Motorola, Inc. made several models of LMR transceivers. Some of them were built with transmitter and receiver units put together as a single unit, either in mobile or (base) station configuration. One was the "Micor" built around the 1970's through the 1980's. They performed with excellence and would last many years in commercial service. Even though they are 30 years old they are still in service. With the exception of an occasional (dried) electrolytic capacitor needing replacement they continue to work well for Amateur service. This for two main reasons; one, they are very cheap (or free) to obtain for Amateur service and two, they are set up for the "normal" +- 5 KHz channel bandwidth, while current commercial systems are migrating to the "narrow" band of half of this. Although there may be unknown "SP" receivers and other sub-bands and chassis types, this document is focused on the more commonly known models in the VHF band.
Definitions"COR" or "COS" terms are used to obtain a DC change from a receiver during valid user input activity to signal associated equipment, such as a repeater's transmitter PTT. For the Micor's audio-squelch board, early days of obtaining a carrier indicate signal used the board's "RUI" or "CAI", meaning, Receiver Unsquelch Indicator or Carrier Activity Indicator, respectively.
Because the Micor Tx and Rx units can be easily taken apart and configured for separate units, this is ideal for SRGs 147.20 repeater design. In the early days of this system build-out, by the Author, the Micor receivers were used for the out lying coverage areas, such as "Wenatchee" and "Omak". In the near future of 2008-09 it's planned to continue setting up these receivers for the "Spokane" area as well. Even though all of them are negative ground, there are some differences you need to be aware of before using these receivers for Amateur repeater service.
The receiver chassis discussed here come in three types and dimensions for width, height and depth. The width, of course is fixed at 483 mm, or the standard 19". The depth takes into consideration of the front panel controls to the rack's rails. For height panel sizes are identified in Rack Units (RU) with one RU being the smallest panel's height being 44mm or the standard 1 3/4". RUs are multiple of this.
Take this into consideration if mounting a receiver in a restricted area such as a cabinet.
Most have an I.F. of 11.7 MHz and the RF-IF boards come in three VHF sub-bands:
You'll need one for the 144-148 MHz Amateur band. Most range available is the mid-band "H" TLD 8273A. It can be converted to a 8272A for the Amateur band, "M" range (140-150.8 MHz). This is done by changing several components, per the parts list and diagram of the radio manual,starting with:
Frequency determining devices
Most receivers (all types of chassis) normally come with a standard IF of (11.7 ). Most Amateur band receivers 142 ~ 150.8 will use the high side channel element for the LO (local oscillator) injection to the mixer stage. This means the fundamental crystal inside the "CE" is multiplied nine times, coming out 11.7 above the operating receiver frequency. The difference, of course, is 11.7 MHz for the IF. This is an important note if you are having IM or image response problems. Also, injection direction will affect the phase of detected AF at the discriminator. Therefore, the discriminator diode reversing is performed because of the high side injection, per the manual. This actually would be important for simulcasting or other systems not capable of correcting phase differences on the audio output. However, for most Amateur use this step can be left out.
If you choose to experiment with a low side injection or already have a crystal on hand for this, it's unknown if this would be successful. Just a side note....read below.....
Having talked about the IF, you need to be aware some special receives have an IF of 11.8 MHz. These receivers are used for special interference problems. The channel element you ordered for the "normal" receiver will not work for these special receivers, of course. Keep that in mind when ordering several channel elements for your receivers.
Just a recap; when ordering a crystal to be installed and compensated in a CE (Channel Element) the following formula should be observed:
Of course, if you are doing your own project (on a different frequency) change the figures in the formula.
Note: As of 2017 most or all of the crystal vendors stop performing frequency compensation as a service. Therefore, you may have to do it yourself. Here's some drawings that may help you. Most of them are from repeater builder's page. Starting with the receiver some helpful pictures. The last one from them is for the transmitter.
