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FAQ: Plectron and Motorola Alert Monitor Receivers

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              Add a Plectron or Motorola Alert
                  Desktop Monitor Receiver
                 to Your Scanner Collection

                    by Bob Parnass, AJ9S

  [NOTE: This article may not be reproduced in whole or in
 part on CDROMS, in bulletin boards, networks, or
 publications which charge for service without permission of
 the author.  It is posted twice monthly on the USENET
 groups,, and  It is also available electronically from
 the ftp archive on the official USENET
 FAQ library
 group/  The author writes a monthly
 "Scanner Equipment" column for Monitoring Times magazine,
 published by Grove Enterprises,
 but views expressed in this article are his own.]

 In the 1960s and 1970s, thousands of firemen and ambulance
 squad members came to rely on their crystal controlled,
 transistor Plectron and Motorola desktop monitor receivers.
 They have now been replaced by battery operated portable
 pagers, and the desktop models are often available at
 hamfests in the $5 - $25 range.  Since many are in rough
 condition and need repair, a hamfest special is better
 suited for hobbyists who like to fix their own radios.

 Plectron and Motorola crystal controlled receivers are
 excellent for dedicated monitoring of local frequencies --
 a task for which you wouldn't want to tie up your 400
 channel programmable scanner.  They are desktop models
 which can be powered from 117VAC or 12VDC with the proper
 mobile cord.  The audio quality of a Plectron P1, Plectron
 700 series, and Motorola Alert Monitor is far better than
 any consumer grade scanner and the sensitivity, image
 rejection, and intermod immunity is outstanding when
 aligned properly.

                Models and Frequency Coverage

 The Plectron and Motorola Alert monitors are single band
 receivers: VHF-low, VHF-high, or UHF.  No single Plectron
 or Motorola Alert monitor can cover the entire 30-50 MHz
 band, so there were versions optimized for each portion of
 the VHF-low band. There were different versions to cover
 low and high "splits" in the VHF-high band, too.  For
 example, low band R8000s come in 30 - 35, 35 - 39, 39 - 49,
 and 49 - 54 MHz versions.  There are two VHF high band
 versions of the 700 and R8000: 148 - 158 and 158 - 175 MHz.

 VHF-low band Motorola Alert monitors come in two splits: 30
 - 42 and 42 - 54 MHz.

 Although UHF versions of the 700 series were made, they are
 somewhat rare and coverage is limited to 470 MHz and below.

 Plectron made several models with dozens of different
 options.  Both the original P1 and later 700 series are
 suitable for restoration, although I prefer the newer 700
 models.  The 8000 series were even newer and very
 desireable, but they are difficult to find at hamfests.
 The Plectron 500 "economy" series is less desirable.

 The P1 series have black cabinets with a blue-green and
 silver color scheme used on the front panel.  They were
 made in the Chief (tone decoder & carrier squelch), Patrol
 (carrier squelch only), and Sentry (tone only) models.  The
 Sentry models lack a carrier squelch and therefore are of
 little use to hobbyists except for scavenging parts.  Model
 number and frequency information appears on a gummed label
 on the rear panel:

            Band (MHz)   Chief   Patrol   Sentry
                 25-54   R19     R15      R17
               148-174   R20     R16      R18
               450-470   R23     R21      R22

                Table 1.  Plectron P1 models

 The R700 series have black cabinets with brown front panels
 and are slightly smaller than the P1 receivers.  R700s were
 made in the Chief (tone decoder & carrier squelch) and
 Patrol (carrier squelch only) models:

                 Band (MHz)   Chief   Patrol
                      25-54   R719    R715
                    148-174   R720    R716
                    450-470   R723    R721

               Table 2.  Plectron 700 models

 The R8000 series are much smaller than the P1 and R700 and
 are housed in metal cabinets of various colors.  The R8000s
 use the same MC3357 IF/detector/squelch IC used in crystal
 scanners, but their crystals are not interchangeable with
 ordinary scanners.  R8000s were made in the Chief (tone
 decoder & carrier squelch) and Patrol (carrier squelch
 only) models:

                 Band (MHz)   Chief   Patrol
                      30-54   R8190   R8150
                    148-174   R8200   R8160

               Table 3.  Plectron 8000 models

 The Plectron FM Receiver/Recorder is a collector's item.
 It is essentially a 700 series Chief receiver with a built
 in cassette tape recorder.  The recorder is carrier
 activated and can tape transmissions while the receiver is
 unattended.  A connector on the rear panel allows for all
 kinds of remote control possibilities.

