COR Board version 7.3




This board interfaces a receiver to a controller or transmitter, while it performs basic link or repeater functions, except for an ID'er. Emphasis on size, simple design, parts availability and easy modifications, limited only to your imagination. It contains simple audio, PTT (Push To Talk) and AGC meter control. Depending on the application you can leave out some parts, while strapping for others, such as cor delay, and polarity on "COR" and PTT output. There are some extra pads on the board for this purpose. 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 operation you might want to check out additional technical books relevant to this documentation.


This board design 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. In the 1990's recent versions of 5.x 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 analyzed these features for future SRG projects. Some of the changes were based on input from other techs as well.

After some consideration in 2008 the Author designed version 6.0, which has repeater functions, such as the timers, etc. with slightly better parts use efficiency. That version is documented separately and was not put into production. However, you are welcome to use the ideas and circuitry keeping the Author as designer. For receiver downlinks 6.0 was unnecessary, therefore, a "cut down" version of 7.0 was designed. Then a "mirror" version (7.1) was designed for mounting the board upside down in a receiver chassis. These versions were not produced, but kept in reserve in case of. It's the bases of later versions documented separately. Version of 7.2 was specifically sized for mounting in place of the PL deck of a Mitrek radio. Because of some minor improvements, mainly in the features it was scrapped and replaced with this version 7.3 . Also note this version is not produced.

Changes for this version of 7.3

They are similar to version 7.2, however, if you are experienced from building earlier versions it's best to discuss them:
Some thought of additional filtering was in earlier versions, even way back to 5.0, however, was not deemed necessary at the time of construction. To be complete all past considerations were incorporated in this version; some are considered optional (to your needs) :

  • The "AUX" AF input can be used for a low-level IDer output. However some IDer boards, including the ones used for the system sounded bad. A high pass filter was added for the "AUX" line. This rounded off the high frequency harmonics from IDer's square wave output.

  • An optional variable sensitivity input on the AGC circuit was added.

  • Optional PTT output protection was added for driving a mechanical relay, if anyone uses such a device anymore.

  • The op-amp pin number assignment are different; that's because the board layout was re-engineered.

  • New board layout was done to expand for an area for a Comm. Spec. TS-32 decoder board, and even a ID-8 by the same company.

    Normal JU1 strapping puts the cor on pin 3 of U1. Pay close attention to avoid construction issues and confusion. Another change from earlier versions is the capacitor added on the AF input to address a receiver discriminator output that may have DC component. Bypass capacitors were added on bias dividers for additional stability of the audio amplifiers in this version and 7.2. (they are optional) Another change was the addition of VR6, to adjust the ACG meter output to for most any meter movement. Also, the audio or PTT (or both) can be setup either for carrier squelch or "AND" (carrier + tone). 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. More details on all of these items are discussed later. Recently it was decided to add PCB eyelets for optional bypass capacitors in the .1~.001 uf range. They can filter out RF that the 100uf will miss. They are mostly around the bias for the operational amplifiers.

    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, (sometimes 1004 Hz in professional telecommunications testing) of 100% system modulation or rated capacity of a channel. Channel TTL generally refers to an commercial FM microwave path modulated by SSB channels in FDM. For this document will discuss system modulation only.

    TTL standard for FM Amateur systems is, + and- 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 TTL. TLP provides easy reference to any parts of the system for measurement and alignment.

    Power is a (ohm's law) formula involving voltage, current, resistance, or power values, expressed in volts, amps, ohms and watts, respectively. For telecommunications a good way for level measurement is using Logarithmics of these values on a 10 based system. 0 dbm is referenced to 1 milliwatt at 600 ohms 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 (expressed in dbm) are sometimes used in diagrams and on the physical equipment. Figures in parenthesis are the TLPs. Non-parenthesis figures are operating levels, and, as mentioned, 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 by the Author 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 (tail) 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 OEM equipment uses a "closed loop" and a HUB for mobiles and base station control during idle receive conditions. Whereas, the major competitor, G.E., is the opposite, being an "open loop" for the same condition. "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 an open audio path.

