"Thomas Wright" <[EMAIL PROTECTED]> made an utterance to the drakelist gang
----------------------------------------------------------------------
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
----------------------------------------------------------------------
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
----------------------------------------------------------------------
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
"Thomas Wright" <[EMAIL PROTECTED]> made an utterance to the drakelist
gang
----------------------------------------------------------------------
I assume you mean finals. I have a set but they're weak.
----- Original Message ----- From: "john" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Saturday, December 01, 2007 1:43 PM
Subject: Re: [drakelist] AC3 Bias
john <[EMAIL PROTECTED]> made an utterance to the drakelist gang
----------------------------------------------------------------------
Sounds like the PA might be oscillating on 20M. Do you have another set
of tubes to try, to see if behaves differently?
John
----------------------------------------------------------------------
Submissions: [EMAIL PROTECTED]
Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body
Hopelessly Lost: [EMAIL PROTECTED] - help in body of message
Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net
----------------------------------------------------------------------
----------------------------------------------------------------------
Submissions: [EMAIL PROTECTED]
Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body
Hopelessly Lost: [EMAIL PROTECTED] - help in body of message
Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net
----------------------------------------------------------------------