 The tape recorder inside the Receiver/Recorder employs two
 rubber belts.  The Projector Recorder Belt Company makes
 replacement belts which work fine, though they may not be
 the original equipment:

  1.  PRB SCA8.6 (218 mm)
  2.  PRB SCQ 2.5 (64 mm)

 I use two Plectron P1s and four R700s in my living room,
 seven R700s down the basement, and several Plectrons and
 Alert Monitors stored in reserve.  The basement Plectrons
 are turned on automatically when someone walks downstairs.
 Power to them is controlled by a modified Radio Shack Safe
 House infrared motion detector, originally sold as an
 intrusion alarm.

 When a rescue squad member from Missouri advertised a
 number of Plectrons on USENET, I bought them -- all 20+ of
 them.  I cleaned, fixed, and restored each one.  After
 recrystalling them on local frequencies, I distributed them
 to members of my scanner club who were glad to get them.


 Both brands of receiver require special crystals.  I
 sometimes use Radio Shack's generic 3rd overtone scanner
 crystals in the Plectrons but they oscillate on frequencies
 far away from their marked frequencies.  That's because the
 Plectron oscillator is designed to be used with a crystal
 which oscillates on its fundamental, not overtone,

 The best source for crystals is:

              International Crystal Mfg Co.
              11 N. Lee Ave
              Oklahoma City, OK 73102
              tel. 405-236-3741, 800-725-1426

 You need not bother with calculating the crystal
 fundamental frequency.  Merely supply International with
 the exact receiver model number, printed on the rear label,
 and the frequency you want to monitor.  They will perform
 the necessary calculations.

                   Crystals for Plectrons

 The Plectron P1 series contain two crystals.  The second
 local oscillator crystal is 11.155 MHz regardless of band.
 On VHF high band (148 - 175 MHz), the P1 series employ this
 formula for the first local oscillator crystal:

             Crystal frequency (MHz)
              = (station freq (MHz) + 10.7) / 4


             Crystal frequency (MHz)
              = (station freq (MHz) - 10.7) / 4

 On VHF low band (25 - 54 MHz), the P1 series employ this
 formula for the first local oscillator crystal:

   Crystal frequency (MHz) = (station freq (MHz) + 10.7)


   Crystal frequency (MHz) = (station freq (MHz) - 10.7)

 There are two crystals (Y1, Y2) used in the Plectron R700
 series, one for each local oscillator.  The crystal
 formulas are:

    RF Range (MHz)   Y1 (MHz)      Y2 (MHz)
    25-29            (f+10.7)/3    11.155
    29-32.6          (f+10.7)/3    11.155
    32.5-35          (f-10.7)/2    10.245
    35-41            (f-10.7)/2    11.155
    41-47            (f-10.7)/2    11.155 (use 10.245 MHz
                                   for Y2 on 44.62 MHz)
    47-54            (f-10.7)/3    11.155
    148-158          (f-10.7)/9    11.155
    158-175          (f-10.7)/9    10.245
    450-470          (f-10.7)/18   11.155

 The economy Plectron 500 series use this scheme for the
 first local oscillator crystal:

               RF Range (MHz)   Crystal (MHz)
               25 - 35          (f+10.7)
               35 - 54          (f-10.7)
               148 - 174        (f-10.7)/4

 Most Plectron 500s use an 11.155 MHz second local
 oscillator crystal.  However, a 10.245 MHz crystal is used
 instead for receivers in these ranges: 33.4 - 33.78, 44.62,
 and 158.78 - 158.82 MHz.

                         Power Cords

 AC power cords for both types of receivers use unique
 connectors and are scarce.  I published the pinout for both
 Plectron and Motorola Alert Monitor receivers in June 1998
 Monitoring Times magazine.

 You can buy the proper replacement power cords but it's
 much cheaper to drill a hole on the rear panel and solder
 on a permanent a power cord.  Make sure you use a grommet
 in the hole to prevent the metal chassis from chafing the
 line cord.

 Many Plectrons and Alert Monitors were equipped with an
 internal NiCd battery pack, intended to power the receiver
 if the AC power fails.  It's unusual to find a used monitor
 receiver with the NiCd pack still capable of holding a
 charge.  More often, the batteries have died and the series
 charging resistor has overheated and burned.  Be sure to
 remove the dead batteries as they often leak.

             Squelch and Minimum Volume Setting

 If you find the squelch on your Plectron 700 series
 receiver has too much hysteresis, replace R96, a 180K
 resistor, with a 560K resistor.  On the P1, the resistor is
 designated R81.