    Setup for DC/Key outputs

    When the cor is active, U1 input translates polarity (depending on your jumper settings) and drives both the audio squelch and PTT circuits. The latter consists of two transistors; one being an inverter. When active, pin 1 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. It also turns on off Q2, turning on Q3. Q3 collector output will key most modern transmitter's "PTT line input".

    Start with the cor point. 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 have its sensitivity adjusted. It also can convert an "analog", varying voltage point to a logic level and if needed, invert the polarity. 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.

    The PTT output normally is set up for relaxed (open) collector during idle and going forced low during activity. However this output mode can be reversed for a forced "high" output during activity. If you wish this reverse Q3's emitter and collector for voltage source and output respectively. You'll need to use a little imagination with an additional jumper for the source.

    Here's some examples of radio models and their two cor polarities:

  • Non-inverting: For a cor that is high or higher on standby; then goes low or lower for active, jumper A-B and C-D . This would be in the case of the Mitrek's cor point " E" . Neither the pull or down resistors are used in this case.

  • Inverting: For a cor that is low or lower on standby; then goes high or higher for active, jumper A-D and C-B. This would be in the case of the Micor's shunt output or many conventional receivers. Use the pull-up resistor on the cor board, but not the pull-down in this case.

    COR points:


  • For "H" method, pin 9 would be the input, and pin 10 would be the bias. You set the bias less than the cor active high voltage. For this arrangement JU1 is set to inverted. Refer the schematic for clarification.

  • For "L" method, pin 10 would be the input, and pin 9 would be the bias. You set the bias less than the cor standby voltage, but more than the active low voltage. For this arrangement JU1 is set to non-inverted.

  • For "E" method, pins 10 and 9 are the same as "L". The difference with "E" is almost a 'logical' change with squelch, rather than a DC analog movement. This method is the most common used, including in the Super Consolette stock configuration.


  • The micor has an audio squelch board with a transistor collector type output called a "shunt" in its diagram. During standby it's "on", pulled to ground. During activity it relaxes. Therefore, a pull-up resistor is required to produce a voltage. Use the (optional) pull-up resistor shown, but not the pull-down. They are the grayed out components in the cor section of the cor board's schematic. If this is unclear (due to graphics) refer to the diagram for version 5.5, found on this site.

    Other (conventional) receivers:

  • Most other receivers may have a similar cor (open collector) output like the Micor receiver. If not, you would be "picking" off a voltage (change) point, similar to the Mitrek receiver configuration. In this case you would not use either the pull-up or down resistors on the cor board.

    Squelch modes (tone or carrier):

    "CON" and "CTCSS" (inputs) have been earlier mentioned in this document. CON can be used to control (off) the associated transmitter as in the case of earlier versions was called "CON-1" or "CON-2". CON can also be used to make an AND squelch, by connection from a CTCSS's decoder output. You should also know that earlier versions called for pink or slate color wires to these points. Because of these colors being difficult to obtain, plus the version is used differently from the earlier ones, violet colored wire was chosen. for these version.

  • Connect the receiver's cor point to the cor input on the cor board if not already done so. To make both the repeat audio and PTT outputs to an AND squelch connect the tone decoder to PLI. The decoder line needs to be an active going relaxed output. A good choice is the Comm. Spec's TS-32 tone decoder, using it's "Out-2" with its "JU2" cut. If the TS-32 is unavailable the TS-64 will substitute. One disadvantage to this arrangement is the increase delay for an audio signal to get through the repeater. Quick user's first word may be cut off. However, this arrangement has the most protection against RFI symptoms.

  • To make the repeat audio on carrier , but the PTT output an AND squelch connect the decoder to CON instead. For this mode the CON is now being used as a PLI.

  • If you wish both the audio and PTT 2 outputs to be on carrier squelch, don't connect anything to CON or PLI (just the cor input). For the Micor receiver the pull-up resistor would still apply.

  • Micor decoder: This cor board version needs to see a decoder's open collector going relaxed for activity. The stock PL deck does not have this feature, however, you have some options by studying the schematic diagram. You could modify the PL deck to act like a conventional decoder by adding a couple of NPN transistor inverters and resistors or the same for the cor board. Another option is to make the micor (remote) receiver's both audio and PTT AND squelch. This is done by using the stock receiver/PL deck with an AND receiver cor going to the cor board's cor input. In this case you would not use CON or PLI. The same pro and cons exist as with any audio AND squelch as mentioned earlier.