This photo (below) by the Author is reverse engineering the K1005A receiver element which is a little different from the (above) 1003-04 ones. Accuracy is not 100% until the Author can find some of the RB folks to proof it however, should get you going in the right direction. Reminder: the Author's images are in semi-thumbnail size; for a large version, click on the image.
Some steps and views of the receiver channel element K1005A:
No channel element on hand ?
If you want to check the front end's bandwidth and didn't have several crystals (CEs) to experiment with here's an interesting alternative. You will need two signal generators; one with a high level output in the 15-20 MHz range and a second conventional operating in the 144-150 range. By using the high-level signal generator you can "replace" or simulate the crystal output.
First, find an old channel element and remove the crystal inside. Then install a 1 Meg. resistor across the crystal tabs. Then install a 30 pf cap on the lower tab. That would be "lower" when the channel element is sitting upright, with the connector pins pointing down. Then connect a piece of coax's center to the far end of this cap. Connect the coax's shield to the upper tab. Drill a hole in the top of the channel element's cover will allow you to reassemble the channel element and plug it into the receiver's CE slot, presumably, CE1 position, with F1 enabled (grounded). The far end of the coax can be terminated with something convenient for patch cords to the signal generator. Since this coax line does not have a direct path to chassis ground you need to do that. A jumper clip from the coax shield, to the ground should do the trick.
With the normal/conventional signal generator, input the receiver's front end with a medium level, say around a -80 dbm carrier, on the operating frequency you wish to check. A 1 KHz modulated tone is optional at this point. It might help you "find" the signal, if you need to.
With the high-level signal generator input the (newly modified) channel element with a +10 dbm level carrier, on the crystal frequency, based on the formula, above. If this generator is an analog type (dial) you'll need to move the frequency around until you hear the resultant signal in the receiver's speaker. (That's where the 1 KHz tone might help). Once you "found" the signal you can lower and play with the first generator to find out it's sensitivity.
This method is not guaranteed to produce spec sensitivity but will get you in the ball park. Using the manual's schematic, remember to align the receiver properly once you obtain the proper (compensated) re-crystaled channel element for the operating frequency you plan to use. Obviously, for this document is 147.800 MHz. SRG used International Crystal Company since 1975. They accept credit cards and possibly even personal checks. MO's of course.
Before building you should understand the workings of each part of the receiver, whether it be the mobile, Compa or Spectra-Tac type. Configuration, wiring, pin-outs, I/O connections, logic and audio levels should be standardized for easy understanding, troubleshooting, equipment replacement and alignment. Some considerations need to be realized and planned before you start construction, starting with........
For all types of receivers the local speaker audio is always on carrier squelch, with JU201 left in. That means when you open the local squelch either with the front panel knob (pot) or push button the local speaker output will have noise. For carrier squelch receivers doing this will activate the PTT output. For tone squelch, it will not. For tone squelch receivers using the stock PL deck has a high pass filter. This is disabled by removing pins 4 and 5 on P201's mobile type, or pins 6 and 8 on P201's Compa type. Full range local audio is useful for testing and monitoring user input signals.
For the tone receivers, the PL deck (or card) DC logic output (PLI) affects the squelch IC IC202 (or U202) pin 8, making it a "OR" squelch. For Amateur use "OR" squelch is not good and you need to change the tone receiver to "AND" squelch. For each type is a different way to accomplish this. For the mobile (new and old) you may find a jumper, or not; if not, cut the run going to IC202, pin 8. For the Compa's audio squelch board, jumper JU204 is to be left out. For the Spectra-Tac's audio control module, jumper JU1 is to be left out. For more information on why the AND squelch you can click here.
