49 MHz to 6 Meters

49 MHz to 6 Meters

Post by Mark A. Garre » Sat, 18 May 1996 04:00:00

Conversion of single channel FM radios to the 6 meter band

There have been many requests for information in conversion of
some of the Radio Shack and Maxon 49Mhz FM radios to 6 meters.
I hope to let others know that it can be done to certain radios
quite easily, while to convert others may not be worth the
effort.  I have converted one Radio Shack TRC-505 and two Radio
Shack TRC-509's to 52.525 Mhz by re-rocking and tune up.
From what I have learned I will pass on to you to help you in
determining if you have a radio that is convertible and a general
overview in how to do it.  
In determining the radio.  It is best that you consider only the
single channel FM radios that are available from Maxon and Radio
Shack and a few others.  These radios will either be the
walkie/talkie type or the headset type.  These radios will have 3
crystals inside.  A transmit crystal in the 16.61-16.63 range a
receive crystal in the 39.13-39.19 range and a 10.245 Mhz
crystal.
Radios that are from Radio Shack and have the name "Archer" on
them may be a AM radio and are designed for the kiddies to bang
around.  These radios only have one crystal in them (usually
49.860 Mhz and a TRF receiver in them.  These radios are not
convertible for 6 meter FM.  If you have a radio that you are not
sure if it's AM or FM, get a scanner and listen to it's
transmission.  If it has a good quality to the audio then it is
FM.  If the audio is low and scratchy sounding then it is
probably AM.  The kiddy radios also do not have usually squelch
and so there is continous white noise out of the speaker.  If you
still are not sure, open it up and count the crystals.
The 5 channel units that have come on the market recently are not
good conversion items either.  Their chips are designed around
cordless phones and although they can be moved into 6 meters by
changing the clock crystal, they do not fall on the 6 meter FM
channels.  They are not worth your time or effort to convert
unless you can reprogram a chip to cover the specific frequencies
that you need.  
Now that you have eliminated the duds from the bunch, I will say
some things that you might want to consider before starting a
conversion these radios.  They were designed with part 15 in
mind.  They only put out around 40 mw or.04 watt and the antenna
system is made to be somewhat range restricted.  If you convert
one to 6 meters you will still only have 1/4 mile range tops if
you do nothing to that dinky antenna.  On a decent 1/4 wave
antenna at a decent height of 30 feet you can expect around 3 to
4 miles range.  These radios would make a excellent low power
base radio if you place a connector on them or as a e***r for a
home brew amp but then you are tied to a coax at the least.  I
have not found a good antenna for these radios that will fit in
the original design.  
Their receivers are ok but are prone to intermod.  I have picked
up a local 50kw FM broadcast station on mine.  This is due to a
lack of a front end on these units.  Another thing you might
consider is to instal a small pot for setting squelch.  This mod
I want to do since the fixed resistor squelch is set somewhat
high on the units that I have converted.
Now if all I have said so far has not deterred you from
conversion then go for it.  You can get schematics for Radio
Shack units if you give them the model number.  For the TRC-509
it runs around $15 and you will have to order it.  It is worth
it.  The schematic from the TRC-505 was included on the owners
manual and it is so tiny that I cannot read the resistor values
even with a magnifying glass (the printing resolution is poor at
this level and they should have taken two pages for the drawing).
For Maxon and other units you might need to contact the
manufacturer directly for schematics.
Recommended equipment:  General coverage receiver, frequency
counter, scope with at least a 50Mhz bandwidth and signal
generator that covers the 6 meter band, FM modulation preferred.
If you do not have the signal generator a low level signal can be
generated from a nearby radio until the first stage has been
tuned, then if you have a programmable scanner you can use the
IF to provide a low,low signal level for fine tuning.
Confirm the radio's specs prior to conversion.  Check receiver
sensitivity, transmitter output (on the scope)or miliwatt meter
(most cb wattmeters wont detect the low level output these units
have even on the 10 watt scale).
Crystal information:  
Transmitter  FO = Fundamental X 3  (F0= frequency of operation)