    Previously discussed was the closed loop for the HUB. This feature can be used for either local or remote mode change. This mode change is handy for testing where you want both audio and PTT on carrier on a temporary basis. A mode switch on the front panel and it's contacts in series with the TS-32 can make this happen. With some thinking you can do this remotely with a (external) controller.

    Load the board with all the components needed, depending on your application. 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 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. Then power up the receiver and board. The green power led should be lit. Remember, it takes about 10 seconds for the board's audio circuits to stabilize on power up. Since repeater service normally is 24/7 on, this should not be an issue. Adjust VR1 for proper trigger level when the squelch is active. Give yourself a little "margin" with this trigger point, for component/aging variances. Observe the yellow LED to watch the transition. For the mitrek +1.00 v is a good starting point on pin 2 of U1.

    Setup and theory for audio

    Some history; Older receivers are dual conversion, with the second IF of 455 KHz, and leak some of the IF out of it's discriminator. (such as Motrac receivers, still in SRG service in 2009). When running a flat system this IF leakage will modulate the associated transmitter, as can be observed on a spectrum analyzer. To prevent this, the earlier version's audio input has a tunable LC filter-trap for that frequency. The tunable range was 420-800 KHz. For other low IFs you could change the L-C values and find the resonant point with by sweeping the trap. However, with most modern receivers such as the (single conversion) Motorola Micor, Mitrek and MX do not leak IF out it's discriminator at any noticeable level, therefore, this filter-trap was not used in this version.

    The 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 associated parts. Otherwise, inject a clean 1 KHz tone and turn up VR3 to just at clipping point observed on the board's AF Out with an oscilloscope. Tune the bias at pin 5 with VR2 for best even top and bottom clip on the output. Re-adjust VR3 as needed to fine tune VR2 adjustment. VR2 will be a one time alignment.

    Next, inject a RF signal, modulated with a 1 KHz test tone into the receiver this board is being set up for and adjust VR3 to just clipping point. This "IDC" mode, is useful for final/system transmitter inputs. You can either change the output pad for the TLP of the system transmitter, or install a pot in place off for fine-adjustments. For links, please continue reading the paragraphs, below. When properly set, you have the option of using it as a nice "IDC" (deviation limit) because it's linear up to that point, then just flat tops with further increase input. Most conventional IDC circuits use diodes, back to back, which start causing distortion before the actual clipping point of the industry standard of +- 5 KHz deviation. Since Amateur stations are not required to have as much splatter control with harmonics, as with commercial stations, this should not be a problem. However, you should be aware of any possible bandwidth limitations in your area, since there is a trade-off between bandwidth and system performance. This board was developed in the Pacific NorthWest were we are blessed with 20 KHz spacing for repeater pairs. In other parts of the country with narrower spacing, make your calculated changes as needed. 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.

    Otherwise, in a passive (linear) mode output pads may not be needed and control the line out with VR3. A good starting point is 0 dbm for an output TLP.

    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. You also will need a cap-res substitution box and a handful of various values of these components or access to such a source.

    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 upper end, say, above 2 KHz, and the second stage, for 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 Motrac receiver. These values were found from early research. This vintage receiver is being phased out with later ones such as the Micor and Mitrek receivers. Another point to remember these latter receivers do not need IF filtering, however, do need a DC blocking capacitor as mentioned earlier. Therefore, the latter cor board versions audio input drawings will reflect these changes. As of publishing of this document research will progress (as time permits) on the "default" values for the latter receivers. Therefore, the only (Motrac) values are:

  • First stage: the series cap is a .0082uf (822), the series resistor is 68K, the shunt resistor is 15K.
  • Second stage: the series cap is 390 pf (391), the series resistor is 68K, the shunt resistor is 10K.

