How to Fix Sideband Distortion in the Palomar Elite 300 RF Amplifier
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Disclaimer:
This post is for educational purposes only, and no guarantees are
offered. Proceed at your own risk.
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Some of you may have purchased the Palomar Elite 300 amp
currently
being sold on Ebay and various other places. Unfortunately this
amp has several
major design flaws. If you do own one of these then the following
post may
be of interest.
If you are thinking of buying a Palomar Elite 300....don't!
The amp has serious issues. A Texas Star 350 appears to be a
better designed
amplifier although I have never owned one.
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How to fix design flaws in the Palomar Elite 300 amp.
Design flaw # 1.
The Palomar Elite 300 amp is a class C zero bias amplifier. In a
nutshell that
means your transmit audio on sideband will become distorted as
soon as
you turn the amp on. No amount of mic adjusting, SWR tuning,
cable
swapping, or anything else will fix it. The problem is in the amp
itself. To fix the problem you or a qualified technician will
need to
bias the amp for class B, AB1 or AB2 service.
The instructions below will show you how to fix your amp. All
you need
is basic soldering skills, and a minor understanding of
electronic parts.
Here is the bias fix.
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You may need to remove the Palomar Elite 300 from the housing to
do
this mod. I took off the front face plate, removed the four allen
screws holding the heat sink to the two final output transistors,
removed the screws holding the two female coax connectors from
the
back of the plate, and then removed the little plastic holder
from around
the main DC wires. The unit slides out of the front at that
point.
1) Look at the top of the circuit board from the front of the
amp.
2) To the left of the smaller input broadband transformer there
is a
ferrite bead.
3) Unsolder the farther leg of the ferrite bead which is closest
to
the back panel. Leave the other leg where it is. If you look at
the
back of the circuit board the leg you want to unsolder is the leg
that
goes to ground.
4) Solder a silicon diode with the cathode (the line) facing away
from
the ferrite bead to the leg of the ferrite bead you previously
unsoldered. I used a Radio Shack 1000 PIV 2.5 amp silicon diode -
catalog # 276-1114.
5) Solder the cathode side of the diode (the white line) facing
down
in the little hole the one leg of the ferrite bead was soldered
in.
See the picture below;
http://www.shadowstorm.com/cb/Palomar-Elite-300/Bias-fix-2.jpg
6) Get a 100uh RF choke from Radio Shack (Catalog #: 273-102).
7) Solder one end of the RF choke to the solder covered side of
the
larger output broadband transformer.
See the picture at;
http://www.shadowstorm.com/cb/Palomar-Elite-300/Bias-fix-1.jpg
8) Solder a 100 ohm 10 watt resistor to the other end of the RF
choke.
I used a Radio Shack wire wound resistor - catalog # 271-135.
9) Solder the other end of the 100 ohm resistor to the junction
where
the ferrite bead and the diode you previously soldered meet.
See the picture at;
http://www.shadowstorm.com/cb/Palomar-Elite-300/Bias-fix-2.jpg
10) Make sure none of the exposed wire from the parts you
added
touches the bottom, top or sides of the metal case.
11) Re-assemble the Palomar Elite 300.
12) You're done!
Your sideband transmit audio with the Palomar Elite 300 should
now be
clear.
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Design flaw # 2.
On another issue, the Palomar Elite 300 does not filter RF from
the
B+ line, so the DC power leads act like an antenna just like the
regular output antenna coax connector. The RF running up the DC
power
chord kept tripping out my power supply until I wrapped the DC
power chord
around a large ferrite core just outside of the amp chassis.
(Radio Shack 273-104) .
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There are other design flaws as well like a nasty impedance
mismatch on the input broadband
transformer when the amp is in medium and low. The bundle of
metal oxide resistors in this picture
is
an attenuation resistive "T pad" with a 50 ohm
impedance. It has an input SWR of about 1.3:1. Since
I am using MRF-455 transistors in my amp instead of the somewhat
flaky SD1446 transistors, I am not
going to make a generic recommendation. The input watts to output
watts curve is different for the
MRF455s than it is for the SD1446 transistors. What is the best
amount of attenuation for the MRF455's
may be too much attenuation for the SD1446 transistors.
The raw data I used to build my resistive T pad can be found here. Sorry, but you're on your own for this mod.
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This amp can overheat when being used on AM. A fan placed on
top of the heat sink
helps keep things cool and working correctly for longer
conversations.
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Design flaw # 3
This little bug had me scratching my head, and I had to get the help of some other smart techs to nail the cause.
Here is the bug;
Hook up you radio, amp, SWR meter, and antenna like this;
RADIO===>AMP===>SWR METER====>ANTENNA
Leave the amp off and check your SWR. Hopefully the SWR is nice and low.
Now turn the amp on and check your SWR again. Even if you
re-calibrate your SWR meter for the increased
output you will find your SWR is now through the roof. Turn the
amp back off and your SWR will return to
normal. What the heck? The SWR meter is hooked up AFTER the amp,
so why is the SWR after the amp
going up so drastically? I swapped out the Palomar Elite 300 with
a Drake L-4B putting out 400 watts
and the SWR did not go up, so why does the SWR go up so
drastically when I turn on the little Palomar Elite 300?
I wondered if anyone else was having this problem, and sure
enough I found a post in the news groups from some
other fellow who was having the exact same problem. Here is his post.
The answer is harmonics in the amplifier's output power that
are outside of the bandwidth of
the antenna. The antenna may have a low SWR at it's design
frequency, but the SWR will be out of sight at
twice the design frequency or more.
You can fix this problem by connecting a 39 pf - 1 KV ceramic disc capacitor from the collector on each final output transistor back to the transistor emitter. Keep the capacitor legs as short as possible. The emitter is connected directly to ground, so the capacitor is connected to ground as well. For the technically minded the 39 pf capacitors are in parallel with the existing capacitors, so the actual capacitance is higher than 39 pf.
Next connect a 47 pf - 500 volt ceramic disc capacitor from the base of each final output transistor to the emitter.
The capacitors act as low pass filters for HF signals, but they send the VHF and higher harmonics to ground. You're going to be a little disappointed when the output shown on your watt meter drops back to about 75 watts, but that was all you were really putting out to begin with. The higher output your watt meter was reading was the result of reflected RF power feeding back from the antenna and in to your watt meter due to the SWR at higher frequencies. Welcome to the world of real RMS watts verses imaginary watts. With the above modification my SWR meter read 1.1:1 with the amp off and 1.3:1 with the amp on.
The modification above significantly reduces the harmonic output of the amp, but it does not eliminate them completely. Even with this modification I recommend using a low pass TVI filter with this amp. When I installed a low pass filter in between the amp and the SWR meter, the SWR stayed at 1.1:1 regardless of whether the amp was on or off.
-Night Ranger