Receiver     FO - 10.7 MHZ = Fundamental

Example:  transmitter and receiver on 49.830 Mhz

Transmitter  49.830/3 = 16.61 Mhz
Receiver     49.830 - 10.7 = 39.13 MHZ
Confirm operation of the crystal fundamentals using these
formulas to agree with your radio.  Use a general coverage radio
and a scanner to find the crystal carriers(Remember the transmit
crystal will not be in operation unless the transmitter is
keyed).
I used a 30 pf load and 25 degree temp celsius in ordering these
crystals.  I used Jan Crystals as my supplier.  No mater who you
use let the crystal company know what you are installing these
crystals in.  They may have information vital to making these
crystals work.  Use HC-18 style crystals with wire leads. If you
are modifying the ht radio you will have to lay the transmit
crystal on it's side to allow it to work with the case on (the
top of the crystal hits the speaker, the receive crystal is ok to
instal upright). Since the new crystals do not have plastic on
them, use some sort of insulation material to keep from shorting
out the upright resistors adjacent to the crystal such as
electrical tape around the crystal body.  In the TRC-505 a
smaller crystal can was used for the transmitter but in the TRC-
509 they used a HC18 style and laid it on it's side already.  The
headset type should not have this problem but check it out prior
to ordering.
Start with the transmitter.  Use the general coverage receiver to
determine crystal fundamental operation.  Frequency accuracy can
be set later.  Use your scope with a 10:1 probe on the antenna
output lead, key the radio and tune the first and second coils
for maximum output on the scope. Then place probe on tip of
antenna and tune antenna for maximum.  If the antenna is
telescopic, extend all the way.  Re tune the entire transmitter
stages again, for maximum output.  Now set the crystal frequency
with the coil next to the crystal and deviation pot(it is labeled
L7 on the TRC-505 and with the transmitter fully tuned at this
point it should be readable on a frequency counter.  You might
also want to drip a little wax down in this coil once tuned.  Wax
is better than coil paint because you can always melt the wax
later and re-adjust, with coil paint you break the coil.
The receiver is a little tricker.  First prepare a small
insulated wire, stripping both ends.  You will have to attach
this wire to the demodulated output of the receiver IC.  In the
TRC-505 it uses a MC3357.  Solder this wire to the demodulated
output on pin 10 and clip your scope lead to this wire, again a 10:1
probe and your scope on AC, internal trigger with your ground
clip on one of the metal coil cans.  You will be looking at a
display of white noise on the scope and the goal of this is that
you will be looking to minimize this white noise as you tune the
receiver up to the signal generator. As you tune in the generator
and the noise is reduced to quieting, you need to reduce the
amount of rf to the receiver, keeping just enough noise on the
scope to determine your progress. If you have a audio tracer it
can be used on this point as well, listening for a reduction in
white noise. Instal the receive crystal and check for operation
on a scanner listening for a carrier (the crystal will be weak so
a frequency counter probably is not useable so that is why you
need the scanner).  Tune the fundamental in on the scanner
(example for radios receiving on 52.525Mhz-10.7Mhz= 41.825
fundamental.  Adjust the receive netting crystal for maximum
quieting on the scanner.  On the TRC-505 this coil is L4 and is
located on lower left of the board (it is the coil closest to the
receive crystal you installed).
Apply a signal with no modulation to the antenna input.  Apply
just enough to cause the noise level to drop.  Adjust the coil
between the rf and 1st mixer stage (L2 on the TRC-505) and work
your way back from this stage to the 1st mixer crystal (L4 on
TRC-505) and repeat.  There are really only two stages that you
need to tune but you will have to repeat tuneup several times,
each time reducing the level of the signal generator to leave at
least a little bit of noise on the scope.  You should be able to
get the receiver down to around .5uv or less.
Once you get the receiver tuned to this point turn on a tone from
your signal generator and set deviation to about 5 khz.  Adjust
the quad coil for maximum audio.  In the TRC-505 this is L3 and
is located near the demodulator chip and the 455 khz ceramic
filter.  If you do not have a frequency generator you can adjust
this coil by fully deviating a radio on frequency by a long
helloooo or other favorite fully deviation tricks, watching your
scope for maximum signal level.   Now that you have done this
your receiver is tuned.  Remove the wire from the demodulator Ic
and apply a drop of candle wax to the receiver netting coil.  Re
assemble the unit making sure that no wires are pinched and the
transmit crystal clears the speaker and does not crush components
in the ht version. You have now completed your conversion.
This article gives a rough guide on how to do a conversion to one
of these units and hopefully your conversion will be a smooth
one.  I have tried to keep this as broad as possible to cover the
different versions of radios out there and the amount of
equipment and skill each person has in doing the conversion.
If anyone has experimented with improvements to the antenna
system with either the hand held or hands free unit or has built
a amplifier to bring these units up in power, let me know.  
If anyone has questions regarding conversion, drop me a line and
I will try and help.  Much of my information that I have here is
limited.  Good luck on your conversion.

Mark Garrett, KA9SZX


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