    For the Micor receiver research is still under way, however, it's believed the typical values for that receiver are:

  • First stage: the series cap is .001uf (102), the series resistor is 39K, the shunt resistor is 15K.
  • Second stage, the series cap is 560 pf (561), the series resistor is 47K, the shunt resistor is 10K.

    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.

  • Diagram version 7.3 Jpeg form
  • Diagram version 7.3 PDF form
  • Frequency response Before (stock).
  • Frequency response After (with eq).

    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. Each chart has two lines, one for minor (1db) changes and the other for major (10db) changes in response. These were plotted in 1980 using a Motorola Motrac at the discriminator point. As of 2009 this receiver is still in service !












    Some words about U1 abilities may be in order. With regulator, U2 as a 7810, the maximum unclipped output of U1 is about a +10 dbm (bridged). The AF output pads was designed to further improve the S/N performance. With them out U1 typically is 52 db, however for a slight increase output you could leave it out. If this is a stand alone station, this value is plenty, however, in multiple links (more than 3) noise can add up, therefore, the pad keeps the noise to a good level. You can operate U1 at higher voltages, say with a 7812 as U2 which will drive U1 output near +14 dbm (bridged). U2 keeps out any small ripple that would be amplified on the system, so most any 78xx series will work, since the audio op amp U1 uses a single end supply with voltage dividers for the "+" reference. Just watch out for the maximum operating limits of U1, and that higher regulator values reduces ripple protection. As of 2003, Author's design settled on a the 7810 (+10v) for the best performance.

    Another point is the DC capabilities of U1. It was thought the output could go forced low enough, even with devices "fighting" the logic, such as a pull-up resistor. This was an issue in early design using a PNP transistor for Q2 on the U1's output which was an inverter and a positive going PTT output option. The transistor required a pull-up to keep it turned off during standby. However, the U1 output also drives Q1 (AF gate). During activity the U1 output did not go low enough, therefore, Q1 did not turn off completely, causing an issue with the audio section. Therefore, the PNP inverter idea was canceled and conventional a NPN inventor (and associated resistors) for Q2 was used. This change permitted the U1 buffer output to go completely low during activity, thus, allowing Q1 audio gate to function properly.

    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, 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. The values of the pad can be changed, depending on the transmitter's TLP. One example is a 6 db pad and run the board's output at 0 dbm for a TLP. Normal SRG configuration is the 1K voltage divider and run the "AF Out" at 0 dbm TLP.

    Another word about VR3 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 pins 6 and 7, 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 you can make the Logarithmic conversion for a more realistic approach on levels. With R1 value and VR3 you can control the stage gain. Typical figures are in parenthesis on the schematic, (bridged) assuming the input TLP is -10 dbm.

    AGC Theory

    An Received Signal Level(RSL) measurement is very useful to checking out a link path either the first time, as a bench mark, or periodic maintenance to check on the performance. This log measurement (in dbm) helps understand system losses and gains, and gives a realistic idea on a path performance in this area. RSL can be measured with an (expensive) spectrum analyzer, or as an (much cheaper) alternative using the receiver's limiter-detected voltage. This voltage normally is directly proportional the RSL; more signal = more limiter voltage. This voltage can be measured with a sensitive meter.

    Most commercial receivers do not have an AGC meter built-in; rather a test set is connected with its own meter for a relative AGC indication. However, it does not have a meaningful scale. Most Amateur receivers do have an AGC meter (they call it an "S meter") however, typically are set too sensitive, giving a false indication of the RSL. (too generous). This is a waste of indication. Explanation may be in order. FM receivers' audio output quite with signals, therefore, you can easily listen for these changes when checking performance. For modern narrow band (10-20 KHz) receivers this occurs less than -100 dbm RF input. When the signal gets almost full-quieting (-90 or greater) is when you need a visual (meter) indication to observe signal strength changes. The circuit in this version will do just that. U1 amplifies the receiver's AGC voltage, then with a strong signal will flatten out with no increase in output. This allows you to plot an AGC curve in dbm.

    U1, pin 12 inputs a fairly wide range of receiver detected/IF amplifier DC meter function. Pin 13 sets the reference (bias) and pin 14 output drives most meters. The output has a resister and pot. in series to limit current to the ACG meter. The values of both can be changed to work with most any meter. Default for this version is 1K and 10K ohms, respectively. Typically you can leave the pot. at minimum (total output resistance 1K) for meter movements in the 100-200 uA range.

