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MX294MOD.TXT
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1994-05-07
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MX294 modification details.
Part1 ... Outline.
The details contained in this text assume that you are mo-
difying the 16 channel version of the MX294, with selcall
tone facilities installed via an internal, add-on board.
This model was recently sold by GWM Radio, and all the mods
were designed to work with this particular model.
GWM supply some information and a copy of the cct diagram,
but take care. There are mistakes in it. Especially the de-
tails for working out the EPROM data.
There has also been an article in Ham Radio Today magazine,
but again there are mistakes in the text.
There are single channel, six channel and 40/80 channel mo-
dels out there, but I cannot guarantee if these mods will
work with those models.
The original set was designed to work on 16 channels max.
with selcall tones on Rx & Tx.
By removing a shorting link from the mother-board, a further
16 channels were activated. So the basic set-up, with the
tone board removed, would give you 32 channels.
This was my original design, 32 channels, 8 repeater & 24
simplex. With a "listen on input" facility for the repeater
channels. A 2716 EPROM is used to replace the existing PROM,
because it is easier to program, and erase, if there were
any errors in the data table.
This basic set-up then evolved to become a 48 channel de-
sign, 8 repeater & 40 simplex channels, with 25khz spacing.
With the repeater listen on input facility.
Freq ranges ... Band 1 .. 144.500 - 144.900 Mhz
Band 2 .. 145.200 - 145.575 Mhz (S8-S23)
Band 3 .. 145.600 - 145.975 Mhz (R0-R7)
The original set had 2 illuminated press buttons fitted to
the front panel, one yellow and one green.
The yellow lamp is used to indicate a busy channel (ie
squelch open) and the green lamp indicates a repeater chan-
nel has been selected.
The yellow switch selects "listen on input" when listening
to an active repeater, and the green switch operates the re-
peater tone burst.
Part 2 ... Getting started.
2.1 Open up the set by removing the screws holding the
covers in place.
2.2 Remove the 2 screws holding the front panel in place,
and gently pull the front panel away from the set.
At this point you should have the front panel, with the
selcall board attached, separated from the main body of the
radio.
2.3 Remove the selcall board from the front panel. Remove
the support fixing screws, and the screws that pass through
the push-buttons; also remove the screw-in locating lugs
holding the small panel, with a connector mounted to it,
that is attached to the board by ribbon cable.
2.4 Separate the small panel from the ribbon cable. Clean
up the solder holes and put to one side.
2.5 CAREFULLY unsolder and remove the push-buttons and
their leds from the board. You could have 2, 3 or 4 fitted.
Remove them all, and put to one side. (check that the
switches are working ok, I have had 3 faulty ones)
2.6 With the switches removed there should be an area of
board approx 95 x 25 mm clear of components. Using a fine
blade junior hacksaw cut out this portion of board. Go as
near to the components along side voltage regulator as you
can. You need this piece of board so that you can use it to
mount the push switches on. It also provides connection
points for wiring to, when you come to connect the new fa-
cility board.
2.7 Remove the screening cover from the radio.
2.8 Looking into the set you will see the original PROM
unit fitted into a small pcb in the bottom left corner.
Unplug the pcb from the DIL socket, remove the RED and
ORANGE wires. To remove the ORANGE wire, cut it from the
board and pull it back through the plastic cable ties.
That is the end of the recovery section. The next thing is
to make up the new facility board and program the data into
your 2716 eprom.
The diagrams accompanying this text, show you the latest
layout I have used. There are no hard and fast rules for the
layout, I have made 4 boards up, and all have been differ-
ent.
NOTE. Most of the connections are made using the pcb tracks,
all other connections are made using single strand wire.
Part3 ... Connecting new facility board.
3.1 If you have managed to follow the diagrams and have
made up a new board, now is the time to connect it up.
3.2 Plug the header plug into the DIL socket used by the
original PROM. Make sure you have the polarity right ie
check that PIN 1 is in the correct position.
3.3 Make the connections to the push-buttons & leds using
any convenient soldering points you find. Check for any ex-
isting printed wiring that is not required, and remove it.
There are connections between the switches that MUST be
taken out.
3.4 Make connections to the small pcb with the socket at-
tached to it, and refit to front panel.
3.5 Fit the single pole double throw, centre off, switch to
the front panel. I fitted mine into one of the dummy switch
positions under the volume control.
Part4 ... Programming the EPROM.
You need access to an Eprom Programmer, for inputting the
data for the frequencies you require.
Each data block contained in the eprom consists of 4 bytes
for the receive freq and 4 bytes for transmit.
The data specifies the number of 6.25 khz channels that must
be added to a frequency offset. The data is made up of 4
bytes, byte D is multiples of 4096 channels, C is 256 chan-
nels, B is 16 channels and A is unit channels (0-15 or 0-F
hex).
NOTE. The freq. offset is also shown as a figure represent-
ing a number of 6.25 khz channels. The RX and TX data use
different values.
The RX freq. offset is 2128 channels, the TX is 3840.
3840-2128=1712 1712x6.25khz=10.7mhz RX I.F. frequency.
Example.
To calculate the RX/TX channel data for 145.500mhz.
RX data = 145.500x1000/6.25=23280
-2128 offset =21152
21152/4096 = 5 .... Byte D
672/256 = 2 .... C
160/16 =10 (A).... B
0 = 0 .... A
so 145.500 Mhz RX data = DCBA = 52A0
TX data = 145.500x1000/6.25=23280
-3840 offset =19440
19440/4096 = 4 .... Byte D
3056/256 =11 (B).... C
240/16 =15 (F).... B
0 = 0 .... A
so 145.500 Mhz TX data = DCBA = 4BF0
However, when it comes to program the data into the EPROM,
the order of the data changes to (RX)BCAD(TX)BCAD
so for 145.500 the data will be ... 0A,02,00,05,0F,0B,00,04
This has to be calculated for all the channels you require.
