Last changed: April 15, 2000                                 |

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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


 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 They have now been replaced by battery operated portable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, contact Reach Electronics:

                    Reach Electronics

                    1311 West Pacific

                    Lexington, NE 68850

                    (308) 324-6661

                    (800) 445-0007

                    FAX: (308) 324-4985

 Reach sells tone & voice pagers, Alert monitors, and

 ENCODERS.  They also do contract manufacturing.

 I currently have no Plectrons or Motorola Alert Monitors

 for sale.  I'm hoarding them. :-)


Copyright 2000, Bob Parnass, AJ9S