COR/audio board version 5.4 - - Documentation update as of 2-9-07

Introduction and History

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. 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. The earlier version 5.3 is the same board. The author felt it appropriate to change "cor" to "cor" for Carrier Operated Squelch, which the board is based on. The older term "cor" is for the old 1960's tube radio that really did use a "relay" for keying a transmitter. Otherwise, the board and schematic is generally the same.

Setup and theory: (and some definitions)

TERMS/DEFINITIONS: (Just to set the record straight)

All TLP's (Test Level Point) are referenced an absolute audio level for a 100% system modulation. 0 dbm is referenced to 1 milliwatt at 600 ohm. For example, a TLP of 0dbm is referenced to a 5 KHz deviated RF carrier into a FM receiver with 0 dbm tone of 1 KHz in frequency coming out of it's discriminator. Another example would be to input a 1 KHz test tone at 0 dbm to deviate a FM transmitter at +- 5 KHz. Obviously, different parts of the world have different bandwidth requirements, so make your appropriate calculations. Most of the TLP's for this receiver project are in the -10 dbm area, however, one 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". The documentation on this page was changed to reflect this onion.

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 final 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 discrib 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 Squlech" 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".


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. Leave yourself enough wire length to work on the board as you will be experimenting with different component values.

Start with the COR/COS (Carrier Op. Squelch) point. Study your receiver's schematic or documentation for the best point and make that connection. The board's cor input is high impedance, therefore, should not affect the squelch circuit of the receiver. For a cor that is low or lower on standby, then goes high or higher, jumper A-B and C-D. For a cor that is high or higher on stanby, then goes low or lower, jumper A-D and B-C . Since the Mitrek point "E" is used (active low) the latter jumpers are used. 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. 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 logic and timers, which drive the open collector PTT outputs, active going low, to key a transmitter. "PTT 1" is active for every squelch open, for example, controlling audio switching in a patch or to key a remote base. The "PTT 2" should key most modern transmitters with timer control for normal repeater operation. Transmitter disable controls are active low inputs. You have some choices. "CON 1" simulates a system time out, causing the tail to finish out its time, then drops the transmitter. This affects both PTT 1 and 2. "CON 2" gives immediate transmitter drop out, but does not affect PTT 1, unless the timeout timer expires (if used). Or you can use both "CONs" for control, depending on your needs. If you need positive going PTT outputs move Q2 and/or Q4 around so the emitter is the output. There are extra pads on the printed circuit board for this application. (JU3) 'CON 1 or 2' are still available for control in this case.

U1, pins 12,13 and 14 are set up as an 'and' gate, which require user activity, but not over activity. This is for the FCC requirement of automatic repeater control. This section of U1 is controlled by voltage dividers, for three possible conditions; standby, active and timeout. In standby, pin 13 is higher than 12 and therefore, pin 14 is a 'low'. When cor causes pin 8 to go 'low', pin 13 goes lower than pin 12, causing pin 14 to go 'high' and starts the tail and transmitter key up. If the cor stops, the tail finishes out its time, then drops the transmitter. If the cor stays active too long, (i.e. 3 minutes) U4 times out and U4-3 goes 'low'. Even though cor caused U1-13 to go lower, now pin 12 is even lower than 13; in this case, which causes pin 14 to return to a low and the tail finishes out its time and drops the transmitter and stays dropped as long as the cor is active. When cor stops, both timers reset, which keys the transmitter and finishes out with a tail. This way the others on the system know when the time-out has cleared. Since the timeout controls the tail timer, add both timers when figuring the system transmitter time out. Time out range is 0-3:52 with a 100uf cap and tail is 0-30 seconds with a 10uf cap. Part list shows for the 150uf for longer time out. Try experimenting.

For down link receiver applications the timers and a lot of their associated parts can be left out. For example, if you don't need a tail, leave out U3 and run a jumper from U1-14 to U3-3. If you don't need a time out, leave U4 out and run a jumper from pin 3 to 4. U1-14 is now an inverter. A link transmitter can be keyed with either PTT output.

Setup and theory:AUDIO

Some older receivers are dual conversion, with the second IF of 455 KHz, and leak some of the IF out of it's discriminator. 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 board's audio input has a tunable LC trap for that frequency. For other low IF's you can change the L-C values and find the resonant point with by sweeping the trap. Most modern receivers, such as the Motorola Micor and Mitrek are single conversion and do not leak IF out it's discriminator at any noticeable level. If this is the case, you can leave these parts out and bypass with a jumper.

The 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. 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. When properly set, you have the option (discussed below) 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 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.

Next, inject a test tone into the receiver this board is being set up for and adjust VR3 to just clipping point and then, VR6 for the TLP (test level point) to drive your transmitter, or controller input. This "IDC" mode, is useful for final/system transmitter inputs. For links, please continue reading the paragraphs, below.

Some words about U1 abilities. With regulator, U2 as a 7808, the maximum unclipped output of U1 is about a +9 dbm (bridged). If you need higher output there are some options. You can leave out VR6, experiment with fixed resistors as a pad/voltage divider, or leave them out all together. VR6 or any pads were designed to further improve the S/N performance. With them out U1 typically is 52 db. If this is a stand alone station, this value is plenty, however, in multiple links (more than 3) noise can add up, therefore VR6 or a pad keeps the noise to a good level. There are some extra pads on the printed circuit board to do this kind of modification. 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 higher regulator values reduces ripple protection. As of 2003, author's design settled on a the 7810 (+10v) for the best performance.

