[LONG] Re: [Kenwood] Replacing filter in TS 870
NR1DX
[email protected]
Tue, 15 Oct 2002 23:45:12 +0000
Duncan
This is a problem I struggled with but never fully resolved, let me share
with you my observations and experience to date.
So far I have found the following to be true on two TS-870 with serial
numbers 705xxx and 706xxx respectively.
The problem:
In crowded high signal (40 to 60db over S9) conditions typical of a large
metropolitan area such as Boston, New York, Washington during a major
contest. Strange things begin to happen to the received audio which are for
lack of a better description the inability of the DSP to respond to
multiple strong signals which it thinks are really in the pass band. This
appears to be a result of the fact that the AGC is apparently very much a
part of the active feedback loop for the DSP algorithms which is why under
"normal" conditions you can really get a lot out of the DSP through
critical adjustment of the AGC depending on the type of noise or
interfernce you are trying to filter out.
Kenwood's Solution:
The response by Kenwood on this problem that they at first tried to deny
existed was to reduce the over all gain in the loop by redistributing the
gain as outlined in one of their technical or service bulletins ( I can
find the number if you really need this). They also suggest that you crank
in the attenuation , AIP and lower RF gain all of which basically serves to
get the out of passband signals down to the level where the dynamic range
of the DSP can handle it, but doesn't really get at (I believe) the
fundamental problem, see the next paragraph.
My observations and proposed solution:
I experimented with trying different and narrower filters to no real avail
which really confused me for awhile, until late one night after being at
this too long I accidently left the filter completely out of the radio
(essentially open circuit)and powered it back up with the signal generator
still hooked up. The generator had been set to produce an "in passband" S
meter reading of 60db over. Much to my surprise with the filter
completely out of the circuit (technically infinite pass band rejection)
the S meter still registered S-9! A series of other experiments were under
taken with the help of Motorola RF engineer K9MK. At first we thought that
the there might be bleed through from the diodes used to switch the filter
but that was eliminated by removing the diodes as well .... no change. The
conclusion we came to that the basic board layout was allowing signals to
couple "around" the filter! The thought at this point is to cascade a 2nd
filter "off board". Technically this should be relatively easy to do by
designing a proper matching network/transformer so that the cascaded
filters don't mismatch each other. Physically this should also be easy to
do by simply inserting the 2nd filter at the point where the coax leaves
the the RF board at CN10 that goes to the 455KHZ mixer on the CONNECTION
board at CN412. The lack of a proper equipment to measure the impedances at
CN10 and CN412 plus the impedances of the filters and the time to do it is
where the project has ground to a halt for the moment.
Suggestions:
Before you go further if your serious about this quest to make an otherwise
good radio into a "super" radio, get a copy of the service manual, get
access to some professional test equipment, study schematic and board
layouts from the 3 KHZ filter to the 455 IF mixer. Do a full radio
alignment once just to familiarize yourself with all the levels and
adjustments, the service menus, especially the ones that center the
passband. Make sure you can solder delicate SMT components with out
butchering the circuit board, find a scrap computer card to practice on
before you take an iron to your expensive radio.
Epilogue:
For those of you who live in the country don't worry about the problem.
When I first bought my 870 I lived in "Silly Corn Valley" (rural
Wisconsin)and I couldn't figure out what people were complaining about.
Then I moved just outside Boston where the radio became literally unusable
during CQWW and ARRL DX contests on CW..... When iI get the time the search
for the grail will continue, perhaps this "maap" will help another knight
blaze the trail before me
Good luck
Dave
NR1DX
At 05:23 PM 10/15/02 +0000, Duncan Cameron wrote:
>Does anyone out there have any experience with replacing the YK88S2 (8.83)
>filter in the Kenwood TS-870 with another filter. In looking at the
>service manual, I feel it would not too badly affect the DSP if the
>replacement filter were only slightly narrower or slightly wider. Kenwood
>parts lists the YK88S2 as a 3K bandwidth filter. I am thinking of
>replacing it with a 4K and a 2.1K filter on a small board with relays and
>a switch. This would allow me to open up the 870 (more like the 850 which
>the 870 takes much of its design from) and narrow it down to reduce the
>adjacent channel interference that has been a bit of a problem in using
>the 870 for contesting. My hope is that the DSP filter selection for xmit
>and recv will operate as effectively with after a change in the filters.
>Any thoughts or tips among list subscribers?
>Thanks
>Scott C.
>
>
>
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Dave
NR1DX
[email protected]
"A man who picks a cat up by the tail learns a lesson he can learn no other
way" .... Mark Twain