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Boost Output Power & Range:RadioShack’s (discontinued) FM Stereo Transmitter (12-2051a) |
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Written by hevnsnt
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Sunday, 26 September 2004 |
{mos_sb_discuss:18}
Boost Output Power & Range:RadioShack's (discontinued) FM Stereo Transmitter (12-2051a)
By The Right Reverend Dr. Woolis Beltway Ph.D, DD
WARNING: READ THE SMALL PRINT: This modification could and probably
will render your Radio Shack Stereo FM Transmitter (12-2051a) illegal
to operate according to relevant FCC rules (for U.S. residents) and
likely according to the laws wherever else in the world you may live.
Due to unforeseen consequences of this mod (or due to my or your
mistakes, or a combination of both), this modification may render your
transmitter irreparably busted, weird-looking, diabolically evil and/or
void of warranty. This information is only for theoretical
consideration, and actual implementation or following of the
information here is done entirely at your own risk as well as against
my recommendation***. That said, be careful and enjoy, and if you start
a pirate station heard by ten or twenty others around you in a small
radius after hacking your transmitter or something really cool like
that, I'd be interested to know (minus the location details, of
course)...
*** I recommend that you refrain from performing this modification.
Introduction & Description
The RadioShack FM Transmitter (12-2051a) allows portable CD players,
MP3 players, radio scanners, etc. to play over a nearby FM radio, such
as in a rental car with only an AM/FM tuner or for any other
application in which making a wired connection between devices is
either impossible or impractical. Figure 1 is a picture of the front of
the one I have, which is several years old and has been heavily
modified (more about that later). It can transmit stereo or mono audio
on one of four FM frequencies, the point being that at least one of the
four should be quiet (far enough away from powerful broadcast stations
on higher or lower frequencies) enough to be transmitted on and
received without interference. There is no provision for external
power, but it requires 3 volts DC via two AAA batteries, which usually
last for at least a few days of 8-hour per day transmitting.
Figure 1
Unfortunately, the transmitter has no lights (the red dot beside the
power switch is a red-painted extension of the switch), so it can be
difficult to determine the state of the batteries. There is quite a
long input cable, at least a meter or so if I remember correctly,
ending with a common headphone-style 1/8" stereo phono plug, and a
short length of wire (6" or so) hanging from the bottom opposite the
input cable, which is supposed to be the antenna. Out of the box, with
fresh batteries, the transmitter covered about 10 meters, actually
pretty good compared to the range on many of the newer models I've
heard about (But haven't had a chance to try myself yet. I'm waiting
for someone to find a hack!! J).
Increasing Range Without The Hack
Before performing the power hack, I increased the range of the device
somewhat by disconnecting the short (obviously non-resonant) 'antenna'
from its soldered pad (fig.2), installed a double-headed female F jack
in a Penguin Mints box (fig.3), soldered a wire to the antenna output
pad on the PCB and poked the wire inside the center hole of the jack,
enabling me to connect ordinary RG-6 or RG-59 coaxial cable to the
transmitter. I didn't run an antenna ground wire to the Penguin box, as
I'm not really sure it'd be worth it, and I've been procrastinating on
trying it to see what effect (if any) a ground connection has on the
range. With a 20' length of RG-6 leading to a RadioShack discone
scanner antenna about 10 feet up in the air (at least somewhat resonant
near 88-108 Megahertz (FM broadcast band), the transmitter covered
about 20 yards/meters. Still very, very weak, but around twice the
stock range.
The Discovery Of The Hack
I was searching for information about this transmitter on the net, and
came across a PDF copy of a trade magazine of some type with an article
discussing (if I remember correctly) the different methods of frequency
generation used by cheap FM transmitters, and the 12-2051A was
mentioned along with a schematic! Since I'm no electronics wiz, I
wasn't able to deduce the hack from the schematic, but the writer, at
the end of his discussion of the 12-2051A, made note that the new 'A'
version had added a 1 kilo-ohm resistor to ground near the output
section, for the purpose of limiting power output or complying with
spectral purity requirements. This, of course, set off alarms in the
overclocking/modding/hacking part of my brain and I immediately opened
the transmitter up again and decided it was time to remove the
sheet-metal pseudo-shielding around the transmitter's PCB (Figure 2)
and get a look at the components underneath.
Figure 2
The Application Of The Hack
First, ensure you have the 12-2051a. I'm not 100% positive, but I'm
fairly sure the older 12-2051 (with no 'a') is different enough not to
be 'hackable' by this method. If you have one, however, and this hack
DOES work, please let someone (me, for instance, via my email address,
which is described near the end of this guide instead of just given in
the usual straightforward manner due to my dislike of SPAM.) Anyway,
after removing the one small screw in the center of the battery
compartment and popping the plastic case open, the two metal shells
shielding the PCB from curious fingers will be seen attached at the
sides of the PCB with T-shaped tabs, two of which are soldered
together. The tabs are also aligned to notches in the PCB, and I highly
recommend desoldering one side at a time and bending the non-soldered
tabs clear of the notches in the board instead of cutting or wrenching
the tabs off since the plastic casing is molded to the shield, not the
PCB, and you'll probably want to keep these shields undamaged to fit
back in the case.
