In this column, Tim answers questions about
all aspects of electronics, including computer
hardware, software, circuits, electronic theory,
troubleshooting, and anything else of interest to
the hobbyist. Feel free to participate with your
questions, comments, or suggestions. Send all
questions and comments to: Q&A@nutsvolts.com.
■ WITH TIM BROWN Q & A
Post comments on this article at www.nutsvolts.
• Scanner Frequency Change
• Simple Remote Control
• Telephone Off-Hook Indicator
Scanner Frequency Change
QI have a RadioShack Pro-7A Police and Fire scanner (eight-channel, crystal-controlled) that operates on 148 MHz to 174 MHz. Would it be possible to change crystals so that it can operate
on the 144 MHz to 148 MHz two-meter ham band, or
would other modifications be necessary?
— Gary Ross
AThe Realistic Pro 7A is a vintage scanner which was marketed by RadioShack in the early to mid ‘70s (I found an ad in a 1973 magazine) which uses eight individual crystals to control its eight
channels which are scanned until the radio finds an active
transmission. I have included photos of the top and crystal
holder portion of the Pro-7A in Figures 1 and 2.
The best I can understand about these early scanners
is that they scan only one frequency per channel (there
are a lot of different users in the 148 to 174 MHz band)
unlike the modern scanners which scan entire frequency
bands. To convert the Pro-7A, you start by deciding which
frequencies you want to receive, such as a 145 MHz
Listening Frequency (LF) in my example. Then, you have to
calculate the Crystal Frequency (CF). This gets a little
complicated, so hang on.
Most RadioShack (Realistic) crystal scanners used the
third harmonic of the crystal to convert the incoming radio
signal to the Intermediate Frequency (IF) and a 10. 7 MHz
IF (if your manual says your unit has a 10.8 MHz IF,
change the number in the formula from 10. 7 to 10.8). Use
CF = (LF - IF)/3
For my example, the CF will be (145 MHz - 10. 7
MHz)/3 = 44.767 MHz. Good vendors for crystals are
Ken's Electronics, Digi-Key and Doug Baird (see Q&A
SIDELINES). These crystals sell for between $10 and $15,
so a full set of eight will cost between $80 and $120. A
new scanner with 200 channels covering VHF Low ( 26-54
MHz), Aircraft (108-136.99166 MHz), Police/Fire (137-
174 MHz), and UHF (380-512 MHz) will cost around
$120. Maybe we can get more information from our
8 January 2016
■ FIGURE 2.
■ FIGURE 1.