"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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