parnass@ihuxf.UUCP (07/08/83)
This article briefly summarizes my experiences with the new
Radio Shack PRO-24 pocket scanner, priced at $99.95. Prob-
lems encountered include:
o+ irregularity in plastic belt clip
o+ misalignment of UHF stages as received from dealer
o+ design deficiency in squelch circuit
o+ incompatibility of recommended AC adapter accessory
Cures for these ills, as well as a modification to increase
the scanning rate are covered.
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Powered by 4 AA penlight cells, this is a 4 channel, crystal
controlled receiver which covers the following bands:
o+ 30 - 50 mHz. (VHF Low)
o+ 144 - 174 mHz. (VHF High)
o+ 450 - 512 mHz. (UHF)
Included with the scanner are tri-band helical and telescop-
ing antennas, a plastic belt clip, and an owner's manual
(with schematic).
The outward appearance of this Japanese made scanner is
identical to previous Radio Shack models (PRO-20A and PRO-
21A) which covered only two bands. The circuitry is similar
to other Radio Shack and Montgomery Wards models in that the
same multi function MC3357P (2nd local osc., mixer, if amp,
detector, and squelch) and T3375 selector chips are used.
The audio amplification in the PRO-24 is performed by an
LM386 chip, whereas pocket scanners in the past have imple-
mented this function through the use of discrete transistor
circuitry.
The PRO-24 uses a plastic belt clip, whereas previous
scanners of the same ilk have used metal. The 2 holes in
both types of clips are the same size and line up, so if the
plastic clip breaks, it may be possible to order the metal
one for an older model from your local Radio Shack store.
The plastic belt clip and two screws are furnished in a
plastic bag. If the owner wants to install the belt clip,
she may find that one of the holes is a bit too small. Do
not force the screw in, the brittle plastic may crack!
Instead, enlarge the hole slightly.
I installed 1 UHF and 3 VHF High crystals. I had no way to
accurately measure the sensitivity or selectivity due the
unavailability of calibrated signal generating equipment.
The sensitivity on VHF High seemed fairly respectable, con-
sidering the tri-band helical antenna. Sensitivity on VHF
Low could not be determined due to a lack of a suitable cry-
stal in my collection.
UHF sensitivity was dismal. In particular, the local police
repeater on 470.3125 mHz, could not be received in many
parts of town. I realigned the RF amplifier and mixer
stages in the UHF circuitry, which improved UHF sensitivity
noticeably. However, the UHF sensitivity still seems a
disappointment. In the PRO-24, the front end stages for the
3 bands are all connected together at the antenna. As an
experiment, I disconnected the VHF Low front end from the
antenna, thinking that the direct coupling of this stage was
loading the others down. I again realigned the UHF stages,
but could detect no improvement in sensitivity.
The squelch control had an inordinately high amount of hys-
teresis, resembling "play" in a car steering wheel. This
forced me to keep the squelch up at a point which prevented
reception of signals of weak to moderate strength. As men-
tioned previously, the squelch function is performed by a
common Motorola chip, which has been used successfully in
several previous scanners. An inspection of the schematic
diagram, provided in the owner's manual, revealed that for
some reason, a low value of resistance (33,000 ohms) was
used for R35, which was placed between the squelch output
and the noise amplifier input (IC1, pins 14 and 12). This
"feedback" resistor was much smaller than the 220,000 ohm
resistors used in previous designs. After changing the
resistor in my PRO-24 to 220,000 ohms, the amount of hys-
teresis decreased markedly, and the squelch worked great.
Both the owner's manual and Radio Shack catalog recommend
the model 20-189 AC power adapter/charger accessory ($6.95)
for use with the PRO-24. This adapter may be used in two
ways:
o+ If plugged into the AC adapter jack on the scanner, the
scanner's batteries are disconnected, and the radio is
powered from the AC line.
o+ If plugged into the charger jack on the scanner, the
radio circuitry is completely disconnected, and the
batteries are charged by the adapter through a current
limiting 27 ohm resistor located inside the scanner.
These two jacks are of the coaxial type, and each contains a
switch mechanism which is activated upon insertion of the
proper size plug.
I bought the recommended accessory, and although it was
labeled 20-189A, I was assured that it was indeed the proper
adapter, and the 'A' meant that a new vendor was supplying
these to Radio Shack. As a precautionary measure, I made a
quick check with my Fluke DMM. This revealed a problem:
when the adapter was plugged into the scanner, the internal
batteries were still connected!
This can cause two unwanted effects:
o+ If I used the adapter to power the scanner, the bat-
teries would be charging at an indeterminate rate.
o+ If I used the adapter plugged into the charging socket,
the batteries would be placed directly across the 6
volt DC power supply (bypassing the current limiting
resistor!) and would indeed enjoy a short life.
The culprit was the connector on the end of the adapter.
The outer diameter was just a bit to small to actuate the
switches in the scanner's jacks.
My problem was solved by buying the Radio Shack 6VDC 150 ma.
AC adapter (273-1454, $5.95). In addition to having the
proper connector, this adapter works well, is cheaper than
the recommended one, and can furnish almost twice the
current!
The specifications in the owner's manual state that the
scanning speed is 6 channels/second, a bit too slow for my
liking. I added another 220,000 ohm resistor in parallel
with R64. This changed the time constant of the scanning
oscillator, increasing the scanning rate.
When fully squelched, a good portion of the battery drain
can be attributed to the light emitting diode channel indi-
cators. When the "manual/scan" switch is in the "man" posi-
tion, my unit draws approximately 27 ma. from the batteries.
Following up on a suggestion from Ed Doubek, N9RF, I always
leave the PRO-24 in "scan" mode. When I want to listen to
just one particular channel, I just lock out the others.
This causes a single LED to blink on and off at about a 25%
duty cycle, thus conserving battery life.
I bought this scanner at a reduced price, and in summary,
after a several hours work, it performed at a level close to
my expectations.
Robert S. Parnass, AJ9S (ARRL Technical Advisor)
Bell Laboratories
Naperville, Illinois 60566
ihnp4!ihuxf!parnass