This board was not produced . This document is considered the manual for this board only, which is a prototype for the "HUB-2", Mitrek radio serial 44. Only one was built. Therefore, this document is obsolete, only for history-reference of research and development of the board. This includes all charts and parts listing. Because of certain features and other ideas you may want to read about the history to better understand the thinking process of the Author's design. This may help you understand and make better choices when setting up a repeater or radio link.
Emphasis is on size, fit, simple design, easy parts availability and easy modifications, limited only to your imagination. It's assumed you have a basic electronics background with some repeater building experience. Understanding schematic drawings is required. If you are new at the repeater building or operation you might want to check out additional technical books relevant to this documentation. The COR/audio board, herein, will be referred to as just "cor board"(any case letters).
Designed occurred in the early 1980's by the Author. Older methods were used in the circuitry such as a passive potentiometer array for equalization and the LM-386 for audio amplifiers. Some of these "old school" boards are still in service as of 2009. The more recent versions of 5.x were scrutinized by the Author as well. This was based on gathering information and constructive input from users and repeater builders as well. In the 1990's these versions utilized a better, two stage audio equalization, and the quieter, LM-324 for the audio amplifiers and other logic circuits. Multi-turn pots were also used for easy alignment (no backlash problem). Since then the Author further analyzed these features for future SRG projects.
After some consideration the Author designed version, 6.0, to help accommodate some of the features they have wanted. This, plus the older equipment change-out to interface electronically and physically with the Motorola Mitrek and Micor receivers. This version is some what larger but made to fit inside them either in the compa or spectri-tac chassis. While this board version is designed specifically for links the original simple repeater functions were included as well. This would include both timers and I/O polarity selection/sensitivity. While the DC circuits are different from the earlier versions, the audio section is basically the same. U1 has slightly different functions, plus, an additional IC had been included (U3) for additional features.
For a version for mounting inside the Motorola Mitrek required a "cut down" version of 6.0, leaving several features out, such as the timers for up and down links only. Versions 7.x were developed. The final version was 7.3, being ready to be put in production, when more ideas were discussed with other tech's on the air. Thanks to their input, such as better bias filtering and PL board mounting room was researched for that version 7.3 for enough room on this board to "daughter" mount a PL decoder, such as the Comm. Spec TS-32. However some of the features, such as the timers on version 6.0 were still desired. In order to build just one more board (version) for universal use it was decided to scrap the entire 7.x versions. Version 6.0 had all the features wanted, therefore a new board layout was designed to fit the Mitrek and have space for the PL decoder.
Changes and additions
This version of 6.2 the same as 6.1, with the exception of the meter output adjust, VR5, is left out. Its replaced with one series and one shunt resistor. The AGC curve image further into this document shows the values for this particular radio and its front panel meter. If you choose to use a fixed output you may need different values with the meter you end up using.
As in 6.1 the following updates are incorporated:
The additional IC (U3) is support for various configurations, such as polarity I/O for both cor input and PTT outputs. If you desire active highs can replace the NPN transistor with PNP with the leads in the appropriate places. (The earlier versions have a few limitations in this area.) U1 now functions as DC interfaces for both the COR and CTCSS inputs (if used). The 555 timers were replaced with a section of this additional IC for better parts count efficiency. Instead of using U1 as a timeout/gate, a set of six diodes control the modes you wish. The diodes take less room on the board. For example, the audio and PTT paths can have separate modes, carrier, tone only or carrier + tone (AND Squelch) dependent or together. This maximizes flexibility for system builders wishing to configure their system the way they wish. Note the (changed) color of wires to connect the board's inputs of the "CTCSS", "CON-1" and "CON" inputs and how they are used. Depending on your configuration you can leave certain devices out. This will be covered later on.
