Elecraft K2 Preselector Mod

Transcription

1 Elecraft K Preselector Mod The elecraft K is an excellent transceiver, but in Europe its receiver benefets significantly, especially on 0m, from an external Preselector. Although most Preselectors are designed with an internal T/R relay, theoretically enabling direct insertion between the transceiver and the antenna, there is a problem when running W, especially when running full QSK. The switching time of the internal relay is too slow to follow full QSK. The best way to avoid this problem altogether is to insert the Preselector directly into the transceiver s RX antenna line, thus avoiding entirely the need to switch the Preselector in and out. This feature has emerging as standard on a few of the more recent transceivers (e.g., Yaesu FT-000, Ten-Tec OMNI-VII), but for older rigs, a very simple modification is necessary. The following is a step-by-step description of how to install this mod:. Install RA Phono (inch) jacks on the back panel of the K. These should be installed in the existing holes for XVTR OUT and XVTR IN. If your radio is an older version, you will have to << AREFULLY >> drill two small holes for this. You can identify the position for the holes from current pictures posted on the elecraft web site.. The preselector must be inserted just before the antenna is applied to the first RF stage of the RX. This is easiest accomplished (physically) by interrupting the RX antenna line just BEFORE K (Attenuator Relay) on the main printed circuit board. The antenna signal is normally applied to this relay through.. Unsolder and lift the leg of which is closest to the relay K.. extend this leg vertically upwards, and use this leg to solder the inner conductor of a short piece of RG-U. Trim the braid (shield) of the coax at this point and insulate it with a small piece of heat-shrink tubing.. Solder the inner conductor of a second short piece of RG-U into the hole from which you removed the one leg of. Trim away the braid and insulate as in above.. Solder the cable from to the jack marked OUT. onnect the cable s shield to chassis ground.. Solder the cable from the printed circuit board (K) to the jack marked IN and also connect its shield to chassis ground. Unless your preselector is a passive device and fully bi-directional, you must pay attention to cable connection: onnect the K s OUT to the Preselector s IN onnect the Preselector s OUT to the K s IN NOTE: When there is no external Preselector connected to the K, you must insert a small coaxial jumper (RG- or RG-) between new IN and OUT jacks. Secondary Benefit of this Mod: You may also insert Noise anselors (e.g., S.E.M. QRM Eliminator, TimeWave AN-, etc.) into these jacks to help reduce the effects of heavy QRN. By Rick Westerman, NJ0IP

2 OUT IN

3 RF 00µH D N00 D N00 RF 00µH.0.0 TX VFO T.0 R.K R9 Q0 N000 T BPF (Sh. ) 0. R Xmit Mixer Attenuator -0dB Buffer.0 U9 LT V -.0 R9 0 V- 00pF K R R U0 NE0 R 00 R9 R9.K R9.K.0 R 0 K W R 0 R 0 V XFIL In T D9 S S B 0 R Q N09 RF Preamp db.0.k.0.0 RP, RP: 00K A d a p t e r.0 R0 0K RX VFO V BIAS-XFIL RP RP RP RP RP J9 9 R X X X9 X0 D0 D D D J V Z TUF- Rcv. Mixer RP X R dbm.0 D J MHz Variable-Bandwidth rystal Filter AUXBUS T /DOT-PTT MI AF SSB ontrol V RFDET D9-D: SV9 A EXT AL (NOTE ) V XFIL W XFIL Out NOTE : Remove when SSB Adapter is installed. R0 0 T.0 R.K V RF 00µH Q N09 Post-Mixer Amp. R9.K (NOTE ) R.9K N NOTE : D0 and D were added to improve handling of extremely D0 strong signals (from nearby transmitters). These diodes must be soldered on the back of the P board (see text). D.0 AG R.Ω IF Amp.0 V AL R 0. V AG.0 R R 0.K R 0 R 9 00 R9 0 R U M0 D L.uH 0-0 BFO Buffer/Attenuator N o i s e B l a n k e r I.F. OUT W NB Bypass.0 D9 MV09 nd Xtal Filter.0 TP Q N000 Q PNA X V HI IP D S V XFIL Q J R0 00K G X AUXBUS A 0. X.9MHz BFO Elecraft J 0.0 R 0 Product Det. µh D SV9 00K R00 0 V- R9 00 -db, Z= 0Ω X L V By W. Burdick Rev. E.Swartz F 0. U NE0.0 0 R90 0 NOTE: If Noise Blanker is installed, R and R90 must be removed, and R9 replaced with a jumper. R0.K 00K D SV9 00K K RF Board J Date Sht. //0 of Appendix B Aux. AF PD PD V BFO 00K.0

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5 9 90 R L 9 Q L J L K K L J D R J 0 J RF Q Q 0 W 9 0 R B R0 E L0 R Z 0m Q 99 K K9 A L9 L P L L B E AUX L9 T T K B RF L T D K0 L 9 9 B Z T K 0 R E Q L K 0 Q Q L0 L R9 L K Q L 0 K U U K (SOKET) L K RF Rev. B L 00 ELERAFT D9 Q Q D Q K K R Q 9 Z D - P R D Q R9 R0 T RF 9 R R TP R AUX V 0 W R R J 0 R T R9 L0 R0 K K K R Z R9 R J9 F R R9 W U R9 RP D9 U9 00K Q0 R9 R90 R X 0 D0 D R D D R X D9 D R R0 R R X9 9 R U R TP D X0 D D0 D D R0 R9 Q 9 J 00K R0 D X 00 R 9 Q R SSB RP 9 T RP 0K 9 R 00K 9 D D W D X J0 P 0 D D U 9 Q9 RF X R TP U X X J RP 0K SPKR X D9 RF U X J R LIFT L R S R0 J J S S D R R X 0 0 R J J R ONTROL BOARD BEVELED ORNER D R NA FRONT PANEL BOARD.M R9 RF Probe Elecraft R P EXT INT Q Q Q D RP K U U0 V Q 0 0 MAX P LM0 OUNTER P U RP.9K Q P 0 RP RP Q INSTALL SOKET FIRST K K 0 U Q R R R9 R0 U PI 9 Q9.K L 9 R0 9 D 9 0 V R 0 U X P Q P Q0 R U V RP RP P J J 0 K U 0 Q K ONTROL Rev. B 00 ELERAFT S S RP 0 DS R E E TIP L.0 R U AND ITS SOKET ARE MOUNTED ON THE OTHER SIDE D D DS R R0 D R D R9 R Q D 9 K Q Q R J R R S S9 S0 S S S 9 Appendix F Parts Placement Drawing, Top S S LINEAR LINEAR D R Q D U RP 0K Z S S S S D J AUDIO LINEAR LINEAR R R R 0 0 R K FP Rev. B 00 ELERAFT