Ham Radio 2000 #2

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From bruce@pixar.com Thu Sep 23 21:51:17 EDT 1993 Here's everything I learned about Motorola MICOR and MITREX radios in a week. The people who contributed information are: "Chris Terwilliger" <Chris_Terwilliger@tempeqm.sps.mot.com> Derek Toeppen <derekt@col.hp.com> Peter_Simpson@3mail.3com.com cdp (Chris Peckham) jreese@NeoSoft.com (Jim Reese) lyman@a3bee2.radnet.com (William_Lyman) Any errors that have crept in are my fault, not theirs. However, nobody but you is responsible if you follow these instructions and something goes wrong. If you have more information on these radios, please send it to me. Thanks Bruce Perens KD6OTD/AE Motorola's parts hot line is 1-800-422-4210. The people at this hot line wanted $11.55 for the MITREX manual and took my credit-card order right away. My local Motorola parts depot quoted $25 for the same manual, did not have it in stock, and then could not figure out what the shipping cost would be so they suggested I drive out to the depot and pick it up when they got it. >From this experience I would suggest that you use the 800 number for parts. The part number for the MITREX UHF manual is 68-81045E75, $11.55 As for channel elements, you can either send them in to one of the many companies that do them, or you can buy crystals directly and solder them in yourself. Companies that will make crystals are: JAN Crystals, 1-800-526-9825, $11.00 per crystal. Bomar, 1-800-526-3935, $15.00 per crystal or $25 to re-crystal your channel element for you. UHF and VHF MITREX elements should be KXN1086 for receive and KXN1088 for transmit. The crystal companies know the specs for the elements, all they want to know from you is the transmit and receive frequencies. For the MICOR, but not the MITREX, there is a company called Channel Element HQ, 1-800-237-9654. They maintain a very large stock of used channel elements. They will sell you one on your frequency for $25.00 or $20.00 with trade in. The only catch is: do they have your frequency? Amateur frequencies are commercial some where else in the world, so they probably do. All you need to give them is the model number of the element, the frequency and, for receivers, if its high or low injection. The advantage of them is there is no waiting for re-crystaling. This modification of a MITREX 440 to a 9600 full-duplex repeater is from William Lyman, lyman@a3bee2.radnet.com: I have successfully completed a assembling a 9600 fullduplex data repeater. I used a Mitrex T44 with an MFJ 1270 TNC and Tapr 9600 baud modem with bit regen. This repeater in now in use at Bethany CT, Portland Ct, and Bridgeport Ct and they are all working GREAT. My first feeling was it wasn't going to work. I though that internal desens would kill the receive. Here are some hits and suggestions for making this work Please remember these mods for a Motorola Mitrex T44 UHF model. 1) Be sure to get the elements temp comped! I didn't at first and as it got used it drifted badly as the unit got hot. 2) I maked 2 of the mods that MoAmps published for increasing high frequency response at the limiter/Detector. If you are on a fringe than this will really help. 2a) Remove C240 a .0022 cap. This can be achieved by physically grabbing it with pliers and breaking it away from the PC board. Or you can remove the shield cover and unsolder it. I did the later and that what I suggest. 2b) Remove C451 a 220 cap between U403a pins 2 and 3. 2c) Replace R231, 75.k resister with 2.2k resister 3) Receive audio is (Det) is available on pin 11 of the control cable connector. 4) Lock receive element on. Please note this is for fullduplex only this will cause unit receiver circuitry to be on all the time. If you did this mod in simplex the modem would false by hearing you only transmitted signal and would take longer time to lock. 4a) Locate Q1 and add a jumper across the e and c junction 5) The following are for transmitter mods 5a) locate pin 15 of the interconnect board. This is not pin 15 where the control cable connects but pin 15 on the connector that connects the interconnect PCB to the main PCB. Solder wire from this connection to pin 1 on the backside of where the control cable connects. With unit facing you (lock towards you) this will be the top right most pin. You will see either a white jumper or a coil (small) that goes from pin 1 to the rest of the circuitry. Remove one side of this jumper. 