Definitions, terms acronyms and semantics
References can be expressed in a few acronyms. Test Tone level (TTL) into a two-way VHF/UHF transmitter or out of a VHF/UHF receiver is referenced to a test tone frequency of 1 KHz, of %100 system modulation. For this standard, that is +,- 5 KHz deviation. Other areas and services have different bandwidths, such as in P-25 systems. A Test Level Point, (TLP) refers to a measurement point, on equipment, for a system, in reference to Test Tone Level (TTL). TLP provides easy reference to any parts of the system for measurement and alignment. 0 dbm is referenced to 1 milliwatt at 600 ohm impedance. Therefore, a transmitter AF input with a TLP of 0 dbm, with a Test Tone Level of 0 dbm tone input, would fully modulate the system. A far end receiver with the same TLP would output a 0 dbm tone as well. A 6 db drop in (voltage) level would reduce the modulation in half, and so on. In general, for these standards, levels are stated in transmit-receive (Tx-Rx) order. Therefore, an audio (VF) "drop" TLP of 0/0 would mean a Tx TLP of 0dbm, Rx TLP of 0dbm. Sometimes operating levels are not at TLP. To avoid technician confusion two sets of numbers are sometimes used in diagrams and on the physical equipment. Figures in parenthesis are the TLPs. Non-parenthesis figures are operating levels which may be at a different levels from the TLPs. Most of the TLP's for this project are in the -10~0 dbm area, however, a resistor value change can be made for your system's requirements.
The term "COR" came from the old tube days of "Carrier Operated Relay", whereas, a tube receiver had a point, when its squelch opened, a tube (switch/valve) drew current through a relay's coil, to give some contact closure, to key the associated repeater's transmitter. As the solid state technology came in the later 1960's the term stayed with repeater operation, even though the Author saw no "relay" in most modern repeaters and felt the "relay" term should have been replaced with the term of "squelch", since it's the receiver's squelch that does the repeating. This would be called" COS", meaning a "Carrier Operated Suelch".
Both terms are true and this gets down to semantics. After careful consideration of modern technology used in the LMR field by Amateurs and professional alike, including recent repeater product terminology and to the fact that repeater stations in the early years were also called "Relays", whereas, the station would "relay" a signal rather than "repeat" a signal, the Author decided to stay with the majority's term of "COR", to avoid reader confusion. Therefore, this and other documentation will reflect this decision.
The term "PTT" will describe an active going "low" for DC functions, such as transmitter keying ("PTT Input"). It also will describe a receiver's COR line driving a NPN transistor, with the open collector being "Receiver PTT Out", or just "PTT Out". "PTT-1" will describe this function, however with a buffer, such as the output of the COR/AF board, which changes state for user signal change of status. This function would be used for audio switching, such as Auto-Patch audio routing. "PTT-2" will describe a buffered, and timed output of the COR/AF board, to keep a repeater's transmitter keyed up for normal back-and-forth conversations of the users of such system(s).
"COS" will be reserved to describe a "Carrier Squelch" as a part of a receiver. "CS" will be reserved to describe "Carrier Squelch" as a receiver's mode of operation, verses "TS", "PL" or "CTCSS" to describe a "Tone Squelch", "Private Line" or "Continuous Tone Coded Squelch System", respectively.
"PLI" means Private Line Indicator (or Input). It's also similar to an CTCSS line out of a tone decoder. "HUB" means Hang Up Box. Motorola's uses a "closed loop" and a HUB for mobiles and base station control. "AND squelch" means it takes both carrier + tone to activate a cor board, transmitter or system. AND squelch is also referred as a variable sensitivity squelch, whereas, the squelch setting affects activity. An "OR" squelch does not, whereas, it "bypasses" whatever squelch setting, using only tone to keep it active.
SRG standards: The receivers have a solid logic high and low, and PTT outputs are normally a relaxed, open collector, active going low to ground. Therefore, some components (many resistors) are left out on the board. The comparator jumpers are pretty simple. However, you may have different preferences and standards for your system. For example, you may be working with a receiver that has a sensitive DC squelch circuit and has an analog state. The board has inputs for either a COR DC input or a CTCSS (PLI) DC input, or both, with any polarity and voltage level change. The schematic drawing includes (optional) pull-up and pull-down 100K resistors you can include, depending on the type of receiver you are working with. The (comparator) PTT output has the same flexibility; active high or low transistor outputs. By setting the input and output buffer jumpers (op-amp/comparator input and bias) this allows just about any arrangement possibility. A set of diodes (D1~D6) make a matrix for carrier squelch, tone or both (AND) for either the audio or PTT path, or both. For easy reference the odd number diodes affect carrier, while the even number ones affect tone configuration. The mode chart may help you as well.
