The P1 wiring harness is made up separately, then installed in the radio for further hook-up. The pictures show the P1 and some of the wiring installed. A good way to do this is hold the P1 in a "jig" such as a little vice and solder all the colored wires on at one time. You will have plenty of them going to the right, towards the middle of the radio. Installing some clear heat shrink around the bundle keeps it manageable, while still being able to trace wires, should the need arise.

For clarification the red and black wires, power and grounds, pins 19 and 17,respectively, are discussed here; For the red leads, will be a total of four; three going to the main board's J10 and one for the cor board. (not to be confused with the large red lead for the PA, on pin 18, discussed later). For the black leads, will be total of seven; three going to the main board's J10 and one for the cor board. One short jumper to the P1 ground ring and one jumper to the chassis (with a soldered ring) plus one more for the PA's "A-". (not to be confused with an additional black wire for the mike "low" which goes to P1, pin 2). Even though these runs are fairly short with little potential differences the Author decided to follow OEM wiring as much as practical. Take all of this into account when applying the heat shrink to the bundle.

The left shows where the new speaker coupling caps can be installed, in this case, radial leads were used Right shows the overall view of the new wiring.

The prototype cor board was built with the tan color type with no silk screening. A silver felt pen marked the holes for easy location for the wiring. The right picture identifies the red LED location.

Production of the local speaker assemblies.........

Overall view of the nearly complete wiring inside the prototype radio. The panel wiring is yet to be done. The right image is from a radio near completion. It's enlargeable as well.

After the speaker assemblies were installed it was decided a better way would be to mount the tie points and other parts, such as the load resistor inside the speaker housing. Therefore, this method was abandoned and the following was an upgrade to production. The left showing wiring and the right the complete and mounted speaker housing keeps better wire management.

Another problematic area was the heat sink thermo switch which tells the cooling fan to activate when needed. A quick way to disconnect the wires going to the fan unit was needed for radio servicing. The original way had a couple of "spade" type female slip-ons crimped/soldered to the fan wires as shown in the left image. While moving, packing or transporting this left the issue of this device catching on a seat, clothing, etc and pulling off. The right shows an improved version by shortening the original tabs and soldering the quick disconnect pins to them. However, this still presented some problem of catching and pulling on outside objects.

Another improvement is to use a "pigtail" for the disconnect. That gives it some flexibility should it catch on outside objects. The right image shows production of these tails. Later, it was decided to install the molex "shell" so the pins stay together. Doing this also improves the reliability, so that one wire does not come loose by itself and unplug. The fan wires were now the mating end. This is reasonably secure and still can be quickly unplugged. On the right is a small production for the switches being "gooped" up with heat sink compound.

Modifications made inside the radio are documented on a copy of the transmitter and receiver schematic diagram, usually penciled in. This is a good time to discuss some of the functions of transceiver switching. In order for the receiver to be protected during transmit, the receiver is disabled, or, "turned-off" during transmit. This can be accomplished a couple ways. For the Mitrek, both the receiver crystal (entire channel element) and the speaker amplifier are turned off during transmit. Most of the other receiver circuits are left on during transmit. The transmitter is "turned-on" by turning on Q701 in the early stages, (along with pin 2 of the transmit channel element) plus a few other power control circuits. The transmitter P.A. is "hot" all the time. Since the P.A. is a class-C device there's no power out during receive. It's important for the receiver to "recover" (turn back on) as quickly as possible. This is usually controlled with values of capacitors on these "control lines". No modifications are recommended at this point; these functions are mentioned in the event your version B has a receiver "recover" problem, such as sometimes noticed with high speed (9600 bps) packet operation.

The stock PL buss connects all the CE's pin-4 to modulate the transmitter. Therefore, the channel element positions will be isolated, since they will be used for various functions. The procedure for isolation is covered further into this document, under the "Configurations" and "Transceive, Duplex Repeater, Duplex Link, and scanning Link" sections. This section covers the transmit audio input which uses the F4 line.

The transmit audio needs to be flat in frequency response, by using the PL input. Only the F4 position is used for this, via J10-21. To do this remove CR604 and install a jumper in its place. Solder a 100uf/25v coupling capacitor across pins 4 and 5 of the F4 Tx position, with the positive lead on pin 4. This is to block the DC on the line from the outside, while maintaining the good low end response. Since the associated diode CR604 was permanently removed, this new cap will be called "C604".

If you are building out version "B" for packet this section will apply.

