[Elecraft] RTTY Filter and low end of BFO range

[Leigh L Klotz, Jr.]

I have enough BFO range as described in the manual and am able to reach 
4912.97 on the low end.  Unfortunately, while that is low enough for me 
to center the RTTY filters on 1000Hz for one of the sidebands, it is not 
for the other.  On the "bum" sideband, the narrowest RTTY filters are 
centered around 7700-750Hz.  Both CW and CWR filters at 600Hz are ok.

My K2 is #3121 (Rev. B) and is at current revision level on ECOs, 
including the KSB2 bandwidth. I built the KSB2 as the current shipping 
revision, but with parts from Mouser, as it was a week before the mod 
kit came out.

I did some searching and saw some posts from 2003 that said that being 
able to reach RTTY filter frequencies of 2200Hz was often possible only 
on one sideband, but I am not able to hit even my target 1000Hz mark.

On the other sideband, I have no trouble, and so I am fine on the lower 
bands, as I can use R, but on the higher bands, I have to switch 
sidebands for digital modes.  That's not a a problem for some modes such 
as PSK but it is a problem for MFSK and its variants.

Are other K2s able to center RTTY filters on 1000Hz on both sidebands?  
Would this small cap Bill suggests help mine without affecting 
stability?  It looks like I need a 250Hz-300Hz move down on the low 
end.  I don't know if that is reasonable and an effect of the wider 
KSB2, or if I have some other, undiagnosed problem.

73 es TNX,
Leigh / WA5ZNU

On Sat, 7 May 2005 7:25 pm, Bill Coleman wrote:
> I recently had to completely reconfigure my filters, as I added the 
> RTTY mode. In order to accommodate my chosen center frequency of 1500 
> Hz for AFSK RTTY, I had to modify the BFO to get sufficient range. I 
> had already added 47 and 100 pf caps across C174 and C173, respectively 
> when I made the A->B mods and the BFO stability mod. I changed these to 
> 56 and 120 pf, but this did not move the BFO frequencies much at all. 
> Then I added a 3 pf cap from X3/X4 junction to ground. This moved the 
> lower frequency about 800 Hz without affecting the upper frequency.