Some commercial receivers either don't have an AGC (S) meter or their "M1" output will not properly drive a meter with a meaningful scale. Most Amateur receivers do have an AGC meter, however typically give an "early" (generous) reading with weak signals. This is a waste of indication. FM receivers quite with signals, therefore, you can easily listen for these changes when checking performance. When the signal gets almost full-quieting is when you need a visual (meter) indication to observe signal strength changes. This circuit will do just that in this version. U3 amplifies the receiver's AGC voltage, then with a strong signal will flatten out with no increase in output. This makes a handy logarithmic voltage change, per RF input changes.
U3, pin 10 inputs a fairly wide range of receiver detected/IF amplifier DC meter function. Pin 9 sets the reference (bias) and pin 8 output drives most meters. The output has a variable resister in series to limit current to the ACG meter and to work with most any meter.
This version is specifically for the Motorola Mitrek receiver. Refer to the diagram for changes from version 6.1. Some of the changes are:
For generic receivers, start by measuring the "M1" or AGC point of the receiver you are setting up. The circuit prefers to "see" much less than a tenth of a volt, DC, or less with no RF signal into the receiver. Change the input resistor for the receiver you are using. Plot a curve with an RF signal generator with levels between -100 and -60 dbm, with no modulation. Starting with a level of -60-ish (hard limiting) adjust VR3 for a mid scale meter indication. Install the output resistors to produce a full scale indication on the meter. Then lower the level around -90 ~ -100 area to observe a zero reading on the meter. In some cases the meter will show a slight swing off from zero. Plot a curve from -100 to -60 on a graph paper. If it's not a usable curve try different settings of the adjustments just described. You'll need to do this several times to get a good curve. A resistance substitution box or two comes in very handy for this task.
Mitrek radio serial 44 was plotted; this is typical for this receiver. As mentioned earlier the meter output resistance is fixed, set only for this radio and meter on its panel:
Here's a twist. In 2010 a prototype (version 6.1) was built. From long-term R & D of earlier versions, all the circuit sections of 6.1 worked perfectly, with the exception of the AGC meter driver circuit. 6.1 uses variable resistors (pots) for both the AGC bias and meter driver output series resistance. An alignment (setup) procedure using these pots was being developed to produce a good AGC curve. However, this was interrupted because of a failing HUB support for the 147.20 repeater. Because of an extreme time restraint for repairs, a "rough" alignment procedure was written and used for version 6.1, then put in service.
To produce a backup (FRU), version 6.2 (this one) was built, being a copy of 6.1, with the exception of the AGC pots. Time permitted development of a good bias and meter driver values. They replaced the pots with fixed resistors values for this model of radio only. This produced a dramatic improvement for the curve, with wider level range for indications. This curve, the values and board version is the bases for the final version to be produced, which is planned to be 6.3, as of March 2011. The better curve as seen (with those values):
Setup for DC/Key outputs
Load the board with all the components needed, depending on your application. This is especially true if you are using this version for something other than the Mitrek radio. You will need to connect at least ground, power, cor input and AF input for alignment and testing. The other connections can be made on final assembly. For your first board you may want to leave yourself enough wire length to work on the board as you will be experimenting with different component values. Later on future boards will go much easier so you can plan your fixed wire lengths to go into the radio on a permanent basis.
If you are using this version for something other than the Mitrek radio, study your receiver's schematic or documentation for the best point and make that connection. The board's cor input U1 buffer is high impedance, therefore, should not affect the squelch circuit of the receiver. It also can convert an "analog", varying voltage point to a logic level and if needed, invert the polarity. Most likely you will not be using any pull-up or pull-down resistors on the COR input. (diagram shows most of them grayed out).