For the mobiles, the receiver chassis is kept more or less intact and mounted on a panel, with stand-offs. The chassis is offset on the panel; on the left side of the panel's edge come in 1 1/4" to start the edge of the receiver's chassis. There will be six holes drilled in the chassis for the six standoffs. They are 8-32 female thread, 1" long, hex, aluminum type. The board's pins are removed and replaced with wires; for a custom harness. Wiring directly to the RF-IF and audio boards increases reliability but defeats the quick board replacement feature. Since mobiles audio boards are not compatible with compa this was felt a moot point. "TB" (Terminal Block) on the front panel provides for all I/O connections except for the (BNC) antenna port. For the BNC connector drill a 25/64" (.390") hole, then file/ream out a little for a tight fit. Install a shielded jumper cable from it to the RCA connector on the RF casting.
A custom accessory group will be added, more or less consisting of a volume, squelch control, a mode (CS-PL) switch (for stand-alone packages only) the cor board, a 9.6v regulator and a local speaker fastened to the chassis.
The receiver will be a self-contained unit; you just add the antenna and power. This means the cabinet's rails will need to be up front . Leave enough room in front of the panel for the panel's controls to clear the front door.
OEM base stations in the 1970s area came in a "Compa" type cabinet, varying in sizes depending on the band, features/options ordered and class of service. For example, the U.S. Secret Service had some cool special production (SP) type of options, while "normal" companies, such as tow-trucks, dispatch companies had pretty much a basic package; a power supply, transmitter, receiver and control shelf. For this type the compa came in either non-unified or unified chassis. For the former, the receiver was a separate and removable sub-chassis; more-or-less. This configuration also allowed for a second receiver inside the OEM cabinet. This was widely used for H.E.A.R. systems in the 1970s.
For the Compa, only the non-unified chassis is used for a remote receiver. (The unified chassis is only used for a complete Tx-Rx unit for a single-site duplex station.) This chassis comes with a volume and squelch control. This is also mentioned for SRG projects because occasionally you will be caught without the OEM covers, shields and other accessories, cables and voltages needed to operate a remote receiver. For example, the compa's package's power supply feeds the receiver both 12 and 9.6 VDC. By removing the receiver from the OEM configuration you'll need to add a "9.6" supply (discussed later). This type will also need the COR board covered in other documents/page on this site. Also, a custom accessory group will be added, more or less consisting of a mode (CS-PL) switch (for stand-alone packages only) the COR board and possibly a local speaker fastened to the chassis. There is less room on the front of the chassis, therefore a greatly reduce TB will be used with only the essential I/O connections. The same BNC connector (as in the mobile) will be a separate port. The receiver will be a self-contained unit; you just add the antenna and power.
If you happen to get a non-unified chassis receiver out of service from another band, such as the Author did from the VHF-Lo band, the channel element cage is located differently. Therefore, some chassis modifications where necessary. With a little help of a 4" grinder the small rivets holding them were removed.
The correct type cage (robbed from an old high-band chassis) was replaced with small bolts and nuts.
This is a general view of the finished modification with the correct boards installed.
The third type, Spectra-Tac is interesting in the fact it's self-contained with its own power supplies (12 and 9.6) a nice shielded chassis, with a control shelf, for four card positions. The previous types, mobile and Station compa uses an "audio squelch board", as previously mentioned. This type uses, instead, an "audio control module", which is a slide-in card in position 2. This card also has squelch and line level controls. The latter will become the local speaker volume control. For tone receivers, the Spectra-Tac receiver uses the TRN6083A PL card. Obviously, this card's pins and physical mounting is different from the mobile or Compa type, previously described. Modification drawings will show this.
The receiver will be a self-contained unit; you just add the antenna and power. The ones available came out of 800 MHz service. Therefore, the RF-IF board needed to be replaced with a VHF one. A custom accessory group was added, more or less consisting of a mode (CS-PL) switch, the COR board and possibly a local speaker fastened to the chassis. This type will require a different rack mount depth from the first two types. Take this into consideration when planning a station with limited space inside a cabinet. Open rack/sites have better flexibility in this area, however are less secure against tampering.
