Again...anywhere the plate current or its surrogate is measured, it MUST
be capable of a dip. Especially if the load is anywhere near 50 ohms and
the LOAD control is at minimum.
Does the voltage over R45 dip to the 192mA value??
Also, if the meter is actually reading RF output, the sensitivity
adjustment will change the reading--is this happening?
Also, and I have not tried this, but if one puts an ohmeter across R45
and the meter switch has the meter in the circuit, one would see some
action on the Drake meter from the ohmmeter test voltage. In most
metering circuits, it is dangerous for the health of the meter movement
to measure a shunt with an ohmmeter.
Curt
KU8L
Garey Barrell wrote:
See below.........
Donley wrote:
Garey, et al
The PA tubes are Sylvania 6JB6, checked on a Hickok 600A and test
good. I have seen no color to the plates of the finals.
OK.
I have been doing all testing on 40 M. I came to suspect one of the
sideband filters so I tried 80, 40 and 20 M., both upper and lower
sideband to see what happened. I got output in excess of 125 W. to
the dummy load in all cases. The cables to the wattmeter and dummy
load are OK, the dummy load (switched to a Bird 8201) is OK.
OK. Is this with the LOAD control at minimum? If you just peak TUNE
and LOAD how much output do you see?
I took the lid off of the PA cage and placed an all purpose
telescoping antenna next to the cage and ran the input into an HP
spectrum analyzer. I got a very nice signal at 7.3 MHz and also very
nice signals at 14, 21 and 28 MHz. There was no sign of any other
oscillation of the finals. I did notice hash from the laptop computer
switching power supply which went away when I unplugged it. Replacing
and grounding the PA lid removed the extraneous harmonics at 14, 21
and 28 MHz. I also listened to the signal with another receiver and
had a very clear sounding tone with no other noise up and down the
dial. I varied the TR-4C VFO to make sure I was listening to the
correct signal.
Cool. I didn't know you had access to 'real' equipment! :-)
I did measure the lead resistance of the Fluke leads. It is about
0.15 ohms, so the resistance of R45 will be about 2.25 ohms, closer
to the nominal value.
OK
I don't think anything is wrong with the output circuitry. If there
was, how could I be measuring power out to the dummy load? The
wattmeter is a Swan 3000. I am not absolutely certain that it is
working OK. I suppose I should try the light bulb technology that I
used as a novice back when I knew it all. Now I have some reasonable
equipment and don't know anything.
No I think you are getting output power, what you are seeing is
commensurate with the actual measured cathode current. I think we're
back to the meter circuit itself, which appears to be reading RF
OUTPUT regardless of the switch position. The voltage you are seeing
across R45 is correct , it's just NOT getting to the meter via the
switch and/or R13. Trace that out and you'll have it. The high side
of R45, to the 'front' contact of the switch, and with the switch slid
'forward' connecting the arm, with R13 and through R13 to the 'rear'
contact of the switch and wire on to the positive terminal of the meter.
Now, the last item puzzles me. I measured the ANT output to ground
(SO-239) and it was 1.4 ohms, no way near 2.2 Megohms. RFC 7 goes to
ground right at the SO-239. I would expect the DC resistance of the
choke to be more than 1.4 ohms but less than 2.2 Megohms. Are we
getting closer to an answer or just more confusion (I know it is the
latter for me)?
No, that's MY mistake. Certainly there is an RF choke directly across
the antenna, as there has been for many years. It's a 'safety' choke
so that if the plate blocking capacitor (C78) should fail shorted,
placing 650 VDC on the _antenna_ there would be a blown fuse, or at
worst a smoke signal from the RFC 7. It also serves to bleed off any
accumulated buildup of static on the antenna from snow or nearby
lightning strikes.
Awaiting further task assignments. I appreciate all the help.
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>
73
Richard
----- Original Message ----- From: "Garey Barrell"
<[email protected]>
To: "drakelist" <[email protected]>
Sent: Thursday, July 14, 2011 10:27 AM
Subject: Re: [Drakelist] TR-4C Bias Adjustment
Richard -
OK. As others have said, we are missing something fundamental, as
transmitters just do not do this!! :-) Verifying that the PA tubes
ARE 6JB6 is a good idea. The Simpson is more than adequate for all
but very low resistance readings, 2 - 15 ohm resistors and the
voltage across them.
You haven't confirmed which band you are using for test purposes. I
suggest 40M. Are the PA plates showing any color that you have
seen? Listen on another receiver and determine if you can hear the
TR-4C on the frequency indicated by the dial reading. Once you find
the signal, does it vary with PTO adjustment as expected?
Since we have already measured the voltage across the three Cathode
resistors and determined that they are approximately equal, nothing
is gained by the risky (dangerous) measurement of 'plate' current.
There is no place else for the current through R45 to come from
other than the plate supply, so measuring the voltage across it is
just as accurate and much less dangerous. Check your ohmmeter by
shorting the test leads to determine if it reads "0.00" or if there
is some lead resistance to be subtracted from the 2.4 ohm reading.