The cor input polarity jumper, JU1 can be set for inverted or straight through. The former can be identified on the schematic drawing by the "criss-cross" lines on the cor input buffer/driver, whereas, it's inverting the cor source logic state. There's a set of jumpers on the board for this purpose. They can be either the push-on (PC) type shorting bars, or just wire jumpers soldered in place. These will work with either mode:
Other (conventional) receivers:
Squelch modes (tone or carrier):
CTCSS decoder types:
The Micor PL deck is a relaxed (turned off) transistor output, going to a forced "high" during activity (decode). Therefore, the PLI input, JU2 will be set for inverted, with no pull-up or down resistors. The same pro and cons exist as with any audio AND squelch as mentioned earlier.
The Communications Specialist, CTCSS decoder model TS-32 has two outputs. Refer the documentation with the unit. Should that be not available the "Out-1" is normally turned off and the collector is an active low if HUB-1 is cut. The Out-2 is normally turned on (low) and is an active relaxed open collector, if JU-2 is cut. If the TS-32 is not available it's believed the TS-64 will substitute.
Power up the receiver and board. The green power led should be lit. Note that it takes about 10 seconds for the board's audio circuits to stabilize on power up. Since repeater/link service normally is 24/7 on, this should not be an issue. Adjust VR1 for proper trigger level when the squelch is active. There is no hysteresis on the input, so you should give yourself a little "margin" with this trigger point, for component/aging variances. Use the yellow led to watch the transition. For the Mitrek set it (U1, pin 13) for + 0.70 volts.
When the cor is active, U1 input translates polarity (depending on your jumper settings) and drives both the audio squelch and PTT circuits. When active, pin 14 goes low, turning off Q1 and letting the AF input through the two stages of equalization and amplification to the "AF OUT" to drive a transmitter or controller. Pin 14 also drives an inverter output on U3, pin 1; which drives both the PTT1 and also drives the tail timer for the PTT-2 output. In the case of no timer needed the PTT-1 can be strapped for either collector out active hi or low. For full features install all components per the schematic diagram available in this document.
The summed cor and tone lines drive a "control buss". All lines need to be logic "low" for the output circuits to be active. Diodes D5, D6, D7 and Q6 make up an "OR gate" for this control buss. Any one of these lines going "high" will stop output activity. That would be from the summed cor, summed tone, time-out or the control input's buffer circuit "CON-1".
Audio setup and some Theory
This section is the same as earlier cor board versions because the audio section is identical. The audio input TLP should be -20 dbm or higher. For different inputs change R1 value, per the TLP chart further into this documentation. If you don't need a squelch you can leave out Q1 and its related parts, too. Inject a clean 1 KHz tone and turn up VR8 to just at clipping point observed on the board's "AF output" with an oscilloscope. Tune the bias at pin 5 with VR7 for best even top and bottom clip on the output. Re-adjust VR8 as needed to fine tune VR7 adjustment. VR7 will be a one time alignment unless a major component change is made. Pin 10 doesn't need bias adjustment because it should be running well below clipping, if you follow the TLP chart to control it's gain.
Regulator, U2 keeps out any small ripple that would be amplified on the system, so most any 78xx series will work, since the audio op amps use a single end supply with voltage dividers for the "+" reference. Just remember its abilities. With U2 as a 7808, the maximum unclipped output of U1 is about a +9 dbm (bridged). The current level points in the chart are based on this supply. If you need higher output there are some options. You can operate U1 at higher voltages, say, with a 7812 as U2 which will drive U1 output near +14 dbm (bridged). Just watch out for the maximum operating limits of the op amp. Also, higher regulator values reduces ripple protection. (If running into clipping can be useful for system "IDC" as mentioned in the next paragraph). As of 2003, author's design settled on a the 7810 (+10v) for the best performance. As time permits the level chart will be updated to reflect this.
For links, each time you limit deviation for each hop will add more distortion. In the past, this had been typical with both commercial and Amateur repeaters, which produces a less than optimal system. For superior audio it is highly recommended to run all your links in "passive" mode and only limit at the last point, such as the system's main transmitter. SRG (system) specification is to set the system output transmitter limit at 6 KHz and let the user's transmitters limit at 5 KHz deviation. This mode requires system management, technician maintenance, discipline and user responsibility. This may require some enforcement on user's part. A circuit to "punish" over-deviated users is possible, however, is beyond the scope of this documentation. You can run the AF output in this mode by setting the clipping point at your input TLP. The output TLP can be adjusted either by changing the output pad or using the optional VR 9 pot and set the TLP that way. Remember, for this mode U1 will be "working" much harder, however, is rated for this type of duty cycle.