For the line out a white wire is connected from the discriminator (P904-15) to the audio board, and out to the COR board. The receiver PTT output will have an open collector output; going low during activity. The receiver AF out will be flat from 20-5000 Hz +- 1 db or less. These two features are accomplished with the additional COR board. For information on this board, go back to the "links" page. There's also link to it, later in this document.
The audio-squelch board has several discreet components and two integrated circuits (ICs).
You will need to know about some differences between mobile and Compa receivers.
For example, the IC designators are different; mobile uses "IC" while the Compa uses "U" for the two ICs on the board.
Another example is the audio-squelch boards and optional PL decks between the mobile and Compa types look similar, but they are not interchangeable
The pin functions are different, so keep them organized. Of course if you just need them for parts board (robbing) that would work for repairs.
Take this into consideration when sorting out your inventory of surplus boards, before building several receivers.
The mobile uses the TLN4310A or B audio-squelch board for carrier squelch receivers. Some receivers use the TLN4725A audio squelch board as well. Some of the pins functions between these mobile boards are not the same functions, such as P903, pin 11's function. Most of this document will cover the TLN4310x board. The TLN4294A or B (and reed) PL decks are also used for the tone receivers.
IC202 is a special IC for dual squelch operation. It has two shunt switches. Both of them are used to mute the (local) speaker audio. Pin 7 mutes the first (lower level audio) stage of IC201 at the junction of JU201 and C205. Pin 6 mutes the second (higher-level audio) stage of IC201 at the junction C210 and C211. It's unknown why two mute switches are needed.
The RUI is on pin 8 of P903 (coming from pin 10 of IC202) and the CAI is on pin 12 of P903, (coming from pin 13 of IC202). There are capacitors on these points (C232, C229 respectively) that could slow the output response. For mobile accessory group features (such as the call light, etc.) these points are satisfactory, however, may be too slow for RF links. This logic change needs to happen quickly. This is especially true for multiple links, where is, any COR delays are added in time. Past SRG remote receivers have used these cor points by reducing or removing these capacitors, where needed.
Present SRG receivers use a "COR" point in place of the RUI. In the past pin 7 appeared to be the best cor point to use. Since the "pull down" 10K resistor, R225 would unnecessarily pull the active voltage down, it was removed. (a pull-up resistor will be provide on the COR board that you can use). Presently, there are three SRG modification versions for the TLN4310B audio board COR points. Refer to the diagrams on this page. For example, version A and B uses pin 6, while version C uses pin 7 for the COR point. At a later date will determine which version to use as a standard.
The Compa-station uses the TRN6006, TRN6007A or TRN5363A audio-squelch board for carrier squelch receivers. The TRN6002A PL deck (and reed) are also used for the tone receivers.
U202 is the special IC for dual squelch for the Compa type. Only one shunt is used for (local) speaker mute. Pin 7 mutes the first stage of U201 at the junction of JU201 and C206. This apparently is enough mute for the speaker audio. From older SRG construction it's possible this pin 7 shunt line was moved to the second audio stage as in the case of the Wenatchee receiver documentation. Future design will leave the shunt in the stock arrangement.
COR point does not use the CAI but uses the RUI as a different point from the mobile. The RUI is a different type than the mobiles. The RUI for the Compa is on pin 8 of P903 (coming from pin 6 of U202). (Pin 10 of U202 is not used). Therefore, the RUI provides a "shunt" (to ground) cor output, active going relaxed (high with a pull-up resistor) state, with a quick change. Also, note there's no cap (like the mobile has) across this type of RUI. Since the RUI does not have that "pull down" resistor (like the mobile does) no changes are needed. It's also assumed this RUI is connected (through the mother board/back plan) to the station's control shelf, presumably, for a repeater squelch gate card function.
As mentioned, past SRG construction had some older ways for obtaining a COR point. They are in need of improvement by simplifying and standardizing the changes. The findings will be discussed in this document. For the mobile type using audio-squelch boards, such as the TLN4310A and B several ways have been done over the years starting with version A. Version B and C are being developed in 2010 and quite possibly version C will be the final solution.
