Plectron and Motorola Alert Monitor Receivers

[Bob Parnass, AJ9S]

Last changed: October 30, 1995 |

Add a Plectron or Motorola Alert
Crystal Controlled 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 transistor Plectron and
Motorola monitor receivers. They have now been replaced by
portable pagers, and 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 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. Although
UHF versions 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 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 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. *

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 1996, Bob Parnass, AJ9S par...@bell-labs.com

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