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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