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http://www.repeater-builder.com/rbtip/sensitronRX.html Modification of Micor "Sensitron" High-Band 150.8 - 162 mc. Receiver RF & IF Board for use in 2 meter ham band. by Kevin K. Custer W3KKC Re-written August 19 2000, May 5 2002, and again on October 13 2002 because of coil availability. Concept: To modify a TLD4073, TLD5273, TLD5773, or TLD8273 high-band receiver for use in the 2 meter ham band. Please read through the entire article before attempting the conversion. You may also be interested in reading a Comprehensive Description of the Micor Sensitron high-band receiver. Background Theory: The Micor 150.8 to 162 MHz. high-band receiver is the most common unit you will find in the used/surplus market. These receivers were built in the day when most radio systems had only one or two channels of operation. They were designed to be highly selective from the standpoint of both the front-end and I-F stages, and were made to operate on a narrow portion of the spectrum. Parts for this type of equipment were specified to purposely limit the tuning range. This allowed for more precise tuning and better stability from both a mechanical and electrical view. Channels were usually paired close together so the receiver would operate without reduction in sensitivity as it was greatly reduced when trying to receive more than a few hundred kilohertz from where the receiver was tuned. Referring to the manual and parts list shows the intended tuning ranges and the parts associated with them. The "Comprehensive Description" link above provides more in-depth theory on how the receiver operates, and its tuning ranges. There are several different approaches of modifying the Micor receivers named above for ham band use. One acceptable solution I found is to change the 150.8 to 174 MHz. preselector coils to the ones that are intended to tune the ham band. Placing longer or larger tuning screws into the helical casting creates two problems. First, the width of the bandpass is greater due to 'pulling' the high band coil to resonance. Second, insertion loss is greater than if the correct coils were placed in the casting. These two things are (in my opinion) unacceptable in duplex repeater service, especially on two meters. Research was done on pulling and other methods of making the 150.8 to 174 MHz casting tune the ham band. Go Here to view the research. Modification Description: You'll need to replace the old 150.8 - 174 mc. coils with ones that tune the 132 - 150.8 mc. range. These are available from me, kust click on the helical resonator ordering instructions link below. From the factory, all 5 coils are identical except for the two end coils have wires soldered to them. Since you are converting an existing casting, just salvage the wires from the old end coils and solder them to the new ones at exactly the same place from the grounded end. This ensures the correct impedance point for the coils. Helical Resonator Casting/Coil Ordering Instructions Click to purchase new 132 - 150.8 mc. preselector coils or have the author convert your casting to two meters. There are some options to choose from. The capacitors in the oscillator multiplier need changed to insure these circuits properly tune the 2 meter ham band. Instructions are given below for their replacement. Modification of Helical Resonator Casting: Remove the "sensitron" receiver "RF & IF" board, from the radio set. Remove the RF preselector casting, by unplugging the small black wire on the output end , and removing the 12 screws securing the casting to the circuit board. Remove the 12 screws that hold the cover on the preselector. Now remove the 5 helical coils by taking out the 2 screws holding each resonator coil, and by unsoldering the input connection from the input coil [leave the little wire connected to the input connector, just unsolder it form the coil.] The use of a quality thin wall 7/32" nut driver will make removal of the screws less difficult. Unsolder or break the coil ground end connections with a pair of needle nose pliers. The output coil has a small wire connected to it that passes through a hole in the casting. This is the output connection of the filter assembly that goes to the mixer. Simply unthread the wire out of the casting then unsolder the wire from the coil and save it for installation on one of the new coils. Modification of RF - IF board: 150.8 - 162 model receivers will require the changing of some capacitors in the oscillator/multiplier section. Change C 108 to a 27 pf., C 109 to a 47 pf., C 110, to a 100 pf. (if already 100 pf and this stage still doesn't tune, make C 110 an 80 pf.), and C 113, and C 116 to 7.5 pF. It is helpful to do these changes with the helical casting and all 5 aluminum coil shields in the area removed to gain access to the capacitors. Refer to the images below: Go here for component and shield locations: Coil Shields. Component Locations. Reassembly: Start the reassembly of the casting by installing the salvaged wire from the old output coil by soldering it to one of the new ones at exactly the same point from the grounded end with a soldering gun or large enough pencil. This ensures the correct impedance point for the coil. Install the new resonator coils by inserting the two screws into the coil base and then carefully dropping the coil, with screws down, into the casting. This makes it easier to start the screws. Install the output coil first by threading the wire back through the hole in the side of the casting and securing the two hold down screws. Install the rest of the coils by securing the screws first then make sure the coils are centered inside each chamber and solder the grounded end of all of the coils to the casting. A propane torch is handy when soldering the grounded end of the resonators to the casting. I use a pencil torch as it works well for this. Let the casting cool so you can handle it. Next, solder the input wire to the input coil at exactly the same point it was attached to the old coil, insuring that the little wire doesn't short out against the casting. Finish by reinstalling the tin cover and place the completed casting back onto the receiver printed circuit board by securing all screws. Be careful not to over tighten the casting screws as the printed circuit board is easily damaged. Tune the receiver as per the Motorola manual. No other modification's will be necessary for the above receivers to perform as specified in the manual when tuned to the 2 meter ham band unless you are using the optional AFC plug-in module. If using the AFC plug-in module with the appropriate channel element, reverse the polarity of diodes CR103, and CR104 in crystal discriminator portion of the demodulator. These diodes are very fragile and it is difficult to remove them without damaging them. Some people have had success in simply cutting the diodes from the board, turning them and tack soldering then back in place. If you are not using the AFC option (most likely) there is no need to reverse the diodes as this would be as useless as reversing the polarity of the speaker leads on a monophonic radio. Crystals, for the receive channel element (K1005), can be purchased for the ham band from Jan Crystals at 1-800-JAN-XTAL Complete channel elements on your frequency can be obtained from International Crystal Mfg. Copyright 1-30-1997 Kevin K. Custer W3KKC Any Comments? Send them here e-mail Kevin Back to Home |
#3
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john private smith wrote:
from the page http://www.repeater-builder.com/rbtip/sensitronRX.html Modification of Micor "Sensitron" High-Band 150.8 - 162 mc. Receiver RF & IF Board for use in 2 meter ham band. [...] How do you modify Megacycles to Megahertz? Is it worth the trouble? Dave KZ1O -- This file is PureMail protected. To reply to the sender, you MUST include this in the subject line: YKXWBSX7I6 01/03/2004 (without that string in the subject, your message will be deleted, unread) |
#4
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Dave Bushong wrote in message et...
[...] How do you modify Megacycles to Megahertz? Is it worth the trouble? Dave KZ1O daves being funny........ "cycles" term for frequency back in the day. "hertz" todays term for frequency i think the switch happened back in the 50's or 60's but i bet dave knew that. |
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