Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Rob -
I find it hard to believe that ALL three tubes are totally dead,
assuming the filaments are lit!
First, have you measured the screen voltage on the tubes? Each tube
should have about 250VDC on the screen pin via a 68 ohm, 1W resistor.
Where are you measuring the plate voltage? Measure at the plate cap of
each tube.
Again, if there is 750 VDC on the plate cap, 250 VDC on the screen, 0
VDC on the grid and 0 VDC at the cathode, each one of those tubes would
draw over an amp of cathode current. To control this current, you
apply a negative voltage to the grid, and typically ~ -55 VDC should
limit that current to ~ 30 mA. IF these conditions are met at the tube
pins, then the only remaining answer is three completely open tubes,
which I still find hard to comprehend. The three tubes are in parallel,
so if any one of them is even slightly good, you should see plate current!
If the cathode PINS are at 0 VDC in transmit, then the RFC, meter shunt
and relay contact are OK. There are two RF chokes in series between the
HV feedthrough under the chassis and the plate caps of all three tubes.
73, Garey - K4OAH
Atlanta
Drake B & C-Line Service CDs
<http://www.k4oah.com>
R. Ton wrote:
Great idea, Garey...
With my meter set as low as it'll go, I'm showing about 0.3 mV across
the shunt resistor in transmit. This thing isn't drawing any current
at all! In receive, I'm getting a cathode voltage of about 155V and
a plate voltage of about 754V. The 754V plate reading is the same
as my AC-4 under no load. In transmit, there is no change in plate
voltage, but the cathode voltage goes to zero.
According to the schematic, the cathodes are grounded via the T/R relay,
the 2 ohm shunt resistor, an RF choke and a 15 ohm resistor (one 15
ohm resistor for each tube). Even through the 15 ohm resistor, I'm
showing 0V on the cathode in transmit.
Wierd. This would sure seem to indicate that the finals aren't
conducting at all. Still, very weak on my tube theory here --
could a grid condition cause this? I'm showing a good -60V on
pins 2 and 6.
I was also thinking that a 1 watt, 2 ohm resistor should be able
to handle about 700ma without cooking. When I got the rig, this
resistor was burned in half. Maybe it's time to pull the tubes
and see if anything is rattling around inside? :(
Is my thinking halfway straight? :) Hoping like heck the plate
current meter isn't shot -- I have a feeling those are hard to
get.
Thanks again everyone!
73, Rob (KFØRT)
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] Behalf Of Garey Barrell
Sent: Sunday, April 16, 2006 9:57 AM
To: R. Ton
Cc: Drakelist
Subject: Re: [drakelist] TR-4C transmitter troubleshooting
Garey Barrell <[EMAIL PROTECTED]> made an utterance to the
drakelist gang
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Rob -
OK, a little more information.....
When the 2 ohm meter shunt resistor opens up, the Final cathode current
is routed through a "slide switch" mounted up on the LOAD capacitor
shaft, (activated by pushing in the LOAD control,) a series resistor
that is selected at test, (mounted on the slide switch terminals,) of
typically ~200 ohms, and the plate meter. It wouldn't take much
current to damage that resistor and/or meter with the shunt resistor
open. Or if the slide switch contacts are not closing properly in the
"out" position of the LOAD control.
Perhaps your finals are drawing idle current, it's just not showing on
the meter!? Measure the voltage drop across the meter shunt resistor.
100 mA should read 200 mV.
By the way, if you're not used to working on tube gear, keep in mind
that 650V (OR 250v) BITES! An AC-4 bleeds down pretty quickly when
turned off, but only your meter knows for sure before you stick your
fingers in there!
73, Garey - K4OAH
Atlanta
Drake B & C-Line Service CDs
<http://www.k4oah.com>
R. Ton wrote:
Thanks to all (!!) -- more coming when I get a chance (life is
getting in
the way of fun right now).
