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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