Conversion of High-Band MICOR Mobile to repeater station

By Kevin Custer W3KKC

The following instructions are provided to help convert a Motorola MICOR 142 - 150.8 or 150.8 to 162 megahertz High-Band Mobile to repeater station.  Generically this information can be used to build a 220 MHz. repeater if the RF conversions are done.  This modification is intended for those who are using a newer type controller that provides a means of muting and de-emphasizing the audio of the receiver.  All NHRC  models provide both functions and therefore an additional interface board is unnecessary.  Some NHRC controllers require a positive COS signal, a simple transistor switch can be fashioned to deliver positive logic to the controller.  I prefer to drive the NHRC controllers audio input with a buffered discriminator signal, a simple transistor buffer can provide this and a schematic is referenced below.  These two circuits are much simpler than building a complete interface board.  This modification was written primarily for the NHRC-4 controller, however the instructions can be followed to attach  any similar controller.  Since the phase modulated exciter is the most common, de-emphasis is provided by the controller and this audio is used to drive the microphone audio input of the exciter.

Explanation of the MICOR Mobile:
The MICOR mobile was designed by Motorola to be the ultimate in crystal controlled mobile communications. The MICOR was designed to operate from 13.6 VDC.  VHF models were created to operate in the 132 to 142, 142 to 150.8, 150.8 to 162, and 162 to 174 Mc. range, with 45, 60, 90 and 110 watts of transmitter power, and 10 watts of audio output. The radios were configured carrier squelch, or PL. The receiver preamp was an available option, and was located above  the control board between the transmitter and receiver.  The VHF MICOR Mobile is a great candidate for building a repeater.

Make sure the radio works in its original band properly before any modifications are made. This will allow you to repair problems first without inducing more trouble resulting in really getting lost.  Afterwards, retune the radio to the ham band before any duplex modifications are made so you'll know what to expect as far as transmitter output power and receiver sensitivity is concerned.

General Conversion:

If your unit was built in the 150.8 to 162 megahertz range, conversion of the exciter and receiver may be necessary for optimum operation when building a 2 meter repeater.

Start the conversion by disassembling the radio set by removing the control head, and outer covers.
Wire a large electrolytic capacitor across the feedthru capacitors on the power amplifier, observing polarity.  Something around 1 to 5 thousand uF @ 35 to 50 volts dc. Double sided foam tape can be used to secure the filter capacitor.

Ground the enable pins of the F1 channel element for the receiver and exciter so Frequency 1 is on all the time. Install your channel elements into the F1 positions and ensure there are no other elements in the radio set.  Since you are making a repeater, and only one set of channel elements will be used, the cathodes of all channel element enable diodes may be connected to ground.  This will allow ONE channel element to be placed into ANY open position of either the receiver or exciter. If you enabled all positions... do not put more than one channel element into either the receiver or exciter...  because both elements will create signals and strange things will happen!

  Transmitter Modification:

Modify the exciter as described in this Motorola MICOR exciter conversion.  Also, be certain to place a jumper (JU405, on TLD5382A exciter) or (JU402, on TLD5282A exciter) from pin 8 to pin 10 on the exciter interconnect pins, (P902). This routes keyed A+ to coil of the ptt reed switch, and applies keyed A- to the exciter. Crystals for the transmit channel element (K1007), for the ham band, can be purchased from Bomar Crystal.

Tune the exciter per the Motorola manual or Go Here for online tuning instructions.

No component changes are necessary to the exciters named above for use in the 2 meter ham band. The power amplifier will also tune and have good efficiency in the ham band. The bandpass filter that exists between the exciter, and power amplifier may be taken apart, and retuned to the ham band, remember the filter changes tuning when placed back into the case, so tune and test, then repeat if necessary.

Transmit antenna connection will be made through the original antenna connection. Consider disabling the antenna relay as described in the Motorola MICOR mobile antenna switch modification.

Receiver Modification:

Modify the receiver "RF & IF board", (TLD5273A and B) & (TLD8273A and B) as described in  Conversion of  MICOR Sensitron Hi-band Receiver.

Activate the receiver at all times. This can be accomplished by jumping the reed relay on the Interconnect / Control Board. This picture should aid in the modification of the relay.

Tune the receiver per the Motorola manual or Go Here for online tuning instructions.

There will be enough room inside for a receive preamp, if the radio does not already have one. If you cannot locate a genuine MICOR hi-band preamp, the Ramsey PR-10,  two meter preamp kit, makes a good replacement. I am not fond of most Ramsey kits as I feel most are poorly designed, and are not of good value, however this cheap preamp kit is a good value. If you have a genuine MICOR preamp and you would like it to tune the ham band properly, Click Here.

An antenna connection can be made from the receiver, or preamplifier to the front panel. An SO-239 style UHF, or BNC connector can be fashioned onto the front panel, in any convenient manner. If an SO-239 is used, also use a "hood" on the back of the connector to totally shield the pin of the connector. If a front panel connection is considered, be certain not to disrupt the cover locking mechanism.

Audio & Squelch Board:

Modify the receiver "Audio & Squelch Board" as suggested in the Motorola MICOR mobile audio & squelch modification.

Either wire the control head or wire individual volume and squelch pots up to the radio set. The "external controls" section at the left side of the diagram below should help. Click on it for a much larger view. Also, it may be unnecessary to wire a volume control to the radio, see the heading of Audio Power Amplifier below.

