----- Original Message -----
From: "Donley" <donley...@comcast.net>
To: <k4...@mindspring.com>; "drakelist"
<Drakelist@zerobeat.net>
Sent: Tuesday, July 12, 2011 8:57 AM
Subject: Re: [Drakelist] TR-4C Bias Adjustment
Garey,
Sorry about the delay in my response. Time gets away from
me.
I acquired the TR-4C several years ago and my normal
procedure is to put a new acquisition on the bench and run
it through a simple use test. Then I tag it with the
information, working or not, and put it on the shelf for
later free time. This one has no tag, so my procedure must
have failed. I don't remember if the bias worked or not.
My guess is not.
Yes, the receiver works just fine. The RV-4C is on the
shelf, not connected. VOX and ANTI-VOX are set at
mid-range.
The transmitter works fine also, I just can't measure the
bias current. Can I assume that the bias current is close
to 100 mA if the bias voltage is ~ -60 VDC? It seems like
there must be an open in the meter circuit when switched
to set the bias. That should involve switches and relay
contacts in the circuit. I am not very good at tracing
circuits on the schematic when they start running through
switches.
In SSB mode, NON-X position, grounding the 'TIP' MIC
connector pulls in the relay.
I should probably order a new relay and try it, but the
old one appeared to work when I had it out and open for
cleaning. One set of contacts was somewhat dirty but I had
good continuity after cleaning.
Richard
Assuming the TR-4C is not much different from the TR-4
the problem you describe is very puzzling. The transmitter
settings for adjusting the bias do not affect the metering
circuit at all. What they do is to energize the output tubes
without any drive.
To set bias the controls are set so that the rig is in
CW but the sideband switch is in the "wrong" position, i.e.
non-X. That shifts the CW signal to the wrong filter so that
it doesn't get through to the driver stages. The final amp
tubes are still energized and draw whatever quiescent
current the bias is set for. If there is excessive bias
there will be no current drawn, however, the transmitter
will still put out a signal and draw plate current when
_driven_. The difference is that the output will not be at
the correct level and there may be distortion.
If you have not done the following do it:
Disconnect the power supply from the transmitter, short the
pins on the socket that go to the AC switch so that the
supply powers up. Measure the voltage at the bias terminal
and see how much difference the bias adustment pot makes.
You should be able to vary it over a rather large range. The
corrct setting for the TR-4/C is about -60 volts.
If this works OK take the short off and reconnect it to
the transmitter. Now, check at the feed through terminals
going to the final cage under the chassis to see if you are
getting the correct bias voltage there. Next, check the
voltage drop across the individual cathode resistors of the
final tubes. I don't remember the correct voltage to see
there but Garey or someone will know. If the tubes are
truely NOT drawing current there will be no or very little
drop across these resistors.
The upper meter is switched between reading plate
current and reading power output by the pull switch switch
which is also the control for its sensitivity for output. If
this switch is not making good contact it might make the
meter read low or not at all in the plate current position,
BUT, it will read low or not at all all the time, not just
for adjusting the bias.
There are two things that can cause a tube to draw no
quiescent current when the plate voltage is present. 1, its
biased beyond cut-off, 2, the screen voltage is too low or
absent.
If the screen is too low the tube is no longer
operating in the intended mode. Its possible for it to have
output when driven but not otherwise. It will act as though
its a Class B or C circuit. So, check the screen voltage
carefully _right at the tube_. Also check the bias voltage
at the tube. Its possible that the maximum bias from the
power supply is sufficient to cut the tube off. Again, it
will still put out when driven but will be operating in the
wrong mode and the output will be distorted. I can't find
data on running 6JB6 tubes other than TV sweep mode
althought I think there is such data on the Drake web site.
It would be useful to know what the cut-off bias is for the
plate and screen voltages in the TR-4/C. In any case we know
what the normal operating voltages are.
I think we have been sidetracked by the meter and
relay.
If this is all way off base I am sure someone will tell
me about it:-)
--
Richard Knoppow
Los Angeles
WB6KBL
dickb...@ix.netcom.com
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