TopBand: TS940 CW filter performance??

[MEL]

Still trying to optimize the rig for upcoming Topband season. I posted 
this to Kenwood user newsgroup but only one reply telling me that I was 
imagining things.

Looking for your actual experiences regarding the TS940. Don't waste your 
time reading all this if you don't have a 940.... most people won't care.

Bought a used 940 (SN 8MM) years ago for 100% CW work. Noticed that the 
front end was quite "open" so added the 455 KC IF 500 Hz filter. (YG455)

This filter did help front end IMD and overload, but it also boosted 
signal levels, both audibly and on the S meter! Not only that, but it 
made the VBT filter a lot of fun to use. With VBT full clockwise (open) I 
got maybe 600 Hz bandpass, full closed it's about 250 or so. With the 
NARROW filter out of the circuit, performance was as before. In the 
circuit, there is still significant front end overload, especially with a 
noise blanker inline. Background noise is only a little improved with 
filter inline. Position of VBT had no effect on signal levels, maintained 
high S meter reading (but narrower bandpass) when full closed. As I said, 
S meter readings and noise level increased with filter IN.

Question 1: Has anybody else noticed this phenomenon (the signal boost 
with just the 455 filter)?


Looking for better CW contest performance, I added the 8.8 MHz IF 500 Hz 
filter last weekend and cranked up for the IARU contest, running the 455 
and 8.8 (the YK-88C-1) cascaded to tighten the front end. I noticed that 
the VBT was a lot more critical, and with VBT wide open I got no more 
than 500 Hz bandpass (at least 6-10 dB skirt). I could close the 
VBT no more than 50% (top center position) before signal strength fell 
off significantly. At this point, bandpass is about 150 Hz and I don't 
need my outboard DSP!!! Front end is more bulletproof, and strong signals 
0.5 Khz away have little effect at this VBT setting. The bad news is that 
the NARROW filter now reduces overall receiver sensitivity, where before 
(with just the 455 filter) I got a sensitivity boost. I like to run CW 
sidetone around 450-500 Hz, which is off the center freq of most IF 
filters. (for example, TenTec peaks their IF center frequency at 750 
Hz).. so there may be a sensitivity drop if running sidetone different 
from 850 Hz, which is top dead center of 940's PITCH control and appears 
to be where the sensitity is best with VBT closed up. I like 500 Hz, so I 
must keep the VBT at 3 o'clock position (about 500 hz B/W) to not lose 
signal strength.

Question 2: anybody else running the 8.8 filter notice this?

It looks like a tradeoff... for contests, the extra IF filter is a plus, 
as sensitivity usually not a big issue for a station running low power 
class in contests. But on 160 and 80 meter CW in winter, sensitivity will 
be more critical ... (maybe not, as the sensitivity on the 940 was almost 
TOO good, and I got a lot of atmospheric QRN crap that I needed to use the 
attenuator on last winter on 160... since I like 400 hz for "ESP" 
contacts on 160, this may still be OK)

Question 3: Has anybody tried adding separate switching to take individual 
IF filters in and out, like newer rigs do now? (Tested an Omni VI+ for a few 
weeks, and I liked that feature... not enough to like the 750 Hz 
sidetone. Also tested the 1000MP and other rigs with same feature)

If so, how did you do it? Without looking at schematics, I am thinking of 
wiring the filter jumper (that you cut when you install the filter) back 
into the circuit via a switch. But wire length and capacitive coupling 
may be a problem. Nothing to lose by trying, but first let's find out 
what others have done. May need solid state switch at the filter.


Question 4: Another option would be a Fox Tango filter at 500 hz or 400 hz 
center freq... anybody tried this on a 940?)

Thanks for your time...

Mel KJ9C



PS.... my trip to China has been postponed .... Still hope to operate BY4SZ 
site before the 160 season starts. The station manager says they have no 
space for a 160 antenna, but you guys have demonstrated that you can put 
a signal on top band from a small city lot. Worst case is that I visit in 
winter and string antenna first. (Yes, I have license in hand... took 
only 2 weeks to arrive via mail... faster than FCC...)


--
FAQ on WWW:               http://www.contesting.com/topband.html
Submissions:              topband@contesting.com
Administrative requests:  topband-REQUEST@contesting.com
Problems:                 owner-topband@contesting.com