Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Tom -
OK. Just wanted to be sure we were on the same page! :-)
It sure sounds like an oscillation in the PA stage. The only thing that
comes to mind is when you have one of the older tubes in there, the
maximum plate current is lower. The fact that the max current and
output peak at a different point than the max receive certainly
indicates that the final is at a different frequency than the receiver.
This is also reinforced by the fact that the Pi-net won't resonate,
indicating that the frequency is outside the range of the 20M band position.
Have you tried to find the oscillation frequency by listening on a
general coverage receiver? Knowing that frequency might give us a
clue. The fact that the RF TUNE control loses control of the output
indicates that the PA is oscillating without drive, once it is kicked
off by some initial drive.
This is a weird one..... Anyone else has an idea, speak up!! I
thought Gary was on to something until we found that the problem follows
the tubes in two separate transceivers. It's looking more and more
like the problem is with the GOOD tube(s)!
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:
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
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