"Thomas Wright" <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Garey:
I think we're saying the same thing. Using two new tubes and each one of
the bad tubes to make the trio, I found that all three of the bad tubes
produced the same effect when each was used in combination with the two good
tubes. I kept exchanging one of the bad tubes in the trio and each time got
the same oscillating. This occurred in both transceivers. The only time it
functioned properly was when I combined the two new tubes with an older tube
which was weak. Then both radios performed normally. The strange thing is
that after I peak the rf tune control for max s-level or noise, when tuning
the transmitter I find that the current and output peaks at a completely
different spot when I turn the rf tune control to peak current. And current
peaks when output peaks, of course, but it won't dip. The xmtr gain control
will not reduce output or current once the load and plate controls are
maxed. And rotating between sidebands makes no difference after the
transmitter is tunes either. Both USB and LSB produce high output. I can't
say what I'm looking for, but logic suggests that the PS must have something
to do with this. Last night I checked at the PS connector and found normal
bias and high voltages (although I get the feeling that the high side is a
bit high), and I found less than .6 volts of AC at any of the three sides.
I've got three new finals on the way, but I have no expectation that those
will act any differently. I did check bias voltages with the transceiver on
and found them normal at all three final sockets. I'll keep looking. And
thanks for your continued comments.
Tom
----- Original Message -----
From: "Garey Barrell" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Monday, December 03, 2007 10:37 PM
Subject: Re: [drakelist] AC3 Bias
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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