 Both Plectron and Motorola Alert monitor receivers are
 designed purposely to have a minimum volume setting which
 is still audible.  The intent was to prevent firemen from
 turning the volume down completely, forgetting it was down,
 then missing an important call.

 This "feature" was usually implemented by having a fixed
 value resistor in series between one end of the volume
 control and ground.  In the R15X5, R14X5, R15X5, and R16X5,
 a 47 ohm resistor, designated R624, was inserted in series
 with the violet wire leading from the volume control.

 You could decrease the lowest volume setting by decreasing
 the value of this fixed resistor.  Alternatively, you could
 replace the resistor with a jumper wire to completely
 silence the receiver at minimum setting of the volume

                     Tone Alert Feature

 I have not used the Plectron tone alerting features so I
 cannot address that except to say that the special coils
 and capacitors for the dual tone decoders vary depending on
 the tone frequencies and are somewhat difficult to obtain.

                Repair and Replacement Parts

 Repair service and replacement parts for the Plectron P1
 and R700 models is offered by:

               Weber Electronics
               PO Box 212
               5138 Laurel Ln.
               Broad Run, VA 20137
               telephone (540) 347-7760

 Alert Monitor parts are available from Motorola.  Phone
 (800)422-4210.  The part number on the service manual for
 my VHF-high band monitor is 68P81048A55-G and my VHF-low
 band manual bears part number 68P81050A95-D.

 John Miller reports the current price is $5.70 for service
 manuals.  Motorola advised John that Alert Monitor parts
 are scarce, with last stocked being in 1995, Knobs are no
 longer available, though there are many that would fit.

            Plectron SM-series Scanning Receivers

 Plectron also sold an SM series of scanners, manufactured
 in Japan by the same folks who made Craig (division of
 Pioneer) scanners.  The SM series are consumer-grade models
 and are included in this article for the sake of
 completeness.  They are crystal controlled radios and
 feature priority scan and a separate trimmer capacitor for
 each channel to net the crystals on frequency.

                 Band       Frequency Limits
                 VHF-low    30 - 50
                 VHF-high   150 - 174
                 UHF        450 - 470 MHz

          Table 4.  Plectron SM Model Band Limits

              Model    UHF   VHF-high   VHF-low
              SM-301    X       X          X
              SM-302            X          X
              SM-303    X       X
              SM-304    X                  X

                Table 5.  Plectron SM Models

 The crystals used in the SM series are quite different from
 the crystals used in the conventional single channel
 Plectron monitors.  Here are the crystal formulas:

 VHF low band:
   crystal 3rd overtone freq = receive freq + 10.7 MHz

 VHF high band:
   crystal 3rd overtone freq = (receive freq - 10.7 MHz)/3

 UHF band:
   crystal 3rd overtone freq = (receive freq - 10.7 MHz) /

 Other crystal specifications for the SM series are:

                CR 25/U holder
                3rd overtone
                20 pF load capacitance
                parallel resonant
                max series resistance 40 ohm
                max drive 2 mW

                    Plectron Alternatives

 Too bad Plectron is out of business.  TCS Communications
 Corp. sells used Plectron receivers, in working condition,
 for $75-100.  If you require crystals or reeds, TCS may be
 able to find some types/frequencies.  Contact:

                  Terry Marengi
                  TCS Communications Corp.
                  tel. 1-800-TCS-XMIT

 If you wish to purchase a new receiver with "Plectron-like"
 signaling capabilities, consider the receivers made by
 Reach Electronics.  Reach sells tone & voice pagers, Alert
 monitors, and ENCODERS.  They also do contract

                    Reach Electronics
                    1311 West Pacific
                    Lexington, NE 68850
                    (308) 324-6661
                    (800) 445-0007
                    FAX: (308) 324-4985

 Another Plectron-like receiver, "The Informer," is made by
 Federal Signal and sold by Brinkley Electronics:

        Brinkley Electronics
        P.O. Box 502
        Wallburg, NC 27373
        FAX: (336)769-4029
        web page:

 I currently have no Plectrons or Motorola Alert Monitors
 for sale.  I'm hoarding them. :-)

Bob Parnass, AJ9S                              

User Contributions:

Gary Richards
Oct 11, 2021 @ 10:22 pm
I have a Motororla Alert Monitor. Has 110v & 12 volt plug in back. Cord has been lost. Like to know if anyone knows the layout of the 4 pins, witch 2 are 12 volt. I can make a cord. The unit is a motorola alert monitor model # m01cnb

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