COR point from P903, pin 12, which is the CAI. Adjust the COR point on the cor af board version 5.3 for proper logic change. It's questionable if the CAI is quick enough for link operation. Presently, some SRG receivers that have this arrangement may be changed to one of the other following versions:
As previously discussed the COR point comes from one of the shunts on IC202. For this version with the TLN4310B board, pin 8, is the COR output. Move one of C211's lead to the eyelet of R225. Install a jumper where the lead of C211 was to the eyelet feeding P903, pin 7. This will be the COR output. No pull-up resistor is at this point; the internal pull-up on the COR board will provide for this. Next, move one lead of R208 down a bit on the run. Move C210 with one lead to the eyelet of R225 and the other where R208 was. Cut the run between R225 and C205. Remove P201, pins 4 and 5.
Using P903, pin 8, for the cor point: Remove C211. Move the one lead of C210 where C211 lead was. Remove R226. Remove C232. Cut the run between IC202, pin 10 and the next eyelet. Install a jumper on the far side of this cut (towards P903 pin 8) and eyelet where C211 was (toward IC202, pin 6. P903, pin 8 will now be the RUI (cor point). It now comes from IC202, pin 6. One questionable issue of the shunt (divider) R226 being removed, may cause the audio too high at this point of IC202. This arrangement has not yet been tested as of May 2010. The nice part of this version is the standard of using P903, pin 8 as the RUI as shown in the stock manual. Remove P201, pins 4 and 5.
Using P903, pin 7, for the cor point will be slightly different than the other two versions. For the newer audio boards, such as the TLN4310B, remove the (stock) jumper between the eyelet further back from this pin to the eyelet between IC202, pin 11 and P903, pin 10. For the older audio boars, such as the TLN4310A, they don't have this jumper, however, do have a delayed mute line. Remove at least the CR201. Optionally, you can also remove R222, C230 and CR202 to clean up the board a bit.
Then cut the PCB run between the eyelets of R225 and C205. Move R225's lead further up the run, towards C205's lead. This will keep the audio shunt in place for the first audio stage of IC202 (was not observed in version B). Now, install a jumper between where R225's lead was, to the eyelet feeding P903, pin 7, which now will be the cor output (from IC202, pin 7). This is a shunt (to ground) during standby and active going relaxed. With the internal pull-resistor on the COR board will produce an active going high cor. If you are not using the COR board install a pull-up at a convenient point to feed your controller. Remove P201, pins 4 and 5. This arrangement has not yet been tested, as of June 2010 however, the plan is for version C for all SRG receivers.
Concerning all the versions, the stock PL deck's output is a low-no voltage for standby and goes "high" on active decode. The interface addresses all of these points. It includes proper inversion of carrier and tone operation. CS or PL mode can be controlled either by the front panel switch or TB1 terminal. Control is standard Motorola "closed loop", whereas, a low to ground enables PL mode. Refer to the interface diagram obtainable near the end of this document.
Cut the run between U202, pin 7 and C206. Best place for the cut is just above C213. Then solder a bridge from the pin 7 side of this run to the right lead of C213. Remove pins 6 and 8 of P201. JU201 will be IN.
There are no board cuts. Remove pins 6 and 8 of P201. JU201 will be IN.
If you wish to study the COR board at this time a separate page is available here .
Other modificationsFor the mobile, a 7810 regulator is installed on the IF board, with a 4-40 bolt in the hole for F4 frequency adjust, with the leads going to the pins of F3; pin 3 for the input and pin 2 for the output, with a common diode in series to drop the 10v+ to 9.4, which is close enough for the stock "9.6v". This run also connects to the rest of the supply "9.6 switched" on the board. An orange wire straps this to the "9.6 continuous". A red wire connects to the "A+" and to F3 select, which is now used for the regulator input.
