K2 App. Note:
17m AGC /
S-meter activation from huge signals in the 18m SW band.
6 Nov 2000
Some European K2 owners are experiencing short-wave broadcast interference from just below the 17m band. The result is that the AGC circuit is slightly triggered, and this causes a slight cutback of RX gain with some degree of S-meter indication. These steps should reduce or eliminate this problem.
If you are not experiencing this problem, these changes are not necessary.
1. Change RFC11 in the collector supply of Q22 from a solenoidal choke to a toroid RFC. Wind 17-20 turns (not critical) on a FT37-43 toroid core and install it flat on the bottom of the RF PCB next to the hole or screw where the NB mounts.
2. In the 15/17m band-pass filter, change C30 and C36 from 330pF to 470pF. Alternatively, you can just add a 150pF cap across the existing
330pF caps. Do this on the bottom of the RF PCB.
3. Change C33 from 3.3pF to a 1 or 2pF cap.
4. Peak the 15m band-pass filter inductors at L10 and L11 at 21.200MHz. Peak the 17m band-pass filter trimmer caps C32 and C34 at 18.200Mhz. Tests indicate this does not affect the K2's performance on these bands.
Strong short-wave signals just below the 17m ham band can produce extremely large amplitude mixer products on the collector of post-mixer
amp Q22. RFC11 radiates this RF into the SSB adapter, which is located just above this choke. Replacing RFC11 with a toroid RFC cuts down greatly on the radiated RF and helps keep it out of the SSB adapter and IF signal path, AGC circuits, etc.
The changes to the 15/17m band-pass filter help narrow the bandwidth and improves the rejection of the short-wave signals. Peaking the band-pass filters 100kHz higher helps to put the offending signals a little further down the slope of the band-pass filter's response curve.