Newsgroups: rec.radio.amateur.misc,rec.radio.amateur.equipment From: bruce@pixar.com (Bruce Perens) Subject: SUMMARY: Motorola MITREX modifications, crystals, etc. Message-ID: <1993Sep22.203131.15446@pixar.com> Sender: bruce@pixar.com (Bruce Perens) Organization: Pixar -- Point Richmond, California Date: Wed, 22 Sep 1993 20:31:31 GMT Lines: 677 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. 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