Concept: To modify a TLD52(73 or 74) (A or B) and TLD82(73 or 74) (A or B) high-band receiver, for use in the 222 megacycle ham band.  The modification of a TLD5274 or TLD8274 will require less component changes, however either split will provide equivalent performance.

Description: I have tried several approaches to modifying the Micor receiver for 1¼ meter (222 mc.) use.  This modification is the result of approximately 40 hours of trying and testing, using an old mobile frame for a test bench.  The only respectable solution I found was to alter the preselector coils in a way that allowed maximum coupling while retaining the use of the original tuning screws.  Placing longer or larger tuning screws into the casting creates two problems.  First, the desired passband is wider due to "pulling" the coil to resonance, second, insertion loss is greater when this or any other procedure was tried.  Wider front end bandwidth and excessive insertion loss are two things that are usually unacceptable in duplex repeater service.

This modification will allow the above receivers to be converted to the 222 mc. ham band while retaining original receiver specifications.  The original 150.8 - 174 mc. coils will be altered for use in the ham band.  Capacitor replacement in the oscillator multiplier stages are offered in 2 splits to allow better tuning and stability.  From the factory, some preselectors used a tuning screw that is ¾" long while some were 1" long.  This modification will allow either screw length to be used.

The receiver channel element still operates at the crystals 3'rd harmonic as original, however the multiplier is now operating on a factor of 4 times instead of 3.
The multiplier provides Low Side Injection, and the crystal formula is F_cr  =  F_o  - 11.7,  divided by 12,
where F_cr  =  the crystal frequency, and F_o  =  the operating frequency.
The local oscillator provides 12 times the crystal frequency to the mixer, instead of the original 9 times.  This brings up a neat situation.  For instance, a crystal cut for 222.34 mc. will also work in a 2 meter Micor receiver on 146.280 mc..  This happens because the two meter Micor operates on High Side Injection and the second multiplier operates at a factor of 3.  If you have any 2 meter elements for a Micor, they will work for testing the 220 receiver.  The corresponding range is 144.525 to 148.275 for 220 to 225 mcs. respectively.

Note: Be certain the receiver operates correctly on it's original operating frequency before attempting to modify it for the new band.  You may want to document the sensitivity of your particular unit, using your test equipment, so you know what to expect after the conversion is done.  Please read through these instructions to become familiar with each procedure before attempting the conversion.  You may also be interested in reading a  Comprehensive Description of the Micor Sensitron hi-band receiver before conversion.

 Modification: Remove the "sensitron" receiver "RF & I-F" 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 to the input resonator (L 101.)  Leave the wire connected to the input connector in the casting. The use of a quality 7/32" nut driver will make removal of the screws less difficult. Unsolder or break the coil ground end connections with a pair of good needle nose pliers. The output coil has a small wire connected to it that passes through the casting. This is the output connection of the filter assembly that goes to the mixer.  Simply unthread the wire out of the casting. Do not remove the mixer wire connected to the output coil.  The output coil is L 105.  Remember where the coils came from and how the input connection was soldered so you can reinstall the coils into their original locations and solder the input connection similarly.

Alter the helical preselector coils exactly as suggested below:

Alteration Notes:  There are similar conversions on the internet for altering the coils, however none of these take full advantage of the original coupling scheme used in the preselector casting.  These coupling iris openings determine the bandwidth and insertion loss of the preselector casting.  Most other coil mods remove turns that are exposed to the iris openings between the sections of the resonators.  I have tried these other methods, and even some of my own that change the iris opening.  Of all of the methods I have tried, the one presented here seems to work best.

Casting Reassembly:  Install the altered resonator coils into their corresponding locations by inserting the two screws into the coil base and then carefully dropping the coil, with screws down, into the casting. Stick your finger in the plastic former to guide it into its location.  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. Use a thin shaft #1 phillips screwdriver so you don't break the solder connection where the two parts of the coil are mated.  Install the rest of the coils by securing the screws.  Now solder the grounded ends of all of the coils to the casting. A small torch is handy when soldering the grounded ends of the resonators to the casting. I use a butane pencil torch as it works well for this. Last, solder the input wire to the input coil at the same distance from the ground (cold) end as it was connected to the original coil (approximately 1/4 turn.) Let the casting cool and finish by reinstalling the cover.  Set the casting aside for later installation back onto the circuit board, as it will be easier to change the capacitors, and inductor in the oscillator multiplier without the added weight and mass of the casting.