So far, I've had the chance to change out the relay -- the ONE
spare part
I had on hand. No difference. The rig does go into transmit
when switched
to CW mode (no key plugged in). Both relays respond and the
receiver mutes.
I just noticed this AM that the .025 plate current reading I
get is there
even with the power off. Meter moves about a needle's width
when put into
transmit and the S-meter reads about 7-8 in transmit -- may be
another clue?
And thanks for the hollow-state theory Garey. It's been a
REALLY long time
since I've worked on any tube gear, but it's slowly coming back
to me. :)
I'll be digging some more in a bit here and will report back. I'll also
see if I can catch the technical net this afternoon. 7.238 @
2000Z, right?
Thanks again everyone!
73, Rob (KFØRT)
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] Behalf Of Garey Barrell
Sent: Sunday, April 16, 2006 8:41 AM
To: Drakelist
Subject: Re: [drakelist] TR-4C transmitter troubleshooting
Garey Barrell <[EMAIL PROTECTED]> made an utterance to the
drakelist gang
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Hi Kevin -
Expertise is just experience clouded by memory! :-)
I don't think I've ever come across a PAIR of finals that "lit up", but
wouldn't draw at least a couple hundred mA if they had the DC
potentials
applied to them. In fact, I can only remember one tube that
was "open",
but lit. I still think it had a broken weld inside it!! :-)
But then I've been wrong before. A favorite med school phrase, "when
you hear hoof beats, think horses, not zebras" comes to mind!
Hopefully Rob will get it fixed in spite of my help and let us know the
truth!!
73, Garey - K4OAH
Atlanta
Drake B & C-Line Service CDs
<http://www.k4oah.com>
kbgluxford wrote:
kbgluxford <[EMAIL PROTECTED]> made an utterance to the drakelist
gang
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I certainly don't have Garey's expertise or long experience, but I did
have a similar problem on one of my TX4 transmitters recently. I
changed out the PA tubes and the problem disappeared.
73
Kevin
VK3DAP / ZL2DAP
Garey Barrell wrote:
Garey Barrell <[EMAIL PROTECTED]> made an utterance to the
drakelist gang
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Rob -
You don't mention anything about "keying" the transmitter. You have
to "key" the Push-To-Talk (PTT) of the microphone to switch it to
transmit to see the idling plate current. You should hear the relay
click when it closes.
If the final tubes have plate voltage, screen voltage and filament
voltage, and the cathodes are returned to the negative pole of those
supplies, they WILL draw plate current! The grid voltage is the
only thing that keeps them from drawing AMPS of current in the
presence of the other voltages.
There are two RF chokes in series between the +650VDC supply and the
plate caps of the tubes. There is a 15 ohm, 1W resistor in series
with each of the final tube cathodes to another RF choke, through the
meter shunt resistor, and then through a relay contact to ground.
73, Garey - K4OAH
Atlanta
Drake B & C-Line Service CDs
<http://www.k4oah.com>
R. Ton wrote:
"R. Ton" <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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I'm finally digging into a TR-4C I bought used some time ago. It
"worked
fine" when it was shipped here, but I haven't been able to get any
power out
of the transmitter. It at least oscillates -- I can hear it on a
receiver
in the same room.
Starting at the beginning...
The manual says to adjust the bias on the AC-4 so that the plate
meter reads
.1 amp. This is with the sideband switch counter-clockwise and the
xmtr
gain all the way down. The meter seems to read about .025 in
receive and
goes up just a little when the rig is switched to CW. Adjusting the
bias
has no effect on the plate meter, but it does indeed adjust the bias
voltage. Setting it to -60 volts is no problem, but I'm unable to
get any
"real" plate current no matter what I do. Any ideas?
I haven't done anything to the radio since I got it except spray the
switches with Deoxit D5 and replace R45 (2 ohms -- plate meter shunt
resistor!!). This resistor was fried in half.
What am I in for?
Many thanks in advance!
73, Rob (KFØRT)
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