me? Do I sense that the only fix is new finals? I hate to
give up such a strong trio.
Tom
----- Original Message ----- From: "Thomas Wright"
<[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Saturday, December 01, 2007 2:44 PM
Subject: Re: [drakelist] AC3 Bias
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