This section discusses specifically all about the coaxial RF connections for the radio. For the radio to properly duplex you need separate Tx and Rx RF connectors for the coax runs to the duplexer (or two antennas). Both connections go out the "side" of the newly arranged unit. The first major modification is the mechanical/chassis. For DUPLEX radio option, you need to remove the T-R relay, 2135 core/tumbler and handle parts. These and the mobile mounting plate are discarded.

The next challenge is to provide for a proper mounting area for both RF connections (Tx & Rx). Since the chassis is aluminum, it's practical to use a reciprocating saw to cut away certain portions, to allow proper surfaces to be fabricated for proper mounting of connectors. If you are very careful you can run the blade between the chassis and board, cover the main board with something, such as 1" foam to protect from the aluminum "dust" and cut the one side, over to the far edge, then stop. The pictures show which way the cut was made, by observing the surfaces where the metal was. Any slight debris can be blown away with an air nozzle. You can perform the cutting with or without the radio electronics mounted to the chassis. In early (prototype) versions the cutting was done the former way. By clamping the radio (using the rear PA heat sink area) in a vice you can perform this careful task. You could remove the entire electronics (boards) to prevent metal contamination. For large production projects (such as the phase-3 backbone) it's better (in the long run) to remove the radio's electronics from the chassis. First, unscrew all the main board screws, unsolder the wires at the feed through caps in the rear, and lift out the main board and RF front end chassis as show in the left picture There may be some miscellaneous straps to unsolder as well. The next pictures point out the areas of this task, cutting it and afterwards, with the board in place.

This is rather time consuming and special care must be observed to protect the parts for re mounting afterward. An alternative method was developed to save time and efforts. Also, another difficult area is cutting the front of the (aluminum) chassis straight, to eliminate the sloping front, which is a bad angle for the (Rx) BNC port to mount, with the nut on the outside of the chassis. Want extensive information on both of these points? YES.

After you get the proper and flat mounting "front" for RF connectors, select your type of connectors for the transmitter and receiver ports. For the development of this project Rx (receiver) port was BNC was obviously selected. The Tx (transmitter) port was type "N" for selection. By using different connector types it's improbable to connect the coax cables backwards, thus preventing radio damage. The Tx's "N" type selection was not so easy. After considerable research and trial and error either the bulk head Fem-Fem (UG-30/U) or the Fem-coax termination was planned to be used. Want extensive information on the connector, and coax types? YES.

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By routing the cables this way you can still use the stock PL deck. For linking applications this is not a factor, since all the tone decoding is performed at the far end, while the links are carrier squelch. Remember that the Rx cable is opposite of what you would think of, having the double shielding. (one would expect the Tx side to have it).

 

 

 

 

 

 

 

A nice way to route it, is make a sharp turn halfway down the preselector. Make a notch in the first casting divider so that clip can be soldered to the outer shield at that point.

 

 

 

 

 

 

 

Next, change R1012 to 1K. Install a red led in the holes where the relay wires were. This is handy as a key light indicator.

While we are "cutting" into the radio, some research was performed on the intermittent problem of failing sensitivity of the radio. This plagued the Westlink in the year of 2006, until enough persistence corrected the problem. As you can see the RF coils are in a "cavity" with a window for RF coupling. Also note each coil (except for the first and last, for I/O) are mounting electrically only in ONE spot; the bottom of the coil has a lead going down to the bottom plate, where it's soldered only at that point. The top of the coil (form) snaps into the top (inside) of the cavity section, with it's four plastic tabs. One needs to be aware of this, in the event excessive heat is applied to the front end section. If you have to access the cavities, remove all the reed-prince #2 screws, unsolder all of the ground tabs (going to the main board) and unsolder and peel up the bottom plate.

 

 

 

 

 

 

 

 

 

It was believed (at first) there was a Metallurgic problem of "whiskers" growing on the inside of these cavities (similar to the Mstr-II G.E. radios) and touching the coils. Whiskers? GE?

For either mode you will need to install some "lost" parts from the interconnect board being removed. C1, C2 and R4, are for the speaker output circuit. The best place the author found to mount them is glued on the inside chassis, just behind the escutcheon. Also, C3 can be fly-soldered on J1, pin 11, on the inside, with the wire tied to it. That's for DC blocking of the detected audio for the volume and squelch pots. The negative lead goes towards the pots.