Another word about VR8 and the LM-324 Op Amp. The 5 Meg Burns pot might be hard to find. You can substitute with the 2M pot with some loss in gain of the second stage. 5 Meg was selected for a highest value. Anything much more would make the amp to go into the differential mode. Without the negative feed back resistance between the in and outputs it's in the differential mode, which is used as a comparator, such as the cor input section. Voltage gain is the ratio of the negative feedback and input resistors, then make the Logarithmic conversion for a more realistic approach on levels. With R1 value and VR8 you can control the stage gains. Typical (bridged)TLPs are shown on the schematic drawing, assuming the input TLP is a +2.5 as in the case of the Mitrek.
Flat Audio setup
If you are not setting up a flat system you can skip this section. (Hopefully your are, so read on). Assuming you are using a conventional repeater controller, you will need to perform some modifications for it, such as pre-emphasis in the voice ID (if used) and auto patch and de-emphasis for the DTMF decoder and auto patch line driver (if used). This way the system should be transparent, while the internal parts will compensate for the user's pre-emped radios, all used in F.M. mode.
Most receivers have high end roll off. This is a conventional method for commercial systems. If you want your system to sound really good (flat) you can extend the system's frequency response. First, plot the receiver's response on a graph, from 10 Hz to 10 KHz. This sounds a little extreme, but this will show how you are progressing.
The board has two stages of equalization with amplification to bring the level back up to a usable level. The first stage will flatten out the middle, say above 2 KHz, and the second stage, for the upper end, say, above 4 KHz. With some experimentation with different values you can extend it out to around 6 KHz +- 1 db. To cut some time and performance you can "plug in" some typical values sometimes found to work with the Mitrek receiver, which are:
Re plot the receiver. If you wish for the highest performance use a resistance and capacitance substitution box for each of the stages and re plot as necessary to obtain a flat response curve. You may have to repeat this procedure several times. Remember for multiple links you need to get it really flat, since imperfections will add up at the far end.
Here's the same 2 charts as above, except with a "preview" as you can see. Click on the image for a larger window. Left is stock, right is equalized. These were plotted in 1980 using a Motorola Motrac at the discriminator point. 30 years later is still in service ! Each chart has two lines, one for minor (1db) changes and the other for major (10db) changes in response.
Early research was performed with U2 being a 7808, therefore, here are the typical operational specs for the board;
some 324 IC's produce lower "and" gate voltages and still work fine, since it's the relative difference between the two input levels that determine the logic output. As time permits 10 volt operation points will be updated.
|Condition||U1, Pin 13||U1, Pin 12||=U1, Pin 14||PTT 1 & 2|
|'cor' active "1"||+2.54v||+3.41v||"high"||Active|
|System timed out||+2.54v||2.03v||"low"||Standby|
|"CON 1" pulled low||.||2.6v||"low"||Standby|
Unless otherwise specified, resistor values are in ohms 1/4 w, 10%, chokes in milli-Henries, caps in Micro-Farads. Notes: Color of wires: Black, red, white, green, yellow, orange, blue, brown, violet, pink, slate. Allow 2 hours labor for building and 1 more for alignment; common tools and solder equipment. AC (audio levels) measurements are made with an HP 3555 meter (in dbm) in bridged (high impedance) mode. DC measurements are made with a Fluke DVM model "77", including the (alternate) way of checking an AC (audio) TLP level in one case. * Used for custom selection for each receiver.
Test Level Points (Bridged and using a 7808 for U2) This is for the audio section. These levels were for a test motrac receiver.
Point of Measurement
Noise floor (S/N)
"Rec Discr." Input
Test tone of 1 KHz
IF Trap output
1st Eq stage output
Junc of 68K & 15 K res
U1 input-first stage
U1 Output-first stage
2nd Eq Stage output
Junc of 68K & 10 K res
VR3 and VR6 at maximum
TLP Chart: For levels other mentioned in the above chart, change R1 value per input TLP. These are maximum TLP's; you can run lower levels and/or lower R1 values, if desired. All TLPs are reference to a 1 KHz tone. A good TLP for the AF output is 0 dbm. In case you don't have an AC volt meter (calibrated in dbm) a DVM can get you in the ball-park. The Author tested a Fluke "77" meter which read 750mv RMS. (true RMS would be 717 mv). Also note this chart, along with this version is obsolete. For a better chart view the later, current version of board.
