Richard -
Strange. We must be missing something obvious. IF there is Plate and Screen voltage applied to the
tubes, and the Cathode is grounded, there WILL be Cathode current, regardless of anything else in
the radio. This current will be whatever the power supply can put out until something breaks,
either the tubes melt, or the fuse blows, or ... :-) This current is LIMITED by the BIAS voltage,
with about -70V to cut off the current completely, and around -60V to limit it to 0.100A 'IDLE'
current. Again, this is regardless of anything else in the radio.
I think your problem is that the relay (which grounds the Cathodes) is not doing that with the MODE
switch CCW, but appears to be with it set to X-CW. I don't see any relationship between the SSB
'Filter' switch and the keying system. It merely selects which of the two filters is in use, and
controls the two lights indicating which sideband is selected.
Have you subbed the 6EV7 tube?
Can you hear the relay closing with the switches set for BIAS setting? The Cathodes are grounded by
a single set of relay contacts, directly to chassis.
Again, if the voltages are on the PA tubes and the Cathodes are grounded, the PA WILL draw current,
regardless of anything else. I guess next is to check the PA voltages when the switches are set for
BIAS setting per the manual. They can all be measured at the underchassis PA shield. The rearmost
terminal is the Cathode lead and should be ~ 0 V when keyed, the next terminal forward is the Plate,
(+650,) the fourth from the rear is the Screen, (+250).
73, Garey - K4OAH Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
Donley wrote:
"My guess is that either a KEY is plugged in and open, or the 'shorting' jack,
isn't! Try just
inserting a phone plug into the KEY jack several times to see if that shorting
contact will close as
it should. Otherwise just insert a shorted phone plug into the KEY jack."
Garey,
Been there, did that. The key is not plugged in but the jack is shorted as it should be. When set
up according to the manual, there is no bias current indicated on the meter when adjusting the
bias control, but there is adjustable voltage (around -60 VDC +/- adjustable) on the PA grids. If
I switch the sideband back to the "X" position, there is current indicated and I can easily vary
it with the bias control and get 100 mA. What is the difference in being in the "X" sideband or
opposite sideband position? Both are done with no transmitter gain and I get current in the "X"
position and not in the other. Maybe there is an open condition on the sideband switch when it is
in the non "X" position.
Question - Is the bias current just plate current with no transmitter gain?
Thanks.
Richard
kc9ub
----- Original Message ----- From: "Garey Barrell" <[email protected]>
To: "Donley" <[email protected]>
Cc: "Drakelist" <[email protected]>
Sent: Friday, July 08, 2011 11:18 PM
Subject: Re: [Drakelist] TR-4C Bias Adjustment
Richard -
I'm not sure I follow your description below.
1. There is only ONE metering circuit, NO switching involved. If it can read PLATE current it
can also read BIAS current.
2. 26 V across the relay sounds low, but if it pulls in that can be left til
later.
The manual says to switch the SIDEBAND switch counter-clockwise (the 'non-X' position) and the
MODE switch to X-CW. I don't have a TR-4C set up at the moment, but I believe the KEY has to be
closed. IF you have a key plugged in, it must be shorted. IF you do NOT have a key plugged in,
there is a contact on the key jack that is 'supposed' to short the key. IF this contact does not
MAKE properly you would see what I think you are describing.
The goal here is to put the transceiver into TRANSMIT mode (X-CW) with the 'non' X SIDEBAND
selected and the MIC GAIN at minimum. This should register the BIAS current of the PA stage
since you're essentially in SSB with no audio applied, resulting in only the suppressed carrier
and no PLATE current.
My guess is that either a KEY is plugged in and open, or the 'shorting' jack, isn't! Try just
inserting a phone plug into the KEY jack several times to see if that shorting contact will close
as it should. Otherwise just insert a shorted phone plug into the KEY jack.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
Donley wrote:
A few weeks ago I started working on putting a TR-4C back on the air. I posed a question about
the bias adjustment. I finally got back to working on the transceiver and still have some
questions. I have followed all the information that has been sent to other people with bias
problems, but that info has not helped me.
My problem is that the bias current is not displayed or changed on the Plate Current meter when
I adjust the bias control. I can measure and adjust the bias voltage on the final grids so I
know the system is working, I just can't measure the current. I have done most of the suggested
things: loosen and retighten most ground screws, pulled and checked all tubes and used Deoxit on
the pins and reinserted multiple times to clean the pins and sockets and cleaned the contacts on
the relays. The 48 volt relay only has 26 volts on the pins, but it appears to pull in okay and
work. Is 26 volts normal?
Using the Drake manual instructions for bias adjust, no plate current reading is detected even
though everything seems to be working. With the sideband switch in the clockwise position
(opposite what the directions say) and transmitter gain full counterclockwise, I can adjust the
plate current with the bias control. I also get full power output and can control the plate
current with tuning and loading.
It seems everything is fine except being able to measure the bias current with the recommended
setup procedure.
Any suggestions would be appreciated.
73
Richard
kc9ub
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