Once the shielding is removed, the solder side of the PCB is accessible
and the hack can be applied. Turn the PCB over to the solder side, and
note the one lonely resistor back here, and note it well. It should be
brown, black, and red with a gold stripe at the end, which means it's a
1kilo-ohm resistor and which also means it's the one that's responsible
for the extremely limited range of the transmitter. I *suppose* it
could be a different color (maybe a change in production?), but pretty
certain no matter what colors are on it, it's the guilty party. It's
very convenient that it was placed on the solder side, sort of as an
afterthought, so we don't have to deal with a lot of other parts in the
way when we go to remove it and/or jumper it and restore the range and
power this little transmitter was engineered with. Since this was
written LONG after I'd hacked my transmitter, my 1k resistor is long
gone, but you can see where it was and the very small piece of wire I
soldered in its place in Figure 3. (Yes, this IS a lot of text and
pictures just to say, "Open the case, take off the shielding, turn over
the PCB and jumper the lone resistor". I guess I'm too dramatic...)
Figure 3
Note in Figure 3 above, the position of the BA1404F IC chip in the
upper left for proper alignment. Also, for reference, I cut off the
bottom of the picture of the PCB to un-clutter the magnified section.
Finally, note the magnified section showing the diagonal piece of wire
carefully (but definitely not professionally) soldered to the pads. The
other solder pads above and below should NOT be bridged (or even
touched), and hopefully you're using a relatively modern, precise, 10
to 30 watt soldering iron with some thin rosin-core solder to apply the
tiny piece of copper wire carefully across the pads where the resistor
was, and NOT one of those ten-pound 'ion cannon' soldering guns and
acid-core (or no-core) solder. If in doubt about the old golf-club
length soldering iron with no plug that you inherited from Great Uncle
Zeke, consult a Radio Shack employee (yes, I know, but they should be
able to point to the soldering stuff on the wall...)
Further Modifications
I didn't stop there, since I wanted to make the transmitter able to be
powered by a filtered and regulated wall-wart power supply (3v DC) to
avoid dead batteries at inopportune times, so I installed a pin &
sleeve jack in the end of the Penguin Mints tin into which I squeezed
the transmitter (it fits very snugly). (Figure 4). Since there was the
possibility of noise being introduced, I scripped a ferrite choke off
the cord of a long-dead Microsoft Intellimouse with Intellieye, ran the
pin/sleeve jack's leads through the center of it and soldered them to
the conveniently edge-mounted power input solder pads next to the
stereo audio input pads and stock antenna pad (Figure 5).
Figure 4
Important Note
Usually, early in one's hacking/modding career, if one is interested in
any type of radio, one is tempted (and gives in) to monkey with the
little potentiometers and capacitors (metal cans) with inviting
adjustment screw heads on the top. That's usually when one learns that
this is a BAD THING, as these little components are carefully aligned
with test equipment at the factory, and once they're out of alignment,
your device is pretty much useless. I'm lucky I learned as a child with
an old half-working 23-channel CB radio (which ended up not-even-half
working after a few hopeful turns of my 'golden screwdriver'), but I
have heard of more enterprising individuals who caused several hundred
dollars worth of recalibration work to be needed on their
ultra-high-end amateur radio stuff. In any case, please take my word
for it, there's NOTHING user-adjustable on the other side of the PCB,
and yes, I know the little Philips-heads are practically calling your
name and promising you magical radio powers if you'd only spin one of
those little can adjustments a few turns left or right. Don't listen to
them, they're all very lazy and they want you to ruin your transmitter
so they can sit idle on the PCB and chat instead of working to put your
signal in the air...
Important Availability Information and a 'Please Somebody, Hook Me Up' request.
This Radio Shack FM Transmitter (12-2051a) is DISCONTINUED, and has
been for at least a year or two as of July 2004. Radio Shack stores,
however, often have discontinued items available that haven't sold
(often at a significant discount), so I emailed Radio Shack and asked
them if they could locate me another one somewhere in the country, but
they claim they have no national product search ability. Before
emailing, I called a local store and asked if they could locate a
nearby store with one in stock (none were available anywhere even close
to my area), so you may want to try calling a store to search other
stores for you. If you do find a couple or more and would like to sell
one (or more), PLEASE let me know (via email -- drbeltway@hotmail.com
), as I only have the one in the pictures, which still works, but long
ago would have been destroyed by rough environmental factors (my car)
if I hadn't "hardened" it by fitting it into a Penguin Mints tin.
There are many new models of FM transmitter out now, however, such as
the iRock (12-2053 at Radio Shack), the various Belkin ones, etc., so
if you have knowledge of these models, whether it's that they
ABSOLUTELY CANNOT be range-increased via a similar hack as this one, or
that they CAN, and you know how, submit the information to
i-hacked.com, please. As a blatant plug, I'd like to remind you, my
fellow i-hacked.com enthusiasts, that this site is a really great
resource for those of us with the odd hankering to optimize, improve
and otherwise modify already functioning hardware, so kindly spend a
few minutes emailing your private, secret hacks to -
hevnsent@i-hacked.com - so we hackers ofd normal stuff
Good luck and good hacking,
-The Right Rev. Dr. Woolis Beltway Ph.D, DD
(see second to last paragraph for email address)
{mos_sb_discuss:18}
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Last Updated ( Sunday, 26 September 2004 )
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