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[aprssig] Kenwood D700/D710 RF Problems

Stephen H. Smith wa8lmf2 at aol.com
Thu Jul 22 01:18:26 UTC 2010


  On 7/21/2010 3:42 PM, Herb Gerhardt wrote:
> Since we are discussing noise issues, this might be a good time to bring up
> my problem with a D700 radio.  I am also changing the title to Kenwood
> D700/D710 RF Problems.
>
>
>
> Yes, we are experimenting with this set up at a number of locations around
> Western WA using both D700 and D710 radios and have not had any equipment
> problems until now.  As soon as I installed this equipment at our site on
> South Mtn and the radio transmitted on either side on high power, the Astron
> SS-25 switching power supply tripps off the line.  Turning the output power
> down to 5 watts (low), seemed to keep the power supply from tripping.  This
> dual digi worked fine for about a week and then the 9600 baud side
> disappeared off my screen.  I had to wait until I had time to make another
> trip up the mountain about a month later.  The 144.390 digi side continued
> to work fine, so that proved that the power supply had not tripped and the
> radio's right side was still operating fine with its external TNC.
>
> What I found when I went back up there was that the D700 internal TNC had
> completely reset itself to parade rest but the radio's memory channels were
> still as programmed.  I brought another power supply and D700 with me, so I
> replaced both the power supply with an identical one and a different D700
> radio.  Well, we experienced the same power supply tripping problem as
> before on high power and  also on medium power.  We did lots of head
> scratching and investigating and it appears that the problem is an RF
> problem of when the radio transmits on high or medium power, it trips the
> Astron SS-25 power supply off line.
>
> I had one torride  core with me and installed it on the head cable with one
> additional wrap through the core.  That helped.  We then moved the D700
> control head around in our enclosure to get it as far away from the power
> supply as possible but that is still less than 6 inches and wrapped the head
> with some aluminum foil that my sandwich was wrapped in.  We also rolled up
> the head cable and put a steel plate over the top of it.  We then set the
> power level to low power on both sides of the radio and so far, so good.
>

1)     Despite the convenience of built-in TNCs, the Kenwoods are an appalling 
choice for remote mountain-top sites.   Besides the buggy firmware prone to 
lockups and needing periodic resets,  the Kenwood receiver front-ends are 
wideband "general coverage" designs extremely susceptible to  overload, 
blocking and inter-modulation from other nearby VHF, UHF and 800 MHz 
transmitters and FM broadcast stations.


[Does one EVER have NOT have nearby transmitters on mountain tops in the West, 
especially on ones overlooking populated areas or Interstate highways?    For 
that matter, here in Los Angeles, I constantly get random squelch breaks with 
mixes of several voices, braps of data transmissions, and the occasional FM 
broadcast breaking through my D700 mobile even on the ground, let alone on an 
RF-saturated mountain top.]


Further, because both bands come come out the same antenna port, there no easy 
way to place band-pass filters or cavities inline between the radio and the 
antenna to reject the land-mobile and broadcast crud.    Unless of course, you 
resort to the kluge of using a 2M/70cm diplexer backwards to separate the two 
bands, insert a single-band filter on each side, and then re-combine them with 
a second diplexer facing the antenna (or use separate antennas).




2)      Neither the control head nor the power supply emits any significant 
RF.    The degree of separation between these two components is not the 
issue.   RF radiated by your ANTENNA (or somebody else's at the same site) is 
going to be THOUSANDS of times more intense than any incidental RF radiated by 
the control head, and is most likely the culprit.   How far away is the antenna 
from the radio and/or power supply?



3)     On the other hand, the control head-to-chassis cable can act as a 
RECEIVE antenna to pick up RF from nearby transmit antennas.    Reduce the 
antenna effect of this cable by making up a custom much-shorter 
better-shielded-and choked control-head-to-radio cable assembly.

One can easily make custom-length cables. Use 4-conductor flat phone cord and 
crimp a normal 6-conductor modular plug to one end (line it up so the cable 
lands on the middle four contacts of the plug with an empty contact on each 
side) onto one end.    Crimp a smaller 4-contact plug (the kind used on 
curly-cord handset cables) to the other end.   Before you install the second 
plug, wind the bare end of the cable through a cylindrical ferrite core several 
times -- it's a lot easier without a plug on the end.    If you really want to 
go all-out, slip a piece of tubular braid, pulled off an RG-58 or RG-59 coax, 
over the phone cable for shielding before you attach the second plug.

IMPORTANT:  View the original cable's ends to ensure the conductors in the new 
one are facing the same sides of the plugs; i.e. you don't want the conductors 
crossed over end-to-end.

The smaller 4-conductor   handset-to-phone   modular plugs are easy to find at 
electronics distributors or Radio Shack, along with the normal RJ-11/RJ-14 
plugs used for   phone-to-wall   connections.   However most common crimp tools 
only do the RJ-11/RJ-14 plugs along with the larger 8-conductor RJ-34 ethernet 
cables.   There is a crimp tool that does all three sizes of plugs available at 
Home Depot for about $30.  Look in the electrical telecomm/video/home 
networking area.

Making up a number of these cables may be a worthwhile club project, if you 
have a lot of D700 users.  Having a short head-to-chassis cable is useful for 
bench-testing D700s without having to remove the painstakingly routed original 
cable from a mobile install.



4)      Finally, analog DC power supplies tend to be susceptible to low levels 
of RF, because their voltage-regulator systems typically work with milli-volt 
level control signals.    On the other hand, switching-mode supplies tend to be 
less affected.  The same RF filtering and bypassing that keeps the supply's 
internally-generated switching noise from leaking out (and causing RFI to 
nearby receivers) also works to help keep external RF from getting into the 
supply.


-------------------------------------------------------------------------------

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Stephen H. Smith    wa8lmf (at) aol.com
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