COR/audio board version 6.0

By AK20


Introduction and History

The COR/audio board, or just known as "cor board" (any case letters) designed occurred in the early 1980's by the Author. 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. After some consideration the Author designed the latest 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. Please note the design is not final and in the alpha stage. The physical clearance issues are being worked out for the mitrek, since it will mount upside down, similar the PL deck.

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 the same. U1 has slightly different functions, plus, an additional IC had been included (U3) for additional features. Read on.

Changes and additions

This additional IC (U3) is support/accommodate various configurations, such as polarity I/O for both the cor and tone inputs. A new AGC meter driver circuit has also been added via U3. (The earlier version has a few limitations to accommodate other configurations.) 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. 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. Not all groups wish to configure their systems like SRG, therefore, the Author is attempting to build this philosophy in this version. For example, the inputs for both the cor and tone can be strapped for either polarity and can work with an analog state, instead of just a logic "1" or "0". Also, the PTT outputs are still available in active and timed active going low. If you desire active highs replace the NPN transistor with PNP with the leads in the appropriate places. Audio and PTT paths can have separate modes, carrier, tone only or carrier and tone (AND Squelch) dependent or together. This maximizes flexibility for system builders wishing to configure their system the way they wish. Depending on your configuration you can leave certain devices out. This will be covered more on. 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. As mentioned before the COR/audio board, herein, will be referred to as just "cor board".

Definitions and Terms

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, 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 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.

PTT Theory

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. This permits easy setup as well.

AGC Theory

Some commercial receivers either don't have an AGC (S) meter or their "M1" output will not properly drive a meter with a meaningfull 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-quiting is when you need a visual (meter) indication to observe signal strengh changes. This circuit will do just that in this version. U3 amplifies the receiver's AGC voltage, then with a strong signal will flaten out with no increase in output. This makes a handy logrimic voltage change, per RF input changes.

U3, 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 in series to limit current to the ACG meter. The value can be changed to work with most any meter. Defualt for this version is 10K ohms.

AGC Setup

Start by measuring the "M1" or AGC point of the receiver you are setting up. The circiut prefers to "see" around a tenth of a volt, DC, or less with no RF signal into the receiver. Adjust VR10 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 VR9 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 discribed.

Setup for DC/Key outputs

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.

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 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.

Here's a couple of examples of radios models and their two basic 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):

  • Connect the receiver's cor point to the cor input on the cor board. To make both the repeat audio and PTT outputs to an AND squelch connect 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. This arrangement does have the most protection against RFI, however.

  • To make the repeat audio on carrier , but the PTT outputs an AND squelch connect the TS-32 to CON 1 instead. For this mode the CON 1 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 1 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. You can modify the PLI. Reverse the diode and move the pull-up resistor to between the pull-down and the diode. Now the PL deck will provide the (divided) voltage for the AND gate's logic. Another option is to modify the PL deck to act like a conventional decoder (as described earlier). 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 PTT output going to the cor board's cor input. In this case you would not use CON 1 (only the cor input). 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.

    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.

    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 8 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 8 also drives an inverter, which drives both the PTT1 and also drives the tail timer for the PTT2 output. In the case of no timer needed the PTT1 can be strapped for either collector out active hi or low. For full features install all components per the schematic drawing, available below.

    Control Buss

    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".

    CTCSS decoder

    If you are using either a tone or AND squelch you will need a separate CTCSS decoder board. There some brands available. A very good one (used for SRG) is the "Comm. Spec" TS-32. Connect the "PLI" to the TS-32 'Out-2', with its 'JU-2' cut. Otherwise, this board can be configured for other brands of CTCSS boards as discussed earlier. The repeate audio and/or the PTT outputs can be configured for either carrer or "AND" squelch (tone + carrier) or both. Refer to mode matrix chart for diode selection.

    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 Rx 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 VR3 to just at clipping point observed on the board's "AF output" 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 unless a major component change is made. Pin 3 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 to remember its abilities, here. 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. 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. For this mode use VR3 to control the AF output. The level will depend on the transmitter's TLP. One example is to install a 3 db pad and run the board's output at 0 dbm for a 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 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 Rx value and VR3 you can control the stage gains. Typical TLPs are shown on the schematic drawing(bridged) assuming the input TLP is a +7 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 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 receiver, which are:

  • First stage: The series cap is .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.

    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 COR board version 6.0
  • Configuration Matrix chart.
  • 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. These were plotted in 1980 using a Motorola Motrac at the discriminator point. 29 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 on 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
    Standby  +5.24v  +3.41v  "low"  Standby 
    '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

    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 Level  Remarks Noise floor (S/N)
    "Rec Discr." Input -10 dbm Test tone of 1 KHz .
    IF Trap output -10 dbm . .
    1st Eq stage output -16 dbm Junc of 68K & 15 K res .
    Squelch Switch -22 dbm Q1 collector -63
    U1 input-first stage -58 dbm Pin 2 .
    U1 Output-first stage -1.5 dbm Pin 1 -62
    2nd Eq Stage output -20 dbm Junc of 68K & 10 K res .
    U1-6 input -59 dbm Mostly noise .
    U1-7 Output +7.2 dbm VR3 and VR6 at maximum  -52

    TLP Chart: For levels other mentioned in the above chart, change Rx value per input TLP. These are maximum TLP's; you can run lower levels and/or lower Rx values, if desired.

    Input TLP Rx 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 .

    COR board Parts list-reminder this list is based on the earlier version. For 6.0 time has not allowed for this list to be "adjusted". Use your comon sense at this point.


    Qyt Description Notes Part Number cost
    IC, Quad Op Amp, LN324  U1, U3  511-LM324AN  00.68 
    1 IC, +10v Regulator, 1.5a 7810 U2 511-L7808CV 00.40
    4 Transistor, NPN, such as 2N3904 Q1-4 625-2N3904 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
    8 Resistor, 100 K, 1/4w, 5%  291-100K 00.42
    2 Resistor, 68K, 1/4w, 5%  291-68K 00.14
    1 Resistor, 33K, 1/4w, 5% 291-33K 00.07
    8 Resistor, 10K, 1/4w, 5%  291-10K 00.91
    4 Resistor, 1K, 1/4w, 5% 291-1K 00.63
    2 Pot, trimmer, multi-turn, 5 Meg, inline leads VR 3 Hosfelt #38-184 01.35
    3 Pot, trimmer, multi-turn, 10K, inline leads VR 2,6 594-64W103 06.00
    4 LED;Diffused,Yel:YY,Grn:MG  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 . 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  FAR Circuits**  ver 6.0  06.00 
    11  PVC colored wire, "6 long, 22-24 gu.  Various colors  See notes below 
    bare wire around 22-24 gu  For board jumpers  . .
    Parts cort, less shipping, etc.  As of March,2000  Total $20.00

    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 2 more for alignment; common tools and solder equipment.

    * Used for custom selection for each receiver.

    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:

    Source: Mouser Electronics (800) 346.6873

    Alternate parts source: 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 2008 and later

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