"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
"Thomas Wright" <[EMAIL PROTECTED]> made an utterance to the drakelist
gang
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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
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Sounds like the PA might be oscillating on 20M. Do you have another set
of tubes to try, to see if behaves differently?
John
At 12:52 PM 12/1/2007, you wrote:
"Thomas Wright" <[EMAIL PROTECTED]> made an utterance to the drakelist
gang
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Garey:
What I found taught me a lesson and has me confused. After rechecking
the PS, and reconnecting to the TR4C, I realized I hadn't tried bias
adjustment on anywhere other than 20m. The bias acts normally on all
other bands, but on 14Mhz, the bias won't go below .25 amp, the rf tune
control has no effect on output, the plate current won't dip, and when
the mode switch is ccw, as on LSB, the PA output is still high. This has
me totally confused. The rig acts correctly on all other bands....even
gives me 140 watts on 28Mhz. Any ideas?
Tom
----- Original Message ----- From: "Garey Barrell" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Friday, November 30, 2007 3:47 PM
Subject: Re: [drakelist] AC3 Bias
Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist
gang
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Tom -
OK. The specified load for the Bias supply is ~ 30k, so a load of this
value would emulate the load of the transceiver.
The bias goes to the PA via the _middle_ feedthrough on the side of
the PA shield under the chassis. These feedthroughs are fragile and
easily cracked if too much force is applied to the terminals. The
"other" points in the transceiver that are tied to the Bias supply are
all very high impedance, so a short on one of those is unlikely to cause
as big a difference as you are seeing.
Another "unlikely" fault would be a Grid-Cathode short in one of the
final tubes.
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 measured the bias while disconnected from the transceiver, and when
swinging the pot from stop to stop it seemed to vary the minus voltage
linearly with no spikes, but of course the Fluke may dampen such a
spike if it's a fast one. I'm thinking it may be one of the two
resistors opening up under heat or current (6.8K and 10K). The pot
measures correct resistance from one end to the other. When connected
to the rig the current was either very low or very high, but when
disconnected, the voltage seems vernier as it should. I thought
that -52 volts or so may be too high a bias voltage, especially if
that's as low as it will go. Thanks for you continued help.
Tom
----- Original Message ----- From: "Garey Barrell"
<[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Friday, November 30, 2007 3:13 PM
Subject: Re: [drakelist] AC3 Bias
Garey Barrell <[EMAIL PROTECTED]> made an utterance to the
drakelist gang
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Thomas -
I just re-read your message......
If the IDLE current is either Zero or 0.4A, with little adjustment,
I'd suspect the potentiometer has a break or "bad spot" on the
resistive element.
That behavior indicates that the Bias voltage is either some high
value, such as 70-80 Negative volts, cutting the final tubes off
completely, or somewhere around 30-40 Negative volts, allowing some
400 ma of idle current, depending upon which side of the break the
wiper is on.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B & C-Line Service Supplement CDs
<www.k4oah.com>
Thomas Wright wrote:
Maybe you guys can direct me a little. My AC3 has been showing signs
of bad caps, especially in the bias section ( one of the bias caps
measured at 700MFD). I replaced all the caps in all sections, using
33MFD 160 volt caps in the bias section, and while I was there did
the diodes. After reassembly the bias was adjustable from 41 or so to
80 volts. TR-4C was running well. Today after operating for an hour
or so, the transmitter started to show very high plate current, so I
checked the bias and found that it was either zero or above .4 amps,
without much adjustment between the two, and of course when higher
the xmit relay was engaging. Reducing xmit gain would not reduce the
plate current. I measured the voltage at the plug and the bias is
now only adjustable from 52 to 94 volts, which I think is too high.
There's not much else to replace in the AC3 except the bias pot and
two resistors, both of which measure within tolerance statically.
Any thoughts?
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