For version "B" C604 should be in the 4.7 uf area or less, because of data waveform's eye pattern gets distorted with high coupling. Want more information on this issue? YES. Otherwise, the following applies to either version:

For the early radios: The idea was to short P905 which put "9.5" to the "Tx SW 9.5 line via CR903. The transmitter oscillator would turn on so you could net it to the receiver. The receiver had its own rudimentary netting function, where you would short "P4" to turn on a mulivibrator circuit to give "M4" of the receiver. With the two netting circuits you could net the transmit frequency to the receiver's. This, of course, only worked if the receiver was already properly netting and was on the same frequency as the transmitter's. For this (duplex) project this method is useless.

More importantly, the early version's circuit had a potential problem. Let's review the (stock) OEM arrangement with the radio and interconnect board for the early version:

As its understood, part of the power control circuit, U901, was apparently designed to "see" 12v power (voltage) at the PA, during receive and transmit modes. The normal path for the "big red lead" ("A+" 12v+) for the P.A. is from pin 19 (OEM) of J1, then to a red lead in the radio (next to the chassis) that goes through C884 in the PA section. This circuit, however, is a secondary path. At the J1 connector, pin 19 (OEM) also runs through the interconnect board. "A+" is applied to pin 17 (OEM), 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 turns on the netting circuit, as previously described. While this condition may not damage the front end of the receiver, the condition 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 condition could exist until discovered at the remote site. Of course, if the radio was set up for repeater (duplex) or cross frequency mode this problem would not be so easy to find. Even though the interconnect board is to be removed (for this project) and certain runs and connections are bypassed or otherwise modified (pin 19 is now not PA power, etc.) this condition still can exist, therefore, you need to be aware of this. For further clarification of this circuitry UHF radio, a copy of the OEM drawings are available:

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. Another note is an led on the front panel indicates PA power. The PA red lead powers this led. When the PA power switch is off there is some leakage from the power control circuit presumedly in the Q901, Q902 area. This was verified by temporarily lifting JU906. This condition does not affect the radio performance. You only need to know this to avoid a false indication because the led will glow dimly when the switch is off. In fact, the dim led might indicate a "normal" condition for the power control circuit when the switch is off.

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 the problem; only the UHF early versions. Now, you can continue with your other construction and modifications; just something to remember on this project.

A:

B:

Configurations

The versions can be configured different ways. Being that a morse code IDer is the main component not addressed with this project, you may want to take that into consideration when choosing the configuration. Otherwise, in some cases, most basic functions can be used, such as timers and controls. Referring to the interconnect diagram has many I/O functions on TB-1. Some of them are dual purpose, depending on which configuration you wish for the unit. Consideration was made not to interfere with the cor board's functions either, nor a TB-1 function conflicting with another configuration. These dual purpose TB-1 connection-functions will depend on which version board you use. For example, TB 1, terminal 13 is either a PLI or mode control line. Same goes for the select lines, such as terminal 17 and 18. These can be used for CON 1 and CON 2 respectively, or for frequency control lines. In addition, the old frequency select lines for F3 and F4 will be modified to for new functions of M1 and transmit audio, respectively. However, they will need to be isolated from the matrix. To do this several jumpers will need to be removed, as discussed below. There are about four configurations for this unit depending on the intended purpose. Even though SRG's main configuration is "Link" the other ones may be useful for the reader, therefore, are discussed. They are:

Use cor board version 7.2 for this configuration. PLI or CON 2 won't be a function in this case. Terminal 17 could be used for a rudimentary CON 1; if so leave JU611 in for single frequency operation. Or terminal 17 could be used for control of F1; if so leave JU611 out for the same reason. Transceiver is used for packet operation. To note: The 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 for both the Tx & Rx side.

For Repeater configuration you will need all receiver circuits operating all the time for duplex operation, therefore, the receiver mute functions need to be disabled. Since the interconnect board will be left out that covers the removal of CR2 on that board, as well. (not to be confused with the second "CR2" described, below). For this configuration do not jumper J10-7 to 14. There's also a receiver channel element off/mute function plus a secondary function of "M4" test. The "M4" circuit is explained on the receiver schematic. It's a poor solution to Rx frequency netting. Also, the mute function goes the way of Q3 and Q1 of the "M4" circuit. Neither will be used and can be disable by leaving out CR1 and CR2 on the main board. One last mute circuit needs to be disable by leaving out CR403 on the main board. The antenna port modifications were covered, earlier. Also, in the PTT circuit, optionally, change R1012 to 1K and install a red led in the holes where the relay wires were. This is handy as a transmit indicator. In the receiver section remove JU606, JU607, JU608, JU609, JU610, CR607 and CR608. In the transmitter section remove JU601, JU602, JU603, JU604, JU605, CR603 and CR604.

