Mel-
Thanks for the input. I was energized to look into the amplifier instead of
focusing on the
power supply. The problem turned out to be R8 a 1 ohm 10 watt resistor from
B- to
ground. The grid meter is looking to measure voltage drop across the shunt
resistor, so
of course it would see B-.
Anyway, I had a bunch of fractional to 1.1 ohm high wattage resistors in the
junque box
from working on my LK-500's. Problem cleared! A good Christmas present for
my
buddy in the form of the good news. I'm letting it sit at idle on the bench
for a while just
to make sure.
As a side note, I have never seen big resistors mounted the way Drake did
it, seem slike
it's just looking for a problem. Usually ceramic insulators and the regular
spring type mount.
Well I live and learn.
I'm not entirely certain the original problem was the series resistor that I
mentioned. The
parasitic chokes check out, the resistor looked OK, I just jumped to a
conclusion. Could have
been open diodes or something. Anyway Harbach makes an attractive
replacement kit.
Just those bleeder resistors and the mounting is a problem. Maybe someone
else will read the
thread and learn something for the future.
Thanks for your help!
ed
-----Original Message-----
From: Mel [mailto:itz_da_police@yahoo.com]
Sent: Thursday, December 25, 2008 4:10 PM
To: Edwin Karl; amps@contesting.com
Cc: famous@cinci.rr.com
Subject: RE: [Amps] L-4B Issue
Hey Ed,
I have given it a little thought but, before I kinda had to make
some presumptions; the .82 ohm resistor was open which usually signals a
problem starting in the RF section with too much current being drawn (VHF
glitch, high voltage arc or, operation outside of the duty cycle).
When you said "No high voltage", (before you installed the Harbaugh
Power Supply) was the power supply blowing fuses? Zero B+ out the old power
supply board could have been caused by open diode(s) in the voltage doubler
circuit or no HV AC input.
The B+ was back after you put the Harbaugh board in (I am not
familiar with the PS Kit but, for the pictures, it seemed to be labeled and
straight forward to install) but, you started having arcing to ground where
(correct me if I am wrong) there was no evidences of this before the Harbagh
PS and the new bleeders were installed.
With no tubes installed and the B+ line disconnected, you are seeing
a voltage appear at your grid current meter that is driving it backwards in
standby; this voltage must be negative with respect to chassis ground.
Under these conditions, the only connection to the grid meter (M2) is from
Pin 6 through the plate amp meter to metering combination of R11 - R16 (in
the RF section) through switch S3 to the meter.
There is a signal coming through pin 6 on your PS plug causing the
problem. Pin 6 is the connection to B- when the amp is in transmit. Pin 5
is the connection to cut the amp off in standby; if for some reason, the
connections at R11 (5K at 7 watts) in the PS were reversed, the power supply
would be '"reaching for ground" via Pin 6 and the grid meter.
You say the cut-off resistor runs hotter if you connect the B+ to
the amp; if the tubes are not in, I don't think it would make a difference
but if, the tubes are in the sockets with B+ applied in standby I think I
can see current being drawn.
I'm not telling you anything you don't already know but, step 1 is
to retrace your work and then trouble-shoot what is happening with Pin 6 and
the wiring around that 5k in the PS.
Hope this helps and, I hope that I am not off the beam, if so
someone will correct me.
Mel (ko0m)
--- On Wed, 12/24/08, Edwin Karl <edk0kl@centurytel.net> wrote:
From: Edwin Karl <edk0kl@centurytel.net>
Subject: RE: [Amps] L-4B Issue
To: itz_da_police@yahoo.com
Date: Wednesday, December 24, 2008, 6:44 PM
Mel-
Do you have any experience with these amplifiers?
I seem to recall something similar with my LK-500, which
essentially
is the same circuit. Changing shunts helped (??) I think, was a
few years ago.
Thanks for your interest
73
ed
Best for the Holidays
-----Original Message-----
From: Mel [mailto:itz_da_police@yahoo.com]
Sent: Tuesday, December 23, 2008 10:22 AM
To: Edwin Karl; Amps@contesting.com
Subject: Re: [Amps] L-4B Issue
Updates, did you find the cause?
Mel
KO0M
--- On Sun, 12/21/08, Edwin Karl <edk0kl@centurytel.net>
wrote:
From: Edwin Karl <edk0kl@centurytel.net>
Subject: [Amps] L-4B Issue
To: "Amps@Contesting.Com" <amps@contesting.com>
Date: Sunday, December 21, 2008, 3:40 PM
I am rebuilding the power supply on a pal's L-4B and am stumped at a
problem that has developed.
Original symptom was no high voltage. Looked like the fuse resistor
had opened in the supply. So I felt the equipment was old lets rebuild the
supply.
First I bought the Harbach Power Supply kit and installed it. I also
replaced
the two 50K 50 watt resistors in the bleeder. All looked Ok.
After about 15 minutes in standby, one of the bleeders flashed over to the
mounting
bracket. Evidently the mica spacers broke down. This is between #1 and #2
bleeder.
So I changed the resistor, adding more mica.
After about 10 minutes in standby, big flash, resistor arcs to the mounting
screw internally.
So I look at this and think about how disappointed I am at Drake
engineering.
Apparently the screw must be exactly centered or bad things happen.
I ponder this and do the following on the replacement resistor. I wind
Scotch #27 tape
around the screw building enough on each end to act as a spacer centering
the screw.
Rated for 250 degrees so I'm not worried the tape will melt making goo all
over. I had also
reinstalled the mica spacers. Now I take nylon washers and screws and lift
the mounting
bracket above ground. I'm now confident the arcing will go away.
Hook everything up and now I see negative grid current, even with no tubes
in the sockets
and the HV not connected. When the HV is connected a 5K 10 watt resistor at
the bottom of the
bleeder string runs very hot. There is 125 or 95 volts at the junction of
the bleeder stack
feeding bias to the tube. This looks normal.
There is 1950 or 2600 volts on the HV output depending on setting of the
CW/SSB switch.
I'm stumped. Help!
ed K0KL
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