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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