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.
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.
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.
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.
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.
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)?
Awaiting further task assignments. I appreciate all the help.
73
Richard
----- Original Message -----
From: "Garey Barrell" <k4...@mindspring.com>
To: "drakelist" <Drakelist@zerobeat.net>
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" <k4...@mindspring.com>
To: "drakelist" <Drakelist@zerobeat.net>
Sent: Wednesday, July 13, 2011 1:42 PM
Subject: Re: [Drakelist] TR-4C Bias Adjustment
Richard Knoppow wrote:
----- Original Message ----- From: "Garey Barrell"
<k4...@mindspring.com>
To: "drakelist" <Drakelist@zerobeat.net>
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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