COR/audio board version 6.1

By AK20



This board was not produced . Only one prototype was built as part of the development of later versions. However, this version was installed as an emergency repair for the HUB repeater in central Washington. Therefore, this document is considered the manual for this board version 6.1 . The AGC section was not perfected. More on that later. 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 6.1 is similar to version 6.0, 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) :

  • This version is designed specifically for the Motorola Mitrek radio, to be mounted upside down (solder side up) in place of the stock Pl deck.

  • 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. Even though there's not room to mount an IDer board on the cor board, an IDer made small enough can be glued inside the Mitrek radio.

  • A capacitor added on the AF input to address a receiver discriminator output that may have DC component.

  • Optional output protection for the PTT-2 was added for driving a mechanical relay, if anyone uses such a device anymore.

  • PCB eyelets were added 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.

  • A new AGC meter driver circuit has also been added via U3. A pot was added to adjust the ACG meter output for most any meter movement.

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

  • The op-amp pin number assignments 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 made by this company.

    Even though they are no longer in production there is a lot of them out in the used market. Plus, current decoder boards can still be mounted on the board in the space provided by drilling a couple extra holes.

    More features

    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.

    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. The mode chart may help you 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 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.

    AGC Setup

    NOTE: As previously mentioned this AGC section was not perfected. Because of a system problem, emergency repairs were made in 2010. Of that was a "FRU" (replacement) for the failing HUB for the 147.20 repeater. Therefore, the AGC curve alignment here, is only a guideline. For more accurate instructions refer to later versions found on this web site.

    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 shunt resistor for the receiver you are using, or install the optional pot. 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. Then adjust VR5 for a full scale reading. 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 adjust the input, VR3 and VR5 several times to get a good curve.

    Specifically, for the Mitrek receiver, with no RF input set U3, pin 9 for + 1.552 volts DC. The input divider is the 1 Meg. resistor in series with the 470 K shunt to ground. With a strong signal (-60) set VR5 for a full scale indication. Now plot the curve on the graph paper. The curve for this receiver turned out steep, but usable as shown:















    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:

  • 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. The next few paragraphs explain more in detail.

    COR points:


  • For "H" method, set JU1 for inverted. Refer the schematic for clarification. You set the bias less than the cor active high voltage.

  • For "L" method, set JU1 for straight though. You set the bias less than the cor standby voltage, but more than the active low voltage.

  • For "E" method, Set JU1 for straight though; 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. Set JU1 for inverted .

    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. As previously mentioned, in this case you would not use either the pull-up or down resistors on the cor board.

    Squelch modes (tone or carrier):

  • To make both the audio path and PTT outputs to an AND squelch (carrier + tone) connect a tone decoder to the PLI. 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, however, have the most protection against RFI.

  • If you wish both the audio path 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.

    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.

    DC Setup:

    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.

    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-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. For example, early research showed the Motrac receiver in some cases needed the following:

  • First stage: The series cap is 390 pf (391), the series resistor is 68K, the shunt resistor is 15K.
  • Second stage, the series cap is 240 pf (241), 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 6.1
  • Configuration Matrix chart (same as 6.0).
  • 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. 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
    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 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).

    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 .

    COR board Parts list-probably accurate, however not verified. Use your common 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
    5 Transistor, NPN, such as 2N3904 Q1-5 625-2N3904 00.50
    1 Transistor, PNP, such as 2N3906 Q6 625-2N3906 00.50
    1 Resistor, 470K, 1/4w, 5% *  If "R1" is that value  291-xx 00.07
    1 Resistor, 2.2 K, 1/4w, 5%  291-2.2K 00.42
    7 Resistor, 100 K, 1/4w, 5%  291-100K 00.42
    2 Resistor, 68K, 1/4w, 5% *  291-68K 00.14
    4 Resistor, 4.7 K, 1/4w, 5% 291-4.7K 00.07
    18 Resistor, 10K, 1/4w, 5% *  291-10K 00.91
    12 Resistor, 1K, 1/4w, 5% 291-1K 00.63
    2 Resistor, 15K, 1/4w, 5% * 291-15K 00.63
    6 Trim-pot, multi-turn, 10K, inline leads VR 1,2,3,5,7,9 594-64W103 06.00
    2 Trim-pot, multi-turn, 2Meg, inline leads VR 4,6 Hosfelt #38-183 06.00
    1 Trim-pot, multi-turn, 5 Meg, inline leads VR 8 Hosfelt #38-184 01.35
    1 LED;Diffused,Green T1 3/4 XC556G Jameco 34761 00.15
    1 LED;Diffused,Red T1 3/4 XC556RB Jameco 94511 00.15
    1 LED;Diffused,Yellow T1 3/4 XC556Y Jameco 34825 00.15
    1 LED;Diffused, Orange T1 3/4 XC556A Jameco 107393 00.15
    7 Diode, switching 1N4148/914 Jameco 36311 00.03
    2 IC socket 14 DIP tin/solder 571-26403573 00.08
    2 Capacitor, Elect, radial, 100uf/25v . 140-XRL25V100 00.21
    2 Capacitor, Elect, radial, 10uf/25v . 140-XRL25V10.0 00.15
    5 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
    12 Capacitor, Mono, radial, .1uf/50v Hosfet 95-192 00.05
    6 Capacitor, Mono, radial, .01uf/50v Hosfet 95-19x 00.05
    1 Capacitor, Mono, radial, 100pf/100v D-K 478-3166-ND 00.05
    1 COR / AF board, AK2O  FAR Circuits**  Ver 6.1  08.00 
    11  PVC colored wire, "6 long, 22-24 gu.  Various colors  See notes below 
    11  bare wire around 22-24 gu  For board jumpers  . .
    Parts cort, less shipping, etc.&nbs p; 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 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.



    During the R and D, some attention was necessary for clearance around L208 and a few other areas of the receiver section. This prototype proved to be satisfactory. The image doesn't do justification since the board was not held straight for the picture. The final product provided plenty of clearance.

    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:

    For future 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.

    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

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