5b) Remove one side of each of the following caps. C507,C508,C509. This will remove the splatter filter allowing you to use Q503 and q504 to drive the element. You could drive the element directly but I found that there was distortion when driving the element hard. This also matches the Impedance on the Element. 6) Xmit audio is now available on pin 1 on the control cable connector. 7) Rec/Xmit ant connections 7a) On my unit the xmit connection to the ANT relay was hard wired on to the final AMP. I didn't have any small coax so I left the ANT relay in. My transmitter connection is at the existing ANT connector. For the receive I unplugged (RCA JACK). Trace back from ANT switch. I taped the jack and left in place. I drilled a hole in the top cover over where the RCA jack plugged in. I then made a cable with an RCA jack thus making this the Receiver connection. I only drilled the hole in the cover because covers are easy to come by if I make a mistake. 8) Cable connections 8a) The following connection to the motorola female connector should be made. pin 17 to pin 6 this is negative pin 4 to pin 12 to + fused at 2 amps (Rec/Xmit enable) pin 19 + fused at 20 Amp pin 13 PTT pin 1 xmit audio pin 11 rec audio 9) Summary 9a) it is very important that the receive cable be of the best quality. Remember it is VERY close to the Xmiter. When you adjust the Xmiter follow the manual very close. You can't have any spurs. 9b) I did experience some desense from the xmiter circuitry in the range of .03 uV. I find that to be almost non existent. There might be other ways or mods. These have worked for me in 3 installations. The mods are made so if you decide to resale the radio. Just replace cover, install splatter caps, Install jumper on pin 1, And remove jumper from pin 15 interconnect to pin 1 on cable connector. And resale. I am running the unit at 50 watts. It get warm, well very warm. I would suggest that you place some blower fans blowing on the heat sink. You could get a T74 (110 watt) unit and run it all day at 50 watts, But they are very hard to come by. 73's and good luck This modification of a MICOR UHF to duplex mobile is from Jim Reese, jreese@NeoSoft.com Why would you want a duplex mobile, you ask? Once you've experienced it, you'll never go back to jap-trac's. This describes how to take a standard UHF micor mobile radio and make it work as a full-duplex mobile radio. Since Texas is high-in, low-out on UHF, this also includes which cap changes are required to make the radio receive below 445 MHz. There are diagrams referenced in this text. If you are unable to figure out what to do without the diagrams, mail me at jreese@sugar.neosoft.com and I'll try to get you a copy of the diagrams. Jim Reese WD5IYT Conversion of Motorola MICOR UHF Radio For Use as a Duplex Mobile This conversion is not for the faint-hearted. Make sure you are very familiar with the operation of the Micor radio before attempting this conversion. As with all modifications of this type, insure that the radio is operating correctly BEFORE any modifications are made. It is much easier to fix it before you hack it up. Do not attempt this conversion without a service manual. You need the PC layouts and tune up instructions from the service manual in order to perform the modification. Useful Motorola Part Numbers 68-81015E70-H Manual for UHF MICOR KXN-1024 5ppm Channel Element for UHF MICOR KXN-1029 2ppm Channel Element for UHF MICOR KLN-6210A PL Encode Reed TLN-8381A PL Decode Reed 66-84690C01 Contact Removal Tool for MICOR plugs TKN-6458A Large Fuse Holder for Primary Power (40A) Crystal Ordering Information When ordering crystals, specify the KXN-1024A channel element number. Always order the crystal on the RECEIVE frequency. If your radio will be a high transmit unit, the crystal will be LOW side injection. For low transmit radios, order the crystals on HIGH side injection. This will flip the transmit offset to the other direction. Conversion Instructions Step 1: Make sure that the radio is operating properly by tuning it up with one of your crystals before any modifications are made. If you are using the radio as a high transmit unit, you must make the capacitor changes in step 2 for the radio to work properly. Step 2: If your radio will be receiving below 445 MHz, change C117 to 27pF, C119 to 39pF, and C125 to 12pF on the receiver board. This is not necessary if the radio will be receiving above 445 MHz. Step 3: You must make some modifications to the control board and the receiver audio/squelch board in order to make the radio full duplex. First, remove JU-905 on the control board. Next, jump pins 1 and 4 of the audio squelch board. On the later version audio squelch board, there is a place for a jumper (JU-202), on earlier units, just make the jump with wire wrap wire. Add capacitors at the following points on the audio squelch board. Add a 100pF cap between P903-5 and P903-6. On IC-201, add 15pF caps between pins 3 and 4, and between pins 3 and 5. Add 100pF caps between pins 6, 7, 11, and 13 of IC-201 and ground. On IC-202, add 15pF caps between 5 and 9, 5 and 13, and a 33pF cap between 5 and 15. This makes the board less susceptible to RF. Keep the leads on these caps as short as possible. Step 4: Carefully remove the front casting from the chassis. This is done by removing the four screws top and bottom as well as two screws on the control head plug. This is kind of tricky, so be careful to remember how you got it apart so you can re-assemble it later. Examine the Power Amplifier section of the radio and notice the miniature connector which connects the output of the PA to the circulator. Unplug this connector from the circulator using a needle nose plier or hemostat. Turn over the radio and remove the power control board. This will expose the top plate of the circulator. Remove the circulator by carefully removing the sensing wires which connect to the power control board and the two screws which hold the circulator in. You will have to unplug the receive antenna coax from the preselector unit in order to remove the circulator. Set the circulator aside for later modification. Step 5: Mount a BNC chassis mount connector on the top side of the front casting on the side opposite from where the lock is located. This will be the receive antenna connection. Be very careful to locate this connector so that it does not hinder the operation of the latch mechanism. Attach a small coax to this connector and route it to the receive antenna jack on the preselector unit. Drill a hole in the front of the radio chassis to pass the coax. This will be obvious once you have examined the unit with the front casting removed. Step 6: This is the toughest part of the conversion, the circulator modification. Remove the cover from the circulator unit. You will notice that there is a circulator, an output filter, the antenna switch, and the circulator reject load. There are three trimmer caps, only one of which has an access hole in the top plate. Measure and drill the top cover so that you have access to all three trimmers from the outside. This is necessary because the cover affects the tuning of the circulator. After drilling the cover, set it aside. You must now remove the antenna relay. This is a small relay on the right side of the circulator. The small dark red or green rectangular unit with a wire coming from the relay is the reject load for the circulator. This is a ceramic 75W 50 Ohm resistor. The relay switches the output port of the circulator between the receiver and the reject load. Be EXTREMELY CAREFUL when soldering on the reject load, as the top terminal can break off of the ceramic very easily. I suggest cutting the wire from the relay, removing the relay, and then removing the wire from the load resistor. Once the relay is removed, wire the dummy load back to the output port of the circulator which is on the common side of the relay. Refer to the manual for the circuit. The easiest way to accomplish this is with a small piece of teflon coax (RG-188). Run from the circulator port to the reject load. You can solder to the circulator case for the shield on the load end of the coax. Replace the cover on the circulator and reinstall it in the radio. Tuning Instructions Before tuning, disable the receiver AFC by soldering a wire from the "AFC OFF" trace on the receiver board to ground. The procedure for disabling the AFC is described in the receiver tuning instructions section of the service manual. Tune the radio per the Motorola manual. Once you have achieved this, you need to tune the circulator. The following procedure should be followed: Remove the power control board, and power the radio with a supply having a current meter. Attach a jumper or clip lead from feedthrough C527 on the Controlled Stage in the PA compartment and feedthrough C536 on the driver stage in the PA compartment. This will force the radio to maximum power output. Key the transmitter and tune the three circulator capacitors for maximum power output. Reinstall the power control board, and preset the drive limit pot fully counter-clockwise. Set the power set pot to the desired power output level. Key the transmitter and tune the center circulator capacitor (the only one accessible from the top of the power control board) for minimum current draw. You should be able