    This version is specifically designed for the Mitrek. "M1" is the AGC point. Per the documentation on the Mitrek conversion brings this point from J10, pin 20 into this board. M1 typically is 180 mv DC with no signal and 490 mv DC with a hard-limiting (strong) RF signal input. Unfortunately, this produces a very steep curve roughly between -116 and -83 dbm RF input to the receiver. Since the AGC voltage comes from U201, pin 13 IC there's no none way to change the limiter AGC characteristics. Two graphs were plotted; a voltage and meter movement-scale indication. They show only a narrow RSL range is usable, typically between -97 and -86 dbm.


























    AGC Setup

    For the Mitrek receiver (with no RF input) set the input sensitivity, VR5, for maximum and the meter output, VR6, to minimum to start with. Set the bias, VR4, just until you start to see a meter movement. Input a -60 dbm RF CW signal to the receiver and adjust VR4 for full scale on the AGC meter. This represents around 450 mV on the wiper of VR4 (not U1, pin 13). Plot an AGC curve starting at -110 through -80 dbm RF input. You will notice the steep curve, however, you can move this curve up or down the axis of the graph by changing VR4 setting and VR6 accordingly. (one affects the other so you'll have to repeat the adjustments a few times). SRG specs set mid scale around -90 dbm.

    You may have read an earlier (old) procedure which was used in the development of this circuit, however, was abanded for the method just described. (note: no VR6 was used). It's left in this documentation for your notes and comparisons:

    With no RF input set VR5 for 100 mv DC on U1, pin 12. Then input a -60 dbm RF signal (no modulation) and adjust VR4 for a full-scale AGC meter reading. You can use this circuit for most other receivers. Start by measuring the AGC point of the receiver you are using and vary the RF signal level from nothing to -60 dbm. The circuit prefers to "see" around a tenth of a volt, DC, or less with no RF signal into the receiver. Adjust VR5 for this range. Then input a high RF signal into the receiver to cause a hard-limiting condition. Typically this would be in the -60 dbm range. Then adjust VR4 for a full scale meter reading. Then you can plot an AGC curve. If it's not a usable curve try different settings of the two adjustments just described. This is a typical curve using the Mitrek with two different M1 inputs, 100 and 179 mv DC inputs for no RF input. When you completed the setup plot the AGC curve for every unit on your meter.

    Other receiver types give a better AGC range with this circuit. For example, in 2009 it was found the Johnson Fleetcom II had a usable AGC of -107 to -63 dbm RF input. 44 db range gave a nice curve on the graph; two curves were plotted for different VR4 settings; since the upper curve contained what the lower had the former was used:















    Test Level Points (Bridged and using a 7810 for U2).

    Point of measurement Level  Remarks Noise floor (s/n)
    Board's AF input -10 dbm test tone of 1 KHz .
    Q1 Collector -30 dbm squelch gate .
    1st eq stage input -40 dbm pin 9 .
    1st eq stage output -16 dbm Junc of 68K & 15 K res .
    2nd eq stage input -20 dbm pin 6 .
    2nd eq stage output 0 dbm pin 7 .
    "AF Out" +10 VR2 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.

    Input TLP R1 Value  Remarks .
    +5 470 K Or lower value .
    0 820 K Or lower value .
    -5 1.5 Meg Or lower value .
    -10 2.1 Meg Or lower value .
    -15 5.6 Meg Or lower value .
    -20 9.3 Meg Or lower value .

    D.C volages-chart:for receiver's cor and cor board's functions, etc.

    Device-Point Standby  Active Remarks........
    COR point "E" input 3.75 .254 board's input
    U1, cor buffer pin 2 2.00 2.00 Bias-fixed amount
    U1, cor buffer pin 3 3.71 .254 U1 buffer input
    P.L.I. 3.75 .254 .
    Q1 base .7041 -.009 .
    Q1 Collector low-Z Relaxed, hi-Z AF squelch gate
    U1, cor buffer pin 1 8.53 -.004 Buffer output
    Q2 Base .736 -.004 .
    Q2 Collector .057 8.48 .
    Q3 Base .057 .770 .
    Q3 Collector 11.95 .041 pull-up from external LED

    Cor board parts list-some parts are not yet determined for pricing therefore, the total is an approximate.

