[Kenwood] re filter mods to TS-870S

Thu, 17 Jan 2002 19:32:23 +0000

I have experimented extensively with changing filters in the 870 to solve 
to problem of strong adjacent channel interference which is a big problem 
up here in the NE during contests particularly on 160 and 80 where the band 
can literally a dozen signals 60db over on the band at one time.  The 
effect is hard to describe but the DSP goes nuts trying to deal with it and 
it sounds sort of like a sound affect for Tinkerbell in a Walt Disney cartoon.

The problem is not with the filters unfortunately but with what is 
sometimes called "IF blow by" . I discovered this quite by accident as I 
had the radio on the bench with the signal generator still hooked up to 
provide a 60DB S meter reading. In the process of changing filters one time 
I powered up the radio by accident while there was no filter in the board 
to my amazement the S meter read S-7. Several other experiments were 
then  undertaken to see if this was caused by leakage through the pin diode 
switching of stray coupling. By grounding other sections of the IF through 
a  .1uf cap  in that vicinity it was determined that the signal path was 
stray coupling or board layout related and not the switching diodes. It 
seems that the only real solution may be to put a 2nd filter IN SERIES with 
cable going from the output jack of the 8.33 MHz to the  455khz mixer 
section. When I get time again I will pursue that approach but it is not as 
simple as just sticking another filter in series with the output of the 
existing 3KHZ filter because of impedance matching problems. which if not 
handled properly will detune both filters. Too bad too this is the only 
problem I see in making the 870 one of the best rigs of all time. Kenwood 
much like a husband caught philandering continues to deny that there is a 
problem, "That's their story and their sticking too it" Their solution is 
turn up the attenuators and turn off the preamp (AIP) to get the RF levels 
in the IF to point that the blow by doesnt tickle the DSP/AGC .

Another point to consider here is that the AGC is very much part of the DSP 
feedback loop. That is what the 870 has a continuously variable AGC 
control. To get the most out of the DSP when you are not plagued by strong 
adjacent signals try adjusting the AGC depending on what kind of noisy 
signal conditions you are faced with. This is not intuitive at first but it 
will give you some additional edge with this receiver.

Dave
NR1DX

PS: Oh yea and before you go playing get a service manual ...invaluable. 
This radio is really pretty easy to get into and play with, but you need 
the manual. Also a lot of interesting theory of operation stuff in there as 
well.

What all this At 11:38 AM 1/17/2002 -0700, Fern Rivard wrote:
>Have any of you folks changed the 8.83 mhz IF filter and the 455 I F filter
>in the TS-870S with sharper filters from International Radio or InRad and
>what were you results like? Also, what is the web site for InRad as I can't
>seem to find it now.
>     Another observation is using the SM-220 monitor scope with the TS-870S
>is that the display on receive incoming signal is very very low if the AIP
>is turned ON on the 870S. I have NOT done any mods to my radio as yet except
>to replace the main eprom located in the front panel which has made some
>improvement to the DSP especially in the receive mod. Have any of you users
>noticed that and do you have a fix for it? I contacted Kenwood service
>department in Toronto this morning and they were NOT aware of that problem.
>     Best of 73's from Fern
>
>
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