Oscillator Multiplier:  There are two tuning ranges for the multiplier depending on the desired receiver operating frequency.
Remove all 5 aluminum shields from the oscillator end of the receiver circuit board as shown in Figure 1.

Capacitor Replacement:  Remove and replace the following capacitors with the values indicated.  There are two tuning ranges for C 113.  Replacement capacitors should be of the same type and style as the original.  The use of a desoldering tool or solder wick is recommended to open up the holes in the circuit board for the capacitors and the shields.

C 107 - .75 pF   (may already be a .75 pF, if not, replace with one. Violet, Green, Gray, Silver)
C 108 - 27 pF.   (will already be a 27 pF if a high split receiver (74) is being converted)
C 109 - 39 pF.
C 110 - 100 pF.  (may already be a 100 pF, if not, replace with one)
C 113 - 4.0 pF for 220 - 223.5 mc,  or  3.0 pF for 223.5 - 225 mc.
C 116 - 3.0 pF.

Inductor Replacement:  The small inductor near the connection of the output side of the helical resonator casting needs checked for color.  If it is white or clear, it's ok, if it is yellowish, remove inductor from board and carefully take 1 complete turn off, and reinstall.

L 112 - White former = 13½ turns in molded plastic former, (opposed to yellowish inductor with 14½ turns)

Additional Part:  Insure R 107 is a 12 K ohm, if not, replace it with one.

Reassembly:  After capacitor and inductor replacement, reinstall the aluminum shields and place the preselector casting back onto the receiver printed circuit board and secure all screws. Be careful not to over tighten the casting screws since the printed circuit board is easily damaged.

Tune the receiver per the Motorola manual.  You should be able to obtain original sensitivity on the new frequency.  The most important adjustment is the discriminator center.  If you have no way to properly check the discriminator adjustment, don't touch it.  It is best to tune the Micor receiver during the day. What?.....Yes, the receiver is so sensitive at the I-F frequency (11.7 mc.) that it is difficult to tune the receiver with the covers removed during night hours due to the pickup of shortwave broadcasts.  This problem makes it difficult to determine the peak in tuning.  I have been able to obtain original sensitivity on the new band from several receivers converted for the purpose of testing and proving this modification.  My test setup includes an IFR FM/AM 1200A service monitor capable of performing SINAD measurements.  I tested several unmodified Micor high band receivers on my test bench to determine a base line sensitivity using my equipment.  The average sensitivity of the unmodified receivers was -116 dBm for 12 dB SINAD, this relates to the book specification of .35 uV.  The sensitivity of the converted receivers ranged from -115 to -117 dBm, practically the same as before.  It is not a necessity to use a preamp with is conversion, however one may provide extra sensitivity if needed or desired.

I have tested the Ramsey PR-20 preamplifier with the converted receiver.  The Ramsey PR-20 will provide an additional 5 dB of useable sensitivity, nearly the same as that provided on high band with an original Micor preselector preamp.  The preamp actually has more gain than this, however the 5 dB is the amount of improvement gained by adding the preamp to this receiver.

Crystals, for the receive channel element (K1005), for use in the 220 mc. ham band can be purchased from Jan Crystals at 1-800-JAN-XTAL.  Specify that the crystal is for a modified receiver, and give them the formula as written in this text, along with the desired operating frequency. They will build a 220 crystal and mark it accordingly.  Crystals are also available from other vendors.

Copyright January 1, 2000  Kevin K. Custer  W3KKC    Questions or Comments? Send them here  e-mail Kevin

Posting of this data on the SEITS site has been approved by Kevin. For more of Kevin's information on this and other repeater related items visit The Repeater- Builders Techincal Information Page.

Updated February 18, 2000 to show correct sensitivity results.  I was using buffered discriminator audio feeding my SINAD meter and this is incorrect.  Sinad meters need fed with de-emphasized audio to read correctly.  A 4 dB augmentation was discovered.

Updated February 21, 2000 to show the additional component changes that may be necessary due to differing RF-I-F boards.  These components include C 107, C 110, L 112.  Also included capacitor suppliers via hyperlink.

Comments or submissions for publication can be sent to muldoon@seits.org.