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HOW TO 'DISCOVER' THOSE NEAT RADIO MODIFICATIONS
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HOW TO 'DISCOVER' THOSE NEAT RADIO MODIFICATIONS
by Bob Parnass, AJ9S
How do the people that discover modifications to radios
go about finding them? Good question!
The first rule of thumb is to obtain service manuals, as
they contain more than just troubleshooting information.
For example, the alignment procedure outlined in the
Regency K500 and M400 service manuals describes how to
circumvent the frequency limit checking firmware, which
allows out of band frequency programming. The Uniden
200/205XLT service manual describes a keyboard sequence
that clears most of the 200 memory channels, and loads
the others with bizarre test frequencies.
Service manuals often describe the circuit changes in
models intended for export to other countries. This can
reveal features disabled for some customers but enabled
for others.
A good library of IC and semiconductor data books is very
helpful, although radio service manuals can also include
IC internal diagrams. Old TTL databooks are no longer
enough. The transition to surface mount components in
radios like the Uniden/Bearcat 760XLT is motivation for
acquiring data books for leadless components.
Although some modifications involve discovery of "hidden"
features, many others involve designing new circuitry or
applying old circuitry from another radio. I can't claim
credit for many "add/delete a diode" modifications, but
here are the factors that accounted for a few other modi-
fications.
Recognize Common Radio Circuits
Modification: Improve the squelch on the PRO-24, PRO-
2004, 800XLT, etc. Motivation: Unsatisfied with stock
performance.
Almost all modern scanners use MC3357, MC3359, or
Japanese pin equivalent chips, which contain the IF,
squelch, limiter, and discriminator circuitry. Older
Bearcat and Regency scanners, like the BC300 and M400,
often hid the identity of their IC with "house numbers"
painted over them. I compared these ICs pin for pin with
the MC3357 and other known radio ICs to unmask their true
identity. Having the Motorola IC data sheet and scanner
service manuals made learning the chip internals easy, so
I found the way to decrease hysteresis involved changing
one resistor.
Be Curious
Modification: Trick the Icom R-71A to tune below 100 kHz.
Motivation: Curiosity.
Tried manipulating several front panel controls at the
same time to see if I could confuse the microprocessor
into doing something neat. I did.
Study the Schematic,
Look for Unused Pins
Modification: Double the memory in the R-7000 (also pub-
lished by another radio hobbyist).
Motivation: Curiosity.
I studied the schematic of the R-7000, and looked up the
memory IC in a data book. Icom grounded an address lead,
so only 1/2 the chip capacity was used. Not having
enough time to try the idea on my own radio, I suggested
the idea Jack Albert, who writes the RTTY column in "Mon-
itoring Times", who used his R-7000 as a guinea pig.
Borrow Circuits from Other
Radio Models
Modification: S-meter circuit for Bearcat scanners
(unpublished).
Motivation: wanted to use scanners for
transmitter hunting.
I looked for other radios that used the same IF/squelch
chip and already featured S-meters (like the Kenwood TR-
2600A, IC-28A, IC228H, etc.) I grafted their S-meter cir-
cuitry to my scanners.
Vary Parameters and
Measure the Impact
Modification: Speed up the R-7000 scan rate.
Motivation: dissatisfied with stock performance.
I studied the radio schematic, found the components that
determined scan rate, and substituted various values of
resistance, measuring the affect of each change.
Apply Simple Theory
Modification: COR light for the PRO-2004. Motivation:
make scanner easier to use in a roomful of other active
radios.
Having studied the schematics of many scanners, I was
familiar with FM receiver and scanner circuitry. I used
service manual and found the proper point in the circuit
where a logic level signal was produced depending on
whether a signal was absent or present. Again, the PRO-
2004 used a popular IF/squelch chip. I used the simplest
transistor switching principles to design a COR light
circuit.
Fashion a Test Harness
Marvin Moss used an interesting approach to explore his
portable scanner. He wired the diodes in the diode
matrix of his Radio Shack PRO-34 to separate DIP switches
so he could experiment with switching in and out dif-
ferent combinations of diodes.
All That Glitters is Not Gold
I always find other peoples' modifications very interes-
ting, although not all are meritorious. For instance,
avoid changing the crystal or RC time constant circuitry
used as a clock for the microprocessor controller in your
receiver. The controller performs many functions, so
this alteration can produce undesirable side effects
which outweigh any small increase in scan rate.
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This file was downloaded from the
HAM on Rye BBS -- (814) 333-4111 -- 2400-8-N-1
**************** Specializing in *******************
Amateur radio, TVRO, scanning, shortwave and clandestine radio
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