    Qyt Description Notes Part Number cost
    IC, Quad Op Amp, LN324  U1  511-LM324AN  00.68 
    1 IC, +10v Regulator, 1.5a 7810 U2 511-L7808CV 00.40
    3 Transistor, NPN, such as 2N3904 Q1~Q3 625-2N3904 00.50
    1 Resistor, 1 Meg, 1/4w, 5%  If "R1" is that value  291-1M 00.07
    1 Resistor, 220 K, 1/4w, 5%  291-220K 00.42
    6 Resistor, 100 K, 1/4w, 5%  Excluding cor pull-down  291-100K 00.42
    2 Resistor, 68K, 1/4w, 5%  291-68K 00.14
    1 Resistor, 15K, 1/4w, 5% 291-33K 00.07
    5 Resistor, 10K, 1/4w, 5%  Including cor pull-up  291-10K 00.91
    1 Resistor, 4.7K, 1/4w, 5%  291-4.7K 00.91
    6 Resistor, 1K, 1/4w, 5% 291-1K 00.63
    1 Pot, trimmer, multi-turn, 5 Meg, inline leads VR 3 Hosfelt #38-184 01.35
    1 Pot, trimmer, multi-turn, 1 Meg, inline leads VR 5 Hosfelt #38-183 01.35
    3 Pot, trimmer, multi-turn, 10K, inline leads VR 1,2,4 594-64W103 06.00
    1 LED;Diffused,green  Sub"xx" for color 592-SLR56xx3 00.15
    1 IC socket 14 DIP tin/solder 571-26403573 00.08
    1 Capacitor, Elect, radial, 100uf/25v U2 filter 140-XRL25V100 00.21
    1 Capacitor, Elect, radial, 10uf/25v . 140-XRL25V10.0 00.15
    3 Capacitor, Elect, radial, 1uf/25v . 140-XRL25V1.0 00.15
    1 Capacitor, Mylar, radial, .0082uf/100v * 140-PF2A822F 00.43
    1 Capacitor, Mylar or Disc, 390pf/50v  * 140-50P2-391K 00.06
    1 Capacitor, Mylar, radial, .22uf Hosfelt #;for U2 15-315 00.18
    1 Board, cor-audio, AK2O-v7.2  FAR Circuits**  ver 7.0  unknown 
    11  PVC colored wire, "6 long, 22-24 gu.  Various colors  See notes below 
    bare wire around 22-24 gu  For board jumpers  . .
    Parts count, less shipping, etc.  As of March,2000  Total $20.00


    Unless otherwise specified, resistor values are in ohms 1/4 w, 10%, caps in Micro-Farads. The color of wires: Black, red, white, green, yellow, orange, blue, brown, violet, slate. Refer to the schematic diagram or other charts for color assignments for the functions of the board. Time: Allow 2 hours labor for building and 2 more for alignment; common tools and solder equipment.

    For a simulcast system it's important to know the audio from "Discr." to "AF out" is non-inverting. That's because of the two stages of inverting amplifiers. Because of this, "AUX AF" (flat) input is inverted from the "AF out". Even thought you normally wouldn't use it for repeat audio it could be an ID'er input or some other alarm indicator input. If it's not used either ground the input or leave out the 1 Meg resistor to avoid noise being picked up and amplified.

    The parts list generally does not include optional circuits and parts, such as the HP filering, etc. Take this into consideration when deciding what parts to order from your supplier.

    This board was designed by Karl Shoemaker, AK2O for: Spokane Repeater Group at:

    This board was not produced. The design-ideas apply to other versions. Therefore, the accuracy of this version is not verified.

    For alternative parts sources contact: Mouser Electronics (800) 346.6873 or Hosfelt Electronics (800) 264.6464

    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.

    Copywrite: AK2O 2006~present viewing date

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