"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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