to make several Amps difference without affecting the power output. Turn the drive limit pot 1/4 turn clockwise, or until power just starts to fall off. That's it. Remember to always set the receive frequency first when setting frequency, as this affects the transmitter also. Set the transmitter with the offset trimmer coil on the exciter board. Make sure that you have adequate cooling space around the PA heat sink fins when the radio is installed. The Micor PA is not easy to fix, and when it blows, it blows big. -- Jim Reese, WD5IYT | "Real Texans never refer to trouble jreese@sugar.neosoft.com | as deep doo-doo" --Molly Ivins This modification of a MICOR UHF to repeater service is from Jim Reese, jreese@NeoSoft.com Instructions for Conversion of Motorola UHF Micor to Repeater Service This modification is not for the faint-hearted. Be sure you are very comfortable with the operation of the MICOR radio before you attempt this modification. A thorough understanding of the way the MICOR radio and control system works is absolutely essential for the success of this conversion. Modification Notes Whenever "Molex Pin x" is mentioned, it refers to the Armadillo Standard connector. This is a small, 9 pin Molex connector which serves as the interface for all of the Armadillo radios. Use a Female connector on the radio end. The standard pinout of the connector is: 1 - Ground 2 - Receive Audio Output 3 - n/c 4 - Transmit Audio Input 5 - COS Output 6 - PTT Input 7 - n/c 8 - PL Decoder Output (Sense) 9 - PL Select Input The COS output and PTT input are open-collector to ground signals. Pin 8 goes to +9.6 V when PL is decoded. Pin 9 is ground for PL, open for Carrier. These modifications allow "PL and Squelch" operation. This means that when in PL, the normal squelch circuit still operates. This avoids long noise bursts upon unkeying. If using an Advanced Computer Controls controller, the signal at pin 8 is run to the "PL Sense input" on the controller. It will drive this input directly. Order the crystal on the repeater RECEIVE frequency. You must specify HIGH SIDE INJECTION. If you do not order a high side crystal, the transmit offset will be 5 MHz above the receive frequency, instead of 5 MHz below. I recommend sending the channel element to the crystal manufacturer for temperature compensation. Use a KXN-1024 element for 5ppm accuracy, or use a KXN-1029 element for 2ppm accuracy. Disable the receiver AFC by jumping the appropriate pin of the channel element to ground. List of Tables and Illustrations There are several illustrations which are detailed below. Refer to them as needed when modifying or constructing pieces of this project. Table 1: This is a complete hookup chart for the control head plug and interface board. Figure 1: Schematic of control head plug, with simple diagram. Figure 2: Board Layout of audio squelch board detailing added capacitors. Figure 3: Detail of circulator unit showing location of major components. Figure 4: Schematic of circulator unit before modification. Figure 5: Schematic of circulator unit after modification. Figure 6: Schematic of interface board. Figure 7: Rough layout of interface board. Useful Motorola Part Numbers 68-81015E70-H Manual for UHF MICOR KXN-1024 5ppm Channel Element for UHF MICOR KXN-1029 2ppm Channel Element for UHF MICOR KLN-6210A PL Encode Reed TLN-8381A PL Decode Reed 01-84307A89 Empty Control Head Plug less cable, complete with pins 66-84690C01 Contact Removal Tool for MICOR plugs TKN-6458A Large Fuse Holder for Primary Power (40A) Step-By-Step Instructions for Modification Step 1: Make sure that the radio operates properly BEFORE any modifications are done. If there are problems with the radio, they will be easier to fix before the mods are done. Step 2: If you are satisfied with the operation of the radio, construct the control head plug per figure 1 and table 1. Use miniature pots and switches inside the control head plug. It is rather tight inside, so be careful to check that the connector will re-assemble before you drill it up. After you have constructed the connector, make the following connections inside the radio: Jump control head pins 3, 8, and 22 to "A+" on the control board. Jump pins 9, 11, and 17 to ground on the control board. Next locate the two feedthrough caps which power the Transmitter Power Amplifier. They are just to the left of the control head plug on the bottom side of the radio. One will have a red wire, and the other a black wire. Jump from the feedthrough with the red wire to the "A+" trace on the control board. Jump from the feedthrough with the black