Or lower value
Or lower value
Or lower value
Or lower value
Or lower value
Or lower value
COR board Parts list-probably accurate, however not verified. Use your common sense at this point.
IC, Quad Op Amp, LN324
IC, +10v Regulator, 1.5a 7810
Transistor, NPN, such as 2N3904
Transistor, PNP, such as 2N3906
Resistor, 1K, 1/4w, 5%
Resistor, 1.8 K, 1/4w, 5%
Resistor, 2.2 K, 1/4w, 5%
Resistor, 4.7 K, 1/4w, 5%
Resistor, 10K, 1/4w, 5% *
Resistor, 15K, 1/4w, 5% *
Resistor, 68K, 1/4w, 5% *
Resistor, 100 K, 1/4w, 5%
Resistor, 470K, 1/4w, 5% *
If "R1" is that value
Trim-pot, multi-turn, 10K, inline leads
Trim-pot, multi-turn, 2Meg, inline leads
Trim-pot, multi-turn, 5 Meg, inline leads
LED;Diffused,Green T1 3/4
LED;Diffused,Red T1 3/4
LED;Diffused,Yellow T1 3/4
LED;Diffused, Orange T1 3/4
IC socket 14 DIP
Capacitor, Elect, radial, 100uf/25v
Capacitor, Elect, radial, 10uf/25v
Capacitor, Elect, radial, 1uf/25v
Capacitor, Mylar, radial, .0082uf/100v
Capacitor, Mylar or Disc, 390pf/50v
Capacitor, Mylar, radial, .22uf
Hosfelt #;for U2
Capacitor, Mono, radial, .1uf/50v
Capacitor, Mono, radial, .01uf/50v
Capacitor, Mono, radial, 100pf/100v
COR / AF board, AK2O
PVC colored wire, "6 long, 22-24 gu.
See notes below
bare wire around 22-24 gu
For board jumpers
Parts cort, less shipping, etc.&nbs p;
As of March,2000
Unless otherwise specified, resistor values are in ohms 1/4 w, 10%, chokes in milli-Henries, caps in Micro-Farads.
Notes: Color of wires: Black, red, white, green, yellow, orange, blue, brown, violet, pink, slate.
Allow 2 hours labor for building and 1 more for alignment; common tools and solder equipment.
AC (audio levels) measurements are made with an HP 3555 meter (in dbm) in bridged (high impedance) mode.
DC measurements are made with a Fluke DVM model "77", including the (alternate) way of checking an AC (audio) TLP level in one case.
* Used for custom selection for each receiver.
Also, several caps of the same value were considered for their diameter and height for these tight areas. The two electric type are from different vintage, the smaller being more current and advanced, while the third being a tantalum is always a good old alternative, plus lasts longer than the former types.
This board was designed by Karl Shoemaker, AK2O for: Spokane Repeater Group at: http://www.srgclub.org
For future purchase, contact the manufacturer, FAR Circuits at:
18 N. 640 Field Ct.,Dundee,IL,60118, or email at: firstname.lastname@example.org,
Or visit their web site. Fred Reimers, KF9GX, is your contact.
Part vender names are truncated; Jameco is Jameco Electronics, Hosfelt is Hosfelt Electronics, Mouser is Mouser Electronics, etc. For some of these sources contact:
Mouser Electronics (800) 346.6873
Hosfelt Electronics (800) 264.6464
Digi-Key Electronics (800) 344.4539
Jameco Electronics (800) 831.4242
All Electronics (888) 826.5432
This may be copied in complete form only for non-profit purposes, such as for the knowledge for the Amateur Radio Service, with AK2O credited as designer. For other arrangements please contact the author. This documented has been updated on Nov 11,2010
Copy write: AK2O 2008 and later