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Transmitter Modifications

If you are building a two channel station, use the "stock" F1 and F2 lines for frequency selection, per the radio schematic. An example would be the SRG "Westlink" (scanner) repeater. Otherwise, most applications will be in single frequency mode, either a pair or simplex. In this case, JU611 needs to be installed. This will enable the F1 channel elements for both the transmitter and receiver. The next two modifications involve the transmit audio. Transmit audio needs to be flat. The F4 select line will be used for the audio route from the outside, through J10, and finally into pin 4 of the transmit channel element, which has a flat frequency response in the communication audio range. To accomplish this, jumper F4 (position) Tx CE pins 4 and 5 together. Then remove CR604 and install a 100uf/25v capacitor in its place, with the positive lead towards the channel element pins 4 and 5. This is just to block the DC on that line from the outside, while maintaining the good low end response. We will now call it "C604". This is for version A. For version B (packet) use a 4.7 uf instead. Want more information on this capacitor? YES.

As you probably found out, manufactures of two-ways radios sometime do strange things to make a circuit work. Motorola is no exception. The Mitrek power control and receiver netting circuits are strange and poor in design. The former has a more serious (potential) problem. Part of the power control circuit, U901, was originally designed to "see" 12v power (voltage) at the PA, during receive (and of course) during transmit modes. The normal path for the "big red lead" 12v + for the P.A. is from pin 19 of J1, then to a red lead in the radio (next to the chassis) that goes through C884 in the PA section. The circuit in discussion, however is a secondary path. At the J1 connector, pin 19 also runs through the (removed) interconnect board. Because of it being removed a bypass wire is installed to take the place of the interconnect board. That's just a simple modification which does not affect this problem we are discussing. The only reason it's mentioned to aid in understanding the voltage path. To continue, "A+" is applied to pin 17, of P10 on the main board. This runs to the power control circuitry, through L901, JU905 and the surrounding components of Q902. With the "A+" applied, Q902 barely has enough bias to keep it turned off. In the event the big read lead fuse is blown, the transmitter may be active (very low power level). This happens from Q902 leaking some voltage, going through (believed to be) CR902, and CR903 , which applies voltage to the "TX SW 9.5" line. This turns on some of the exciter stages. While this condition may or may not damage the front end of the receiver, one affect will be that the receiver will be "hearing" a local signal all the time. This is more obvious in simplex operation and was observed on the bench with one of the radios. This this condition could exist until discovered at the remote site. Of course, if the radio was set up for repeater or cross frequency mode this problem would not be so easy to find.

As you know, one of the modifications in this document is to have a separate PA power switch on the front panel. This is very handy for testing, transmitting without any power going out to the antenna for netting or other testing, however, complicates the (previously mentioned) condition, thus, when this switch is off is the same condition as the (previously mentioned) red lead fuse blown condition. In the stock circuit arrangement, P905 is a test function to turn on the Tx channel element, with the Rx netting circuit on. This works in conjunction with the "M4" function. After considerable research, another modification can be performed to correct this threat. Since the receiver channel element frequency (netting) can be accomplished by sweeping the front end with a signal generator. The "M4" function can be disabled, which is a preferred method over the (almost useless) "M4" netting function. Therefore, CR903 can be removed, so no voltage gets to the Tx sw 9.5 line during receive mode, with the PA power turned off (or fuse blown). One more point on this; the VHF version does not seem to use this CR903; at least the version of radios and manuals available to the author at time of this printing. If this is the case, then the VHF radios don't have this problem and you can continue with your other construction and modifications. Just something to remember on this project.

Remove CR608. this isolates the Rx channel element matrix from the Tx AF line (the old F4 select line). This is a completion of the for mentioned Tx audio path modification. Unless you are doing something special with frequency control, you can (optionally) remove CR607, CR608, JU610, JU609, JU607 and JU606, just to clean things up a bit. As mentioned above, JU611 will be in to enable both Tx and Rx elements for single frequency operation. (pair or simplex) As in the transmitter section, that web site (TAPR.ORG) recommends: for some RFI protection on the 9.6v line, install a .1 uf disc cap. on Rx #4 channel element pins 1 and 3.