wire to the ground trace on the control board. There is a reverse polarity protection diode on the control board which can be used to make these connections. Next, plug your control head plug into the radio and verify proper operation with the new control arrangement. If there are problems here, troubleshoot them NOW. Do not wait until later, as you may be chasing more than one problem. Step 3: You must make some modifications to the control board and the receiver audio/squelch board in order to make the radio full duplex and to make preparations for the interface board. First, remove JU-905 on the control board. Then, jump the F1 channel element to ground as described in the Motorola manual. Next, jump pins 1 and 4 of the audio squelch board. On the later version audio squelch board, there is a place for a jumper (JU-202), on earlier units, just make the jump with wire wrap wire. Next, remove a jumper on the audio squelch board which goes from pin 3 of the PL decoder to IC-202 pin 8. This is near the pins which the PL board plugs into. Next, ring from pin 3 of the PL decoder to pin 11 of the audio squelch board with an ohmmeter. There should be continuity here. If not, add a jumper. The later boards have a trace from pin 3 of PL to pin 11 of audio squelch, on earlier boards, pin 11 is unused, and you should jump from PL decoder 3 to audio squelch 11 with some wire wrap wire. Add 15pF capacitors between the following pins on the two chips on the audio squelch board. Figure 2 details the location of these caps. On IC-201, add a cap between pin 3 and 4, and between pin 3 and 5. On IC-202, add caps between 5 and 9, 5 and 13, and two caps between 5 and 15. This makes the board less susceptible to RF. Step 4: Next, carefully remove the front casting from the chassis. This is done by removing the four screws top and bottom as well as two screws on the control head plug. This is kind of tricky, so be careful to remember how you got it apart so you can re-assemble it later. Next, examine the Power Amplifier section of the radio and notice the miniature connector which connects the output of the PA to the circulator. Unplug this connector from the circulator using a needle nose plier or hemostat. Next, turn over the radio and remove the power control board. This will expose the top plate of the circulator. Remove the circulator by carefully removing the sensing wires which connect to the power control board and the two screws which hold the circulator in. You will have to unplug the receive antenna coax from the preselector unit in order to remove the circulator. Set the circulator aside for later modification. Step 5: Procure a BNC chassis mount connector for a receive antenna jack. Mount this connector on the top side of the front casting on the side opposite from where the lock is located. Be very careful to locate this connector so that it does not hinder the operation of the latch mechanism. Attach a small coax to this connector and route it to the receive antenna jack on the preselector unit. Drill a hole in the front of the radio chassis to pass the coax. This will be obvious once you have examined the unit with the front casting removed. Step 6: This is the toughest part of the conversion, the circulator modification. Figures 3, 4, and 5 detail the conversions. Remove the cover from the circulator unit. You will notice that there is a circulator, an output filter, the antenna switch, and the circulator reject load. There are three trimmer caps, only one of which has an access hole in the top plate. Measure and drill the top cover so that you have access to all three trimmers from the outside. This is necessary because the cover affects the tuning of the circulator. After drilling the cover, set it aside. You must now remove the antenna relay. This is a small relay on the right side of the circulator. The small dark red rectangular unit with a wire coming from the relay is the reject load for the circulator. The relay switches the output port of the circulator between the receiver and the reject load. You must remove the relay and wire the dummy load back to the output port of the circulator which is on the common side of the relay. Refer to the diagram for the circuit. The easiest way to accomplish this is with a small piece of teflon coax (RG-188). Run from the circulator port to the reject load. You can solder to the circulator case for the shield on the load end of the coax. Replace the cover on the circulator and reinstall it in the radio. Step 7: Construct the interface board using the schematic and board layout in the