As most of you know, a couple days ago, Wayne talked about a mod to
improve the VLF sensitivity of the K3 after the KSYN3A synthesizer
installation. I volunteered and he sent me the information on what
parts were needed and where they went. After finding out that I didn't
have the right parts in my stock I went looking. Wound up ordering them
from Mouser with a delivery date of this coming Tuesday. Being the
impatient type that I am, I went looking. We have a local (former)
Radio Shack franchisee that is also our area ham radio store. I finally
called him and found out that he did indeed have some surface mount
capacitors of the necessary value and also the electrolytic caps that I
was out of.
Parts in hand, I proceeded to open up the K3. Since mine was a factory
built one, though I'd done some repair and installation of the KSYN3A
synthesizers, I hadn't had it disassembled to the points necessary to do
the mods before. Having worked on many unfamiliar radios in my US Army
and Commercial 2-Way Radio careers, I jumped right in (read the book
later, sort of). It was necessary to remove the KRX3 sub receiver and
the KBPF3 General Coverage Bandpass Filter in order to perform the
modifications. If you have the KSYN3A synthesizers and want to do the
mods, you'll have to get them from Wayne though. He gave me permission
to post my experiences & results, but not the total details of the mods.
Prior to doing anything a "before" MDS (minimum discernable signal) test
was requested on several of the VLF frequencies (137 KHz and 472 KHz)
requiring a calibrated signal generator that will output -107dBm signals
on those frequencies. Mine would not so I used a 0 to 69 db precision
step attenuator to take the output down from -73 dBm (50 microvolts) to
the required -107.
Prior to the modifications, the MDS for those frequencies in MY K3 (your
results will most certainly vary a bit) were 137 KHz, MDS -82 dBm and
472 KHz -98 dBm somewhere close to what it was determined it should be.
After completing the modifications which consist of adding a couple of
components on the main RF board, removing a couple of inductors and
replacing 3 surface mount capacitors on the KBPF3, as I was reassembling
the radio, I had all but completed the job when I accidentally dropped
one of the screws and washers used to hold the stiffener bar to the PA
shield. It IS necessary to remove this if you have a KRX3 sub receiver
in order to get the KRX3 out. That errant screw dropped down into the
guts of the K3. When I turned it upside down and shook it, nothing
appeared to come out and I didn't hear it rattling around either. Scary
stuff, I had to completely disassemble things again, including the PA
which was over the area the screw & washer fell into. Still finding
nothing, I was really worried now so I got up to get a bigger flashlight
to help search. As I stood up, the screw and washer fell out of my lap.
Much ado about nothing, but I sure as heck didn't want to reassemble
the K3 and have a screw floating around on top of the main RF board.
Word to the wise, if you do the mods, keep CAREFUL track of all screws,
washers and other parts so they don't drop where they shouldn't and you
won't have to do what I did.
Fortunately, after the scare, everything went back in place and
everything worked when I re-connected things and powered it up.
Again, it was requested to do an "after" MDS on those two frequencies
and the results (on MY K3, serial# 5027) are as follows with 2
additional freq's in the range added.
137 KHz - Wayne got -114 dBm on the lab K3, I got -116 dBm on mine.
160 KHz - Wayne got -117 dBm, I got -119.
250 KHz - Wayne got -126 dBm, I got -125.
472 KHz - Wayne got -133 dBm and I got -132.
These are well within the tolerances of my personal test equipment (Sig
gen was a SARK-110 Vector Impedance Analyzer containing a nicely
calibrated signal generator that goes from 1.0 KHz through 230 MHz but
minimum output is -73 dBm so I used a recently calibrated Telonic
precision step attenuator to get it down to the necessary -107 dBm
called out in Wayne's MDS measurement procedure.
This morning, I hooked up my short PAR EndFedZ 40-20-10 meter QRP
antenna which is strung up just above the house roof - far end 30 feet
high near end/feedpoint 15 feet high with about 20 feet of RG-58 to the
radio and tuned around the Aircraft Beacon band from 250 KHz through
about 420 KHz and heard the following stations well after sunup when the
noise level on those frequencies is quite a bit higher than during the
night time.
Heard on VLF this morning well after sun up - 8:30-9:00 Central Daylight
Time in Park City (Wichita area), KS.
Freq, Identifier, Location and approximate Distance from my QTH
332KHz IC Wichita KS 12 miles
350KHz RG Oklahoma City, OK 141 nautical miles
356KHz PTT Pratt, KS 68 nautical miles
375KHz DW Tulsa, OK 117.5 nautical miles
395KHz CA Newton, KS 15 nautical miles
407KHz HRU Herrington, KS 58.8 nautical miles
414KHz EGT Wellington, KS 28.8 nautical miles
It looks like the modifications really do improve the receive
sensitivity immensely on these frequencies. I tried listening for some
of these beacons after installing the new synthesizers, but could only
hear the Wichita and Newton beacons which are quite close to me.
Jim Sheldon, W0EB
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