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. For this mode, install a pad in place of VR6 and use VR3 to control the AF output. The values will depend on the transmitter's TLP. One example is a 10 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 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 make the Logarithmic conversion for a more realistic approach on levels. With Rx 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.

NOTE: The schematic (to date) was scanned before the test figures were measured. If you really need them they are in the TLP table, here. The "text" snail mail version has them on the schematic.


If you are not setting up a flat system you can skip this section (FLAT AUDIO). Assuming you are using a conventional repeater controller, you will need to perform some modifications for it, such as pre-emphasis in the voice (if used) ID 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 Mitrek receiver, which are for the first stage:
The series cap is .0082uf, the series resistor is 68K, the shunt resistor is 15K. For the second stage, the series cap is 390 pf, the series resistor is 68K, the shunt resistor is 10K. Re plot the receiver. If you choose 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. Remember for multiple links you need to get it really flat, since imperfections will add up at the far end.

Other Notes:

The "AUX AF" is a flat audio input, which is inverted from the output and un-squelched. It could be an ID'er input. If its not used either ground the input or leave out the 1 Meg resistor to avoid noise being picked up and amplified.

For "AND" squelch (CTCSS repeater) use a tone decoder with an open collector output, normally low on standby. A good one to use is the "Comm. Spec" TS-32 Connect the "CTSS" to the TS-32 'Out-2', with its 'JU-2' cut.

Assuming U2 is a 7808, 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.

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 +0.03v "low" Standby
"CON 1" pulled low  +0.6v  "low" Standby

Test Level Points (Bridged and using a 7808 for U2).

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 .


  • Unless otherwise specified, resistor values are in ohms 1/4 w, 10%, chokes in milli-Henries, caps in Micro-Farads.

  • IF trap tunable range is 420-800 KHz;

    cor input for Motorola Mitrec only:

  • For "H" method, pin 10 would be the input, and pin 9 would be the bias. You set the bias less than the cor active high voltage.

  • For "L" method, pin 9 would be the input, and pin 10 would be the bias. You set the bias less than the cor standby voltage, but more than the active low voltage

  • For "E" method, pins 9 and 10 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 and SRG stations.

    cor/Audio Board Parts list
    Qyt Description Notes Part Number cort
    IC, Quad Op Amp, LN324  U1  511-LM324AN  00.34 
    1 IC, +8v Regulator, 1.5a 7808 U2 511-L7808CV 00.40
    2 IC, Timer, 555 U3,U4 511-NE555N 00.72
    5 Transistor, NPN, such as 2N3904 Q1-6 625-2N3904 00.50
    2 Resistor, 1 Meg, 1/4w, 5%  One is "Rx" value  291-1M 00.07
    6 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
    1 Resistor, 15K, 1/4w, 5%  291-15K 00.07
    13 Resistor, 10K, 1/4w, 5%  291-10K 00.91
    1 Resistor, 2.2K, 1/4w, 5% 291-2.2K 00.07
    9 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
    2 Pot, trimmer, multi-turn, 2 Meg, inline leads VR 4,5 594-64W205 04.00
    3 Pot, trimmer, multi-turn, 10K, inline leads VR 2,6 594-64W103 06.00
    4 LED;Diffused,Red:VR,Org:DU,Yel:YY,Grn:MG  Sub"xx" for color 592-SLR56xx3 00.72
    4 Diode, 1N4148 or 914 type,400piv,150ns . 583-FR104 00.40
    2 IC socket, 8 DIP tin/solder 571-26404633 00.16
    1 IC socket 14 DIP tin/solder 571-26403573 00.08
    1 Capacitor, Tantalum, Radial, 150uf/15v Hosfelt Part # 15-238 01.75
    3 Capacitor, Elect, radial, 100uf/25v . 140-XRL25V100 00.21
    2 Capacitor, Elect, radial, 10uf/25v . 140-XRL25V10 00.10
    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 Capacitor, trimmer, 20 pf 455 KHz IF trap 242-3810-23 00.88
    2 Choke, radial, 4.7 mh,9mm.dia. 455 KHz IF trap 434-01-472J 02.28
    1 Board, cor/Audio, AK2O  FAR Circuits**  ver 5.3  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 $29.09

    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. Use other values of chokes for other IF's i.e. 11.2 MHz, etc
    If using a fixed output pad and not VR6, add 1 more 1K resistor, or the appropriate value(s).

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

    To purchase, contact the manufacturer, FAR Circuits at:
    18 N. 640 Field Ct.,Dundee,IL,60118, or email at:,
    Or visit their web site. Fred Reimers, KF9GX, is your contact.

    For alternative parts sources contact:

    Source: Mouser Electronics (800) 346.6873

    Alternate parts source: Hosfelt Electronics (800) 264.6464

    Schematics, strapping options for both Repeater and Link usage: (These will open a second browser window).

    Repeater mode Standard schematic & strapping.

    Link mode For remote transmitter or receivers, no timers, etc.

    Link mode Inproved drawing for Version 5.4.

    PDF Format Standard schematic & strapping

    Here are two photos of the board, with most of the components mounted. (The two chokes are not needed for non 455 KHz IFs).

    Large Photo 1280x960 pix

    Smaller Photo 600x450 pix

    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 and later

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