Garey Barrell wrote:
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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Yes..the whole PA going into oscillation could do that as you described
although I've never seen a Drake PA do that on 40M or below.
It would also be weird if it was oscillating on tune and also had near
zero bias current at idle.
I have seen badly oxidized meter switch contacts but they always seem to
just mess up the RF out mode, not the PA current.
Anyway..sounds like the right track.
PS...make sure it is the PLATE control for the dip and not the the LOAD
control during tune-up. Getting this in reverse will really skew the
result! :)
Curt
KU8L
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