Use cor board version 5.3 for this configuration for internal control/timing. Also, if you are also using the stock PL deck, you may be using the mode function on terminal 13. You also may be using CON 1 and CON 2 on terminals 17 and 18, respectively. If so, leave JU611 for single frequency operation. If you are using an external controller use cor board version 7.2. In this case you won't be using CON 2 or the "mode" on terminals 18 and 13 respectively. Also, you would not be able to use the stock PL deck.

For Link configuration you will need all receiver circuits operating all the time for duplex operation, therefore, the receiver mute functions need to be disabled. Since the interconnect board will be left out that covers the removal of CR2 on that board, as well. (not to be confused with the second "CR2" described, below). For this configuration do not jumper J10-7 to 14. There's also a receiver channel element off/mute function plus a secondary function of "M4" test. The "M4" circuit is explained on the receiver schematic. It's a poor solution to Rx frequency netting. Also, the mute function goes the way of Q3 and Q1 of the "M4" circuit. Neither will be used and can be disable by leaving out CR1 and CR2 on the main board. One last mute circuit needs to be disable by leaving out CR403 on the main board. The antenna port modifications were covered, earlier. Also, in the PTT circuit, optionally, change R1012 to 1K and install a red led in the holes where the relay wires were. This is handy as a transmit indicator. In the receiver section remove JU606, JU607, JU608, JU609, JU610, CR607 and CR608. In the transmitter section remove JU601, JU602, JU603, JU604, JU605, CR603 and CR604.

Use cor board version 7.2 for this configuration. Most of the TB-1's connections go to P1, then various point in the radio and cor board. Being that some of the TB1 terminals have variable assignments/functions there are some points to consider:

For Scanning Link configuration you will need all receiver circuits operating all the time for duplex operation, therefore, the receiver mute functions need to be disabled. Since the interconnect board will be left out that covers the removal of CR2 on that board, as well. (not to be confused with the second "CR2" described, below). For this configuration do not jumper J10-7 to 14. There's also a receiver channel element off/mute function plus a secondary function of "M4" test. The "M4" circuit is explained on the receiver schematic. It's a poor solution to Rx frequency netting. Also, the mute function goes the way of Q3 and Q1 of the "M4" circuit. Neither will be used and can be disable by leaving out CR1 and CR2 on the main board. One last mute circuit needs to be disable by leaving out CR403 on the main board. The antenna port modifications were covered, earlier. Also, in the PTT circuit, optionally, change R1012 to 1K and install a red led in the holes where the relay wires were. This is handy as a transmit indicator. In the receiver section remove JU606, JU607, JU608, JU609, JU610, CR607 and CR608. In the transmitter section remove JU601, JU602, JU603, JU604, JU605, CR603 and CR604.

Use cor board version 7.2 for this configuration. In this case you won't be using CON 1, CON 2 or the "mode" on terminals 17, 18 and 13 respectively. Also, you would not be able to use the stock PL deck. Most likely a link will be carrier squelch. If not, you could use an external controller and/or decoder. In this case terminal 13 might be a PLI for repeat audio being AND squelch. You need to leave JU611 out. An external "scanner" will control the F1, F2 lines. Ideally this controller could be build to do all three; scanning, control/timing and tone decoder in the case of co-channel RFI). The unit would now be a "control station" for two distance "HUB" repeaters in opposite directions. This configuration will have two control pairs, adjacent channel, especially when using a single duplexer. This is an advantage where a site owner charges per radio, when you need to link to other (zero-tail) repeaters together. This is the case with SRGs eastlink repeater.

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", "E" OR "H", 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 cor 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, E and H, each have their own characteristics. Earlier mentioned was the cor board and the versions, depending on what configuration you are doing. Refer to the cor board documentation about polarity of the cor input buffer. Its found on this site.

"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 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 un squelched. 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 between 1.0 and 1.5 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 intended 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 spec 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 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 might be on the outside of 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 that or 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.

Tip: When you suck out the solder in the holes they make a handy eyelet location tool. (you can find the open hole on either side of the board for referencing other eyelet locations).

Many of the wire and circuits may appear to be redundant and not needed. Wiring color and functions were selected for most any configuration you may need. Either install them or leave them out. The former is preferable to avoid "tearing" into the harness, management shrink and glue holding the wire. Another possible option is to use the extra wires for another function. This is another reason redundant (black) wires for additional grounds were installed in the original design specs.

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. More information on this subject can be found by clicking here.

Part list for Mitrek mods. Note: Parts for DUPLEX listing are mostly in green boxes