I replaced my resistors because I didn't like the excess heat generated. I
built a separate power supply that just fits inside the Drake P/S. Works a
treat!!
Russ W6OHM
--- On Thu, 12/25/08, Edwin Karl <edk0kl@centurytel.net> wrote:
> From: Edwin Karl <edk0kl@centurytel.net>
> Subject: Re: [Amps] L-4B Issue
> To: itz_da_police@yahoo.com, amps@contesting.com
> Cc: w7xu@iw.net, famous@cinci.rr.com
> Date: Thursday, December 25, 2008, 8:47 PM
> 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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