packet. Install this board on the three unused mounting tabs near the rear center of the radio. These tabs are above the control board. Wire the board up as described in table 1. At this point, the radio will be operating full duplex, and you should be able to put the radio in PL by flipping the switch on the control head plug to the PL position. Verify that you have +9.6 volts at the 9 pin molex connector PL SENSE pin when the correct PL code is being sent. Also verify correct receive audio gating and COS action. Tune Up Instructions Tune the radio per the Motorola manual. Once you have achieved this, you need to tune the circulator. The following procedure should be followed: Step 1: Remove the power control board, and power the radio with a supply having a current meter. Step 2: Attach a jumper or clip lead from feedthrough C527 on the Controlled Stage in the PA compartment and feedthrough C536 on the driver stage in the PA compartment. This will force the radio to maximum power output. Step 3: Key the transmitter and tune the three circulator capacitors for maximum power output. Step 4: Reinstall the power control board, and preset the drive limit pot fully counter-clockwise. Set the power set pot to the desired power output level. I recommend no more than 50W out for a 75W radio, and no more than 60W out for a 100W radio. The 25 and 45W radios are rated continuous duty at 25W output. Step 5: Key the transmitter and tune the center circulator capacitor (the only one accessible from the top of the power control board) for minimum current draw. You should be able to make several Amps difference without affecting the power output. Step 6: Turn the drive limit pot 1/4 turn clockwise. That's it. you are now ready for major repeating action. Remember to always set the receive frequency first when setting frequency, as this affects the transmitter also. Set the transmitter with the offset trimmer coil on the exciter board. Make sure that you have adequate forced air cooling on the PA at all times during operation. The Micor PA is not easy to fix, and when it blows, it blows big. This modification of a MICOR VHF to repeater service is from Jim Reese, jreese@NeoSoft.com: Instructions for Conversion of Motorola VHF Micor to Repeater Service This modification is not for the faint-hearted. Be sure you are very comfortable with the operation of the MICOR radio before you attempt this modification. A thorough understanding of the way the MICOR radio and control system works is absolutely essential for the success of this conversion. Modification Notes This modification was developed for radios having receiver RF/IF board series TLD8270A; audio/squelch models TLN4310A-2 thru TLN4310A-5; and exciter models TLD8261A, TLD8262A, TLD8261B, or TLD8262B. If your radio differs from these part numbers, all or part of this modification may be different from the descriptions. Use your judgement to alter these instructions as necessary. Whenever "Molex Pin x" is mentioned, it refers to the Armadillo Standard connector. This is a small, 9 pin Molex connector which serves as the interface for all of the Armadillo radios. Use a Female connector on the radio end. The standard pinout of the connector is: 1 - Ground 2 - Receive Audio Output 3 - n/c 4 - Transmit Audio Input 5 - COS Output 6 - PTT Input 7 - n/c 8 - PL Decoder Output (Sense) 9 - PL Select Input The COS output and PTT input are open-collector to ground signals. Pin 8 goes to +9.6 V when PL is decoded. Pin 9 is ground for PL, open for Carrier. These modifications allow "PL and Squelch" operation. This means that when in PL, the normal squelch circuit still operates. This avoids long noise bursts upon unkeying. If using an Advanced Computer Controls, S-Com or other commercial controller which requires an active high PL sense signal, the signal at pin 8 can be attached to the "PL Sense input" on the controller . It will drive this input directly. On the S-Com controller, the pull up resistor on the PL sense input must be removed for proper operation. List of Tables and Illustrations There are several illustrations which are detailed below. Refer to them as needed when modifying or constructing pieces of this project. Table 1: This is a complete hookup chart for the control head plug and interface board. Figure 1: Schematic of control head plug, with simple diagram. Figure 2: Board Layout of audio squelch board detailing added capacitors. Figure 3: Schematic of interface board. Figure 4: Rough layout of interface board. Useful Motorola Part Numbers 68-81008E40-D Manual for VHF MICOR K1007A TX Channel