Sensitive controls and tampering:
The volume and squelch controls are mounted on the front panel for mobile and Compa types. There's some options here. Conventional knobs can be used for best convenience. This is satisfactory in enclosed cabinet arrangement. For open rack mounts short shaft, screwdriver adjust may be an alternative to avoid bumping them from their settings. This also discourages benign tampering of these controls. A third option for the mobile mount is leave the volume control on the front panel (one can't hurt something) and "hide" the squelch control inside, for example, near or on the COR board.
A local speaker switch is normally left off except for working on the equipment. This also prevents distractions to others while at the site, working on other tasks. A 15 ohm 1 watt resistor is installed on the speaker line to provide a load when the switch is off. Some handy LED indicators are installed on front panel, such as PLI, speaker in on position, 9.6 power and PTT out active. The reasoning with the 9.6 indicator is that it shows both 12v and 9.6v is operational, since the former supplies the latter. A panel indicating fuse protects from an overload on the 12v input side. When blown an LED indicates this condition only when power is applied to the input terminals. The stock PL deck (depending on receiver type) uses the stock PL reed; the "vibersponder" (not the "sender").
Previously mentioned, there is a small speaker mounted in the chassis for monitoring. To avoid changing the squelch setting, just to check for receiver noise, a red push button is on the front panel as a "monitor" function, similar to the GE MVP "monitor" slide switch. The button puts a 100 ohm* resistor from the squelch wiper to ground to disable the squelch. The speaker output is balanced, however, can be easily made single-ended by grounding the "speaker low" line making an easy measurement point. In fact, the interface drawing shows "speaker lo" in the grounded mode (TB terminal 14). For measuring sensitivity you need a de-emphasized point. TB is provided for this and other tests.
List of audio boards, and versions. Most of these files are large (for you dial-up people).
List of other boards and receiver sections:
NOTE: If a diagram is not satisfactory for printing out and you are serious about building a receiver a better copy is available at a modest cost.
* In the case of the "Omak Rx" a 10 ohm was used for the monitor function. Speaking of that package, the following parts list is based on it.
1 19' rack panel #2; Bud radio#PA-1102-WH Micor front end parts list for conversion to "M" (142-150.8 MHz)
21-82133G29 C108 18 pf (some manuals disagree with the value of C108) This document may be used for Amateur purposes with the Author given credit for it's content.
This document may be printed and passed on to other Amateurs, with the understanding not to modify or change the content of the document without
the Author's permission. Notifications of typos or errors are welcome.
General Part list for band change
6 standoffs, 1/4" hex body, 8-32?? female threads
6 machine screws PPH 8-32, 1/2"
6 machine screws PPH 8-32, 1/4"
1 cor board Ver ver. 5.1
2 25K LT pots w/switch
1 n.o. push button
1 led, red
2 10K resistor
2 1K resistor (one for Tx switch)
1 4.7 K resistor
mics wire for hookup around 22 gu, various colors, black,red, yellow, etc
some heat shrink to wrap the above wires as a harness
1 terminal block 140-6 (later versions will use 10 position or more).
21-82610C44 C109 47 pf (some manuals disagree with the value of C109)
21-84494B03 C110 80 pf
21-82133G14 C113 7.5 pf
21-82133G14 C116 7.5 pf
24-84070C01 L101 (w/tap)
1-80713B52 L105 (w/tapped black wire)
Change R107 to 82K
Change R122 to 10K
Change R123 to 15K
Reverse CR102 and CR104 (because of high side injection, now)
1 19' rack panel #2; Bud radio#PA-1102-WH
Micor front end parts list for conversion to "M" (142-150.8 MHz)
21-82133G29 C108 18 pf (some manuals disagree with the value of C108)
This document may be used for Amateur purposes with the Author given credit for it's content. This document may be printed and passed on to other Amateurs, with the understanding not to modify or change the content of the document without the Author's permission. Notifications of typos or errors are welcome.