2.4 ohms is 20% high, but absolute current isn't critical at this
point.
I believe that we have determined that the meter IS measuring
current correctly, the current just isn't behaving normally. It
appears that 'something' is wrong with the output circuitry of the
PA, preventing it from transferring power from the PA plates to the
antenna, or the 'antenna' (dummy load) isn't accepting power.
Measurement of the dummy load would be a good next step. If it
measures near 50 ohms, then measure again at the transceiver end of
the cable to verify that the cable is good. Measure the resistance
of the ANT output jack on the radio, (power off, dummy load
disconnected,) it should read about 2.2 Megohms to chassis. Rotate
the PLATE and LOAD controls through their range with the ohmmeter
connected, watching for anomalies.
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:
Garey , Richard I, et al
Since I am the student here, I can only be Richard II.
I measured the resistance of R45, in circuit with the relay pulled.
I'm not sure that made any difference. It measured 2.4 ohms on a
Fluke 179.
Since it is difficult to adjust for MAX or MIN on a DMM, I changed
to a Simpson 260 (I know, maybe not too accurate).
Then set SIDEBAND X, MODE X-CW cranked in some XMTR gain and tuned
PLATE for MAX on the PLATE CURRENT meter (because tuning for MIN
didn't work). Max reading on the PLATE CURRENT meter again occurs
about mid-range and falls off on both sides. The PLATE METER read
200 mA and the voltage on R45 was 0.46 VDC. Since R45 measured 2.4
ohms from before, the calculated current is about 192 mA. Close to
the 200 mA on the PLATE METER.
Then I tried tuning PLATE for MIN voltage on R45 with the Simpson.
I could not tune to MIN, there was a MAX value about mid-range and
it falls off on either side of MAX. The voltage on R45 was very
close to 0.46 VDC, the same as above. Again, the calculated current
was about 192 mA.
Plate current or RF output?
I checked the slide switch on the LOAD control. I took resistance
measurements in each position and it seems to be working. I added a
little DeOxit.
I looked at the circuitry for RF output on the relay board ( I have
most of Garey's Drake CD's). If the problem is in there, it is
going to be a real pain to get to. I hope someone comes up with the
"Golden Screwdriver" solution. When I was working, our solutions
were "Silver Bullets". Of course, that was before the price of
silver skyrocketed.
Richard II
----- Original Message ----- From: "Garey Barrell"
<[email protected]>
To: "drakelist" <[email protected]>
Sent: Wednesday, July 13, 2011 1:42 PM
Subject: Re: [Drakelist] TR-4C Bias Adjustment
Richard Knoppow wrote:
----- Original Message ----- From: "Garey Barrell"
<[email protected]>
To: "drakelist" <[email protected]>
Sent: Wednesday, July 13, 2011 6:19 AM
Subject: Re: [Drakelist] TR-4C Bias Adjustment
Richard -
I don't recall a C73 reference.? Possibly C74, which is the
Cathode lead feedthrough?
Rats, I thought we might be on to something. The fact that the
meter reading peak results in
The other Richard I think. C-73 was my blunder from looking at
the TR-4 schematics. Obviously the TR-4C has considerable
differences. I meant to find a point where the collective voltage
drop across the cathode resistors could be measured independandly
of the panel meter. My idea was not to get a precise reading but
only to see if the current there behaved normally, i.e., dipped
at reasonance. The peaking of of the meter when adjusting plate
tuning sure suggests its looking at output not plate current.
Maybe not a bad switch but it could be something in the wiring
there. At any rate an RF amplifier just can't behave the way the
meter says its doing.
I have enountered bad slide switches but not many, it can't be
discounted with something as mysterious as this. It could be
something as simple as a solder butch or something having been
mis-wired in the past.
The idea is to isolate the problem and I think using an
independant meter would help.
There has been a lot of tail-chasing over this problem and I
am afraid I made it worse by making a fundamental mistake, that
is making an assumption, in this case that the two versions of
the TR-4 were mostly similar.
My only excuse is that I am on a medication for the after
effects of Shingles that makes me a little dumber even than usual.
Richard -
Unfortunately, I don't have medication as an excuse! I did have
low blood pressure after rehab this morning, does that count?!? :-)
C74 would be a reasonable place to measure the drop across R45 to
check for a dip coinciding with maximum output. I was trying to
set the BIAS voltage correctly by measuring across the Cathode
resistors, and the resistor's values, 'accurately'. Richard came
up with ~ -50 VDC, which I'm afraid could result in
over-dissipation of the PA, -60 to -65 is more typical for 100 mA.
THIS RF amplifier _MAY_ behave just as described, (max output
and max current coinciding,) with 'some' PA tubes and/or
improperly adjusted neutralization, by oscillating on its own.
Yet another problem is that this transceiver has NEVER worked for
this owner, so a 'golden screwdriver' may yet turn out to be the
problem!!
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>
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