Element for VHF MICOR K1005A RX Channel Element for VHF MICOR KLN-6210A PL Encode Reed TLN-8381A PL Decode Reed 01-84307A89 Empty Control Head Plug less cable, complete with pins 66-84690C01 Contact Removal Tool for MICOR plugs TKN-6458A Large Fuse Holder for Primary Power (40A) Step-By-Step Instructions for Modification Step 1: Make sure that the radio operates properly BEFORE any modifications are done. If there are problems with the radio, they will be easier to fix before the mods are done. Step 2: If you are satisfied with the operation of the radio, construct the control head plug per figure 1 and table 1. Use miniature pots and switches inside the control head plug. It is rather tight inside, so be careful to check that the connector will re-assemble before you drill it up. After you have constructed the connector, make the following connections inside the control head plug: Jump control head pins 3, 8, and 22 to +12 V. Jump pins 9, 11, and 17 to ground. Plug the newly constructed control head plug into the radio and verify proper operation with the new control arrangement. If there are problems here, troubleshoot them NOW. Do not wait until later, as you may be chasing more than one problem. Step 3: There are some modifications required on the control board and the receiver audio/squelch board in order to make the radio full duplex and to make preparations for the interface board. Remove CR201 on the audio/squelch board. Then, jump the F1 channel element to ground as described in the Motorola manual. Next, jump pins 1 and 4 of the audio squelch board. On the later version audio squelch board, there is a place for a jumper (JU-202), on earlier units, just make the jump with wire wrap wire. Next, remove a jumper on the audio squelch board which goes from pin 3 of the PL decoder to IC-202 pin 8. This is near the pins which the PL board plugs into. Check for continuity from pin 3 of the PL decoder to pin 11 of the audio squelch board with an ohmmeter. If there is not continuity, add a jumper. The later boards have a trace from pin 3 of PL to pin 11 of audio squelch, on earlier boards, pin 11 is unused, and you should jump from PL decoder 3 to audio squelch 11 with some wire wrap wire. Add 15pF capacitors between the following pins on the two chips on the audio squelch board. Figure 2 details the location of these caps. On IC-201, add a cap between pin 3 and 4, and between pin 3 and 5. On IC-202, add caps between 5 and 9, 5 and 13, and two caps between 5 and 15. This makes the board less susceptible to RF. Step 4: Next, carefully remove the front casting from the chassis. This is done by removing the four screws top and bottom as well as two screws on the control head plug. This is kind of tricky, so be careful to remember how you got it apart so you can re-assemble it later. You will expose the antenna relay with its two associated coax cables running to the transmitter and receiver. Unplug the receive coax from the antenna relay. Step 5: Procure a BNC chassis mount connector for a receive antenna jack. Mount this connector on the top side curved surface of the front casting on the side opposite from where the lock is located. Be very careful to locate this connector so that it does not hinder the operation of the latch mechanism. Attach the receive coax to this connector routing the cable through a hole which you will drill in the front of the radio chassis. The method will be obvious once you have examined the unit with the front casting removed. Step 6: Construct the interface board using the schematic and board layout in the packet. Install this board on the three unused mounting tabs near the rear center of the radio. These tabs are above the control board. Wire the board up as described in table 1. At this point, the radio will be operating full duplex, and you should be able to put the radio in PL by flipping the switch on the control head plug to the PL position. Verify that you have +9.6 volts at the 9 pin molex connector PL SENSE pin when the correct PL code is being sent. Also verify correct receive audio gating and COS action. Tune Up Instructions Tune the transmitter and receiver per the Motorola manual. Follow the procedures exactly. Adjust the power set for the desired power output. I reccommend no more than 60 Watts for 110 Watt radios, 40 Watts for 60 Watt Radios, and 30 Watts for 45 Watt Radios. That's it. you are now ready for major repeating action. Make sure that you have adequate forced air cooling on the PA at all times during operation. The Micor PA is not easy to fix, and when it blows, it blows big.