Control Head Connector:

Using an old Control Head Plug or complete cable will aide in connecting signals entering and leaving the radio set when doing initial testing, however simply strap these connections with jumper wire inside the radio set for a permanent way of powering the control and RF stages without the need of any cable or Control Head.
The procedure we use at Repeater Builder is to completely remove the Control Head Connector from the radio set, remove the gold plated pins, and replace the plastic block back into set.  This allows you to use the original block for a strain relief (for the heavy power supply leads) and it frees up space to do the jumpering of the connections below.

Jump control head pins 3, 8, and 22 to +12 V.
Jump control head pins 9, 11, and 17 to ground.

PTT will be placed into control head pin 16.
Be sure to wire a PTT Disable switch somewhere like on the front panel to instantly disable the transmitter so you can test for desense and be confident the controller won't speak its peace when you have the transmitter connection in your hand!

COS is available from pin 8 of the Audio & Squelch Board.  This is an "active low" signal and is not routed anywhere in a mobile. This pin can be tied to one of the control head connector pins by re-assigning one that was never used or is is no longer used like pin 6, 7, 12 or any other.

All NHRC controllers require an "active high" or plus voltage to indicate a COS condition, if your controller requires a positive logic use the circuit below to create it.  If connecting an NHRC-2, or 3... change the 4.7 K ohm resistor in the image to a 1 K ohm.  This will ensure proper drive to the opto-isolator used in those controllers.  If you are using an NHRC-2, or 3 and want PL access, this circuit may not be needed; read on.

PL   If the optional MICOR PL (ctcss) decoder deck is installed, a positive going PL signal is available from the Audio & Squelch Board pin 11 in some units.  Some units did not have A&S pin 11 wired to anything.  If this is the case, wire the PL logic to pin 3 of the PL deck.  Again, re-assign one of the control head connector pins to get this signal out of the radio set.

Since an external repeater controller is used to manage the functions of control, the radio will actually be run in carrier squelch mode. The PL output is simply used to feed a logic signal to the controller and the controller manages the function of controlling the keying and audio. The NHRC-4 and 10 controllers have a dedicated PL input provide the And Squelch function, so additional circuitry is not needed.  Also, since this particular controller handles all audio and signaling.... this modification should be done to eliminate squelch crashes when running the repeater in PL.

The NHRC-2 and 3 controllers can be wired to provide either Carrier Access or PL access, but switching between the two is not a remote controllable function as part of the controller.  This means that if the repeater is to be switched from carrier access to PL access, a manual switch or another tone remote control will be needed.  If you only want PL access, you can simply wire it this way.  The method stated in the referenced article will provide the "and squelch" logic function.  So, if you want to add the PL option to the NHRC-2 or 3, go to the nhrc 2 and 3 pl schematic.

If you are using the NHRC-4 controller and the original MICOR PL deck, go here for modification to the controller to use the original decoder and allow the repeater to operate either Carrier Access or PL Access by touch-tone control-op command.

Buffered discriminator receiver audio is available from pin 3 of the Audio & Squelch board.  The audio available here is not squelched or de-emphasized but most newer controllers handle this.  Again, re-assign one of the control head connector pins to get this signal out of the radio set.  I build the Discriminator Buffer amplifier so the controller is driven with a low impedance source to provide some isolation between the controller and the rest of the squelch circuitry.  If the NHRC-4 controller is used, I found that changing the de-emphasis capacitor (C11) on the controller to a .0015 uF resulted in better transmitted audio.  Audio seemed a bit dull with the original value (too much de-emphasis).

Transmitter Audio will be fed into the exciter at pin 15 of the control head connector or directly into pin 12 (Microphone Audio) on the exciter board.  Audio will be fed through a .047 uF capacitor from the controller to the transmitter.

Audio Power Amplifier  Be certain to have either a speaker, or 8 ohm five to ten watt load resistor connected to the speaker terminals as the audio output stage will oscillate with no load. If you are using something else than the radio to monitor receiver audio (controller with onboard audio amp) then consider removing the Audio Output Transistor assembly, as it will conserve power, and it won't oscillate.

In many cases I have found the original audio power amplifier is not happy in duplex service.  It seems to do funny things in the presence of RF like making noises when the transmitter is keyed.  This is true even if the Audio & Squelch bypass modifications are done.

If I need a way to monitor the receiver activity locally, I simply remove the original Audio Output Transistor assembly and use this circuit to provide audio amplification.  Just remember to use a larger output coupling capacitor that that shown for driving a speaker, and a 10 to 50 K pot for the volume control of the added board.  The input of the audio amplifier can be connected to the transmitter audio output of the controller so the speaker audio is de-emphasized and squelch gated.  This also allows you to hear the responses of the controller when programming locally.  The original volume control can be eliminated.

Additional Notes   Remember to install a PTT disable switch in a convenient place, like on the front panel. This switch can be a simple SPST toggle, and can be inserted in series with the PTT lead from the controller.
Or you can go to a full featured switch and use a SPDT center-off switch.  Wire the armature to the PTT lead, one side to ground, and the other side to the wire from the repeater controller that would normally connect to the PTT lead of the transmitter.  With the switched centered, the transmitter is disabled.  With the switch to the ground side, the transmitter is locked on.  With the switch in the other position the system is in normal repeat.

If you need PL filtered, and/or squelch gated audio for your controller, consider using the MICOR Muteboard for developing the receiver audio.  Again, this or any type interface board is not necessary when using a newer controller.

July 28 2000  Kevin K. Custer  W3KKC

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