Don't forget to put the 600khz shift in for the repeater
channels. I have included a table of the frequencies I have
used, so you don't have to work them all out again.
You will also notice that each data word is made up of two
Bytes. On ALL the simplex channels the FIRST Byte is set to
"0" and ALL SPARE memory locations are programmed as "00".
On the repeater RX Channels, this first Byte is set to "1".
This allows on output from the eprom to drive the REPEATER
LED on the front panel. The TX channels are programmed with
the FIRST Byte set at "3". This allows the LED to operate,
and also provides an output to drive a transistor switch.
This switch would then be used to supply the Tone-burst cct,
ensuring that the tone could only work on a REPEATER TX
freq. Prevents accidental (or deliberate) use of tone-burst
on Simplex frequencies.
You will have to provide the necessary circuitry yourself,
if you want to use it that way.
Part 5 .... Testing and tuning up.
5.1 Having built up the new pcb, and programmed & plugged
in your eprom, it's time to try it out. To isolate the pcb
and prevent contacts to any other surfaces, insulate the
board by using a plastic bag, or any other non-conducting
material. I use rubber bands to hold mine in place.
5.2 Connect the loudspeaker and provide power to the radio.
I removed the original snap connector and used a terminal
block instead.
5.3 Turn up the volume, and adjust the squelch pot until it
is fully open.
5.4 Check to see if the PLL LOCKED LED is lit. Marked LED1
on the screening plate, or LED2 on diagram.
5.5 Locate TEST-POINT 3 on the right-hand side of the large
multipin IC11. Connect voltmeter positive to test-point and
negative to any earth. You want to get 6.5v on the CENTRE
frequency of the tuning range you have programmed. I have
used 145.200, channel 1 on band 2, to set up my sets.
5.6 Adjust core in RX VCO until 6.5v is measured. If the
LED was not lit, it should now come on.
5.7 The same adjustment must now be done for the TX.
Connect a dummy load to the antenna lead and connect the
mike to it's lead. Operate the PPT switch, and again measure
the voltage. This time adjust TX VCO until a reading of 6.5v
is obtained.
Again if the LED indicator goes out when switching to TX,
setting it to 6.5v, the LED will re-light.
To operate the PTT use a rubber band to hold it closed.
5.8 Now when you switch channels from 144.500 - 145.975 you
should see the voltage gradually rise on the test-point, and
the LED should stay lit all the time. If it should go out on
any single channel - suspect data errors in the eprom. If it
goes out on a band of channels - check wiring.
Check BOTH RX and TX though all channels.
On the repeater channels, operate the REVERSE REPEATER
(Listen on input) switch, and check the LED is lit on RX and
NOT lit on TX. TX on Repeater OUTPUT freq not enabled. You
will have to provide that yourself if you want it?????
5.9 Now the RX and TX VCO is set-up, it is time to tune up
the RX and TX stages.
*** BE CAREFUL ***
The cores in the RX coils are very fragile. Use a NON-ME-
TALIC adjusting tool. The cores may have locking compound on
them, making them difficult to turn, and if you break any,
you will have a difficult job trying to remove or alter
them.
*** YOU HAVE BEEN WARNED ***
5.10 Use a signal generator, or local signal, to provide a
signal source, and tune coils L1 - L9 for maximum strength.
If you are using a signal generator, reduce it's output as
each stage peaks up.
5.11 Go back to L1 and start again to make sure every stage
is at it's best point.
5.12 You should now be able to set the preset squelch pot
for quieting, and if you connect a suitable antenna, tune
about and see if it opens and closes, as signals go on and
off.
I have found that the cutoff tail is a bit fussy, and a
extra turn of the pot may be required to ensure the squelch
closes properly when signals go off.
That completes the RX, now for the TX.
5.13 Connect power meter/dummy load to antenna and micro-
phone to it's connector. Operate the PTT and check what re-
ading you get. It should be approx 10 watts.
5.14 Adjust POWER CONTROL pot, RV4, for maximum reading.
5.15 Adjust C126, C129 and C138 variable capacitors for
maximum output. You should get between 35 - 40 watts.
5.16 To ensure a long life for your PA transistors, use VR4
to adjust the power down to 20-25 watts. At around £40 each
it makes sense to protect the Power Amplifier transistors.
That is the complete modification package.
Part 6 ... Future developments?
What do you want the set to do? Now you have a basic set-up
you can add whatever you want.
The tone-burst on Repeater TX only was an after-thougth, but
I programmed the eprom to make it available if required.
All the spare memory locations were programmed with "00", if
you wanted to, you could put the data if for 145.500 for
example. So if you operate the Reverse Repeater switch on
any channel, other than a Repeater, you could listen to S20.
I also made a boot-mount MX294. There was no room to fit the
original set in the car, so I made a small control box using
a 40 channel CB radio type switch.
The eprom in this set was programmed to cover 144.500 -
144.9875 in 40 12.5khz steps, and then cover 145.000 -
145.975 in 40 25khz steps.
The control box plugs into the MX294, either direct, or via
a 25 way computer type RS232 cord. So depending on the
length of the lead, the set can be fitted remotely.
I have one set working on Packet, using the 15 way D cord to
pick up the necessary connections to the TNC. This means I
can have the TNC, and a microphone connected, and use
either.
I have one set for use as a base station, and I modified one
for my father to use.
Other local amateurs have used this design to modify sets of
their own, and have used them on RAYNET exercises without
any problems.