Ten-Tec Orion Sub Receiver VHF Filter Modification Prepared by Rick Williams, VE7TK

Transcription

1 Ten-Tec Orion Sub Receiver VHF Filter Modification Prepared by Rick Williams, VE7TK (Note: Those undertaking this modification do so at their own risk. The procedures outline a method of installing an International Radio # MHz filter in the Orion 565 sub-receiver. This procedure has not been approved by Ten-Tec, Inc. or International Radio.) Under no circumstances will the author be liable for any damage to your amateur radio equipment resulting from the installation of this modification. First a warning This modification is not trivial and requires removal of parts from the Orion sub-receiver as well as construction of an impedance matching network. Those wishing to undertake the modification should proceed with appropriate caution. International Radio makes a 45 MHz filter that can be used to replace FL-1 in the 1 st I.F. of the Ten-Tec Orion sub-receiver. The specifications of the 902 filter are: Bandwidth: 4-5 khz typical Insertion loss: 5 db typical Input/Output match: 50 ohms Operating level: 0 dbm maximum Isolation: -60 db typical Filter type: 6-pole crystal filter The stock FL-1 filter, made by Siward, has a bandwidth of 15 khz and is a 2-pole monolithic crystal filter. The 902 filter is much larger than the stock filter (Figure 1) and it is NOT direct replacement for the stock filter. Input/Output Impedance Matching: Figure 1: Siward Filter and InRad 902 Filter The InRad #902 filter has an impedance of 50 ohms on both the input and output. The stock filter (Siward 45S151AT), according to Ten-Tec, has an input impedance of approximately 2,600 ohms on the input and output. Revised: June 21, 2010 Page: 1 of 6

2 After numerous iterations with a broad-band transformer as an impedance matching device, and much assistance from VE7BFK, it was decided that a simpler L/C matching network would be more appropriate. Figure 2 below shows the design used with the high impedance source on the left and low impedance source on the right. Figure 2: Generic Matching Network (Source: ) In the above schematic the following apply: R g is the 2,600 ohm load circuitry that fed the original filter R l is the 50 ohm load of the new InRad #902 filter L is 1.27 µh C is 10 pf Two identical matching networks for the input and output to the InRad filter were built on a single piece of copper clad perf board (Figure 3). In addition, the input filter is in series with a 220 pf DC decoupling capacitor (see Figure 9 for additional details). Figure 3: Circuit Board with 2 Matching networks (Inductors are wound on Amidon T37-6 yellow powdered iron toroid cores) Inductor L was constructed by winding 19 turns of #30 enamelled magnet wire on an Amidon T37-6 (Yellow) powdered iron core. (Calculations suggested 20 to 21 turns were needed but actual measurements indicated a better match was obtained at 45 MHz with 19 turns.) Revised: June 21, 2010 Page: 2 of 6

3 Testing was done by building the matching circuit onto a chassis mount BNC connector. A 2,700 ohm resistor replaced R g and an MFJ-259 antenna analyzer replaced R l. The number of turns on L was modified until the SWR at 45 MHz reached a minimum. (In my case 19 turns was optimum.) Sub-Receiver Board Removal: Figure 4: Inductor Testing Platform 1. Remove the top cover and locate the sub-receiver board located at the rear of the Orion next to the antenna jacks. 2. Unplug all the cables to the board (NOTE: J4, the 45 MHz scan out, has no connection!) 3. Remove the 4 screws at the PC board corners 4. Slide the PC board away from the centre of the chassis to disengage connector U6. 5. Some boards MAY have a resistor soldered to U6 (visible in Figure 5 covered by the white insulation ). Slide the insulation back from the resistor and unsolder the connecting wire from the resistor. (This will keep the resistor attached to U6.) 6. FL-1 is located on the left side figure 5, circled in red about half way up the PC board. Figure 5: Sub-receiver Board Revised: June 21, 2010 Page: 3 of 6

4 Stock Filter Removal: 1. On the back of the sub-receiver board carefully remove the shield covering FL-1 (see X in figure 6 below). Typically the shield is soldered at the 4 corners. Apply heat with a soldering iron to the corners at one end and gently lift that end with a jeweller s screw driver. Repeat the process at the other end. Figure 6: X marks the Shield that needs to be removed 2. Carefully unsolder and remove FL-1. This takes patience if you plan on saving FL-1. (The 3 pins are circled in RED in Figure 7.) Figure 7: FL-1 pin location (shield removed) 3. IF you do not intend to save FL-1 filter, cut the leads on the top side of the board and carefully unsolder the remaining wires from the underside of the board. (The chances of damaging the printed circuit board are greatly reduced with this method.) Revised: June 21, 2010 Page: 4 of 6

5 Locating the 902 Filter & Matching Network: In an effort to keep the interconnecting leads as short as possible (as recommended by International Radio) the filter was located close to the impedance matching circuit board and attached to the main board with double-sided tape (Figure 8). The high impedance side of the matching circuitry is connected to the original location of the 45 MHz Siward crystal filter. The low impedance side connects to the 902 with RG-174 coaxial cable. Connecting the filter: Figure 8: Locating the 902 filter Installation of the impedance m atching circuitry and 902 filter was accomplished using double sided carpet tape. Figure 9 shows the circuit board in grey and the off-board connections. Figure 9: Matching Network Interconnections Revised: June 21, 2010 Page: 5 of 6

6 The stock FL-1 connections, listed as 1 to 3 on connector J in Figure 9, are numbered from the rear of the Orion. That is, pin #1 is closet to the antenna SO-239 jacks on the back of the Orion. A 220 pf capacitor is connected to pin #1 and then to impedance matching circuitry at the junction of the 10 pf capacitor and the 1.27 µh inductor. Pin #3 is connected to the second impedance matching circuit at the same location. Pin #2 is Ground and it attaches to the other end of both 10 pf capacitors and the coax shields of both cables feeding the 902 filter. The centre conductor of the RG-174 coaxial cables feeding the 902 filter attach to the other end of each 1.27 µh inductors. The RG-174 shields attach to the ground connections on the PC board. The 902 filter is symmetr ical (there is no In or Out) and an RG-174 cable goes to each end of the filter. The centre conductor connects to the insulated pin on the 902 and the shield connects to the pin that is connected to the 902 filter case. Final Assembly: Solder the shield in place on the underside of the board. (Tack the shield in place by soldering the 4 corners.) Install the sub-receiver board back into the Orion in the reverse order. Connect the 4 coaxial cables to the correct connectors. (Remember there is no connection at J4.) Solder the wire to the exposed resistor (if it was present) and replace the insulation. Make one final check and then reinstall the Orion s top cover with the 9 torx screws and 4 Phillips screws. Acknowledgements: Thanks to the following for their assistance and guidance during the development of this modification. Joe Young, VE7BFK, for his assistance in the design and testing of the impedance matching circuit. Trey Garlough, N5KO, and Mark Oberm an, AG9A, at International Radio Corporation Paul Clinton, WD4EBR, at Ten-Tec Service Mark Hansen, VE7CA, who provided the matching network calculator (located in the Design Utility section of his website). Barry Gross, N1EU, for his comments and encouragement. December 2009 Rick Williams VE7TK Revised: June 21, 2010 Page: 6 of 6