FAQ: Plectron and Motorola Alert Monitor Receivers

[Bob Parnass, AJ9S]

Last changed: March 31, 1997

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.]

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They have now been replaced by battery operated portable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.

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 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.

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,
frequency.

AC power cords for both types of receivers use unique
connectors and are scarce. You can buy them new but it's
much cheaper to drill a hole on the rear panel and attach 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.

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.

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

Woodson Electronics
Plectron Place
P.O. Box 960
Imboden, Arkansas 72434

telephone 1-(501)869-7985

Some replacement parts are currently available from Federal
Signal in University Park, IL, phone (800)524-3021. You
can also get Plectron repairs and parts from:

Northeast Communications
4 Oak Ridge Rd
Newfoundland NJ 07435
201-697-8804

Alert Monitor parts are available from Motorola. Phone
(800)972-3226.

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

--
==============================================================================
Copyright 1997, Bob Parnass, AJ9S par...@bell-labs.com

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