The M1 function is picked up from the junction of R222 and C233, then processed externally with the link board's built-in limiter DC amplifier, then goes back out through J1 to dive an external meter (panel mounted) to indicate Receiver's limiter. To aid in RSL measurements an AGC curve can be plotted with a RF signal generator. This is handy for checking path/antenna alignment, RFI or even tuning the Rx side of a band pass cavity.

As you probably know, when a signal enters a FM receiver, it quiets the receiver, which activates the noise operated squelch. This squelch has several circuits to handle this condition, which also provide several voltage points that changes DC level. There is a choice of using one of the three "cor" points, "L", "H" OR "E", all which are controlled by the panel squelch pot, of the point at which the local speaker and repeater squelch gate opens. (In the Micor repeater the squelch gate has it's own noise amp and switch for independent opening point). One of these points can drive a high impedance DC buffer/amplifier. The SRG link or cor/af board has a DC comparator to perform this function. In order for this buffer to sense carrier activity, a reference voltage (bias) need to be adjusted on a one-time basis, depending on which squelch point is used. cor points of L, H and E, each have their own characteristics. Refer to the link board documentation about polarity of the cor input buffer.

"L" is a negative going active point (less positive). Being a DC "analog" point, it sits about 1.8 volts positive with the squelch closed, but near the threshold. As a signal quiets the receiver, this point goes less positive, to .04 volts with a full quieting signal. (Never goes negative). This point is DC analog, therefore, you have a "quieting" choice where the cor will change logic state on the control board. This might be handy to set the cor and local speaker activity points differently. This arrangement is similar to the "repeater squelch gate" used in the Micor station repeater. If using this point, set cor board bias (U1, pin 9) at un-squelch, at desired level of quieting, but less than the cor standby voltage, but more than the active low voltage. It's at the junction of R410 and R411 and the base of Q405.

"E" is a negative going active point (less positive). Being an almost completely logical point, it sits about 2.8 volts squelched and 0.16 unsquelched. It's at the squelch switch and used for the stock consolette interface board's carrier indicator. The advantage is time and "stock" proven for reliability. If using this point, set the bias for 1.0 volts. Point "E" is at the junction of R430 and C418 and at collector of Q406

"H" is a "low" in standby (squelch closed) and goes positive on squelch open. Being a logical point, it sits about zero squelched and 6 volts unsquelched. Point "E" drives the input of U401 which is acting as a DC comparator to switch 'on' the audio. Point "H" is pin 4 of U401 which is one side of the balanced audio output to drive the local speaker. The advantage is this active high point will drive any cor/circuits you might already have in mind, and is simple to set up. The disadvantage being audio is riding with the "cor" voltage, so if you crank up the local volume too high, the cor/PTT function will drop out erratically. If using this point set the bias around 4 volts (lower than the cor active voltage).

As previously mentioned the squelch time constant can be shorten (5-10 times as less) to get away from linked additive long bursts. Depending on which 'cor' point is used, will determine how many caps are needed to change to a lower value. You have the option of performing all the cap changes to allow cor pick off changes in the future. All the SRG link radios are indented for long haul links, therefore, most or all the changes are performed. For the short squelch constant change the following capacitors:

Additional information

The OEM Rx audio output TLP is speced at about a +7 dbm. This is at the "detected audio output" from pin 9 of J10. For amateur standards this point is fairly flat. Further improvement for frequency response can be provide by using the (separate) cor/af board designed by the Author. If you wanted to standardize Tx and Rx levels, such as 0/0, you could install a simple pad on this output, before it gets to the external equipment. A good place to perform this would be on the I/O connector, J1 pins.

The Tx AF TLP was based on the channel element's "IDC" set at maximum (which no longer functions as a deviation limiter). As previously discussed, the Tx audio input TLP can be either set up for 0 dbm as well. Otherwise, if you choose to leave the Tx TLP a little higher (ie. +5 dbm) thus, producing some headroom for minor level adjustment (using the IDC pot) this will allow for little differences in crystal characteristics.

One more item. There was a minor inconsistency in the Motorola Manual. It's about the dropping resistors for the volume and squelch. The documentation, here, by the author, is correct for this application and will work fine. Want the full story (with schematic/images) on this? YES.