INTRODUCTION
Recently here has been a great influx of the Philips FM900
VHF transceivers on the market, at a price too cheap to ignore.
This is really good for amateurs as the FM900 is a quite sophisticated
25 watt FM commercial unit that readily converts to operation
on the 2 metre band.
The set has two digit display which allows the user to select between
channel 1 and channel 99. Rather than having to fit a small bucket load
of crystals, a small pre-programmed EPROM memory chip can be inserted so
that up to ninety nine of your favourite frequencies can be associated
with the ninety nine channels available.
After a bit of a tune up you can be on the air for the price of a good
bottle of rum. (This is not to imply that you should buy a radio in lieu
of rum, it is feasible to purchase both, just don’t use too much
of either at the same time.)
Note that this guide does not pretend to be a comprehensive tune up
procedure, it focuses on the minimum changes that are required to convert
a unit that was already operational on the original commercial frequencies.
Unless the unit was defective before you started, you should finish up
with a transceiver that will equal or excel the performance of anything
else you have in the shack.
Confession time. Until recently acquiring a unit of my own, I knew nothing
about them. I had never even pushed the button on one. Why then, you may
ask, am I writing a manual about them? Well after purchasing a couple of
these units for myself and being faced with the need to get them going,
I collected as much data as possible from a variety of sources and successfully
put it all together. After listening around to many others who were keen
on getting the units going, I thought it best to compile this information
in a ‘minimalist’ form to help others to follow suit. So here we are. Sit
back, sip a (small) glass of rum and rotate a few slugs…
The transmitter (including the antenna
changeover switch) is fully solid state, capable of delivering about 25W
rms to a 50 ohm antenna.
The rigs recently released come in two versions, MK1 and MK2. The MK1 receiver
is Single Conversion with an IF of 21.4 MHz only and the MK2 receiver is
double conversion with ‘IF’s on 21.4 MHz and 455 KHz.
(Note that some of the early MK1 units had a 10.7MHz I.F., which not
unlike some contemporary politicians, suffered from some terrible image
problems)
While both rigs seem to perform approximately the same, you can easily
pick the MK1 (1984 release) from the MK2 (1987 release) by looking at the
transmitter PA module, as the MK1 version is missing a separate shielding
cover
A single PLL loop circuit controls both the transmit carrier frequency
and the first local oscillator frequency.
The audio amplifier can deliver up to 3.5watts of audio to the speakers.
While many of these sets were made with a control panel integrated with
the main chassis, most of the recently distributed units have a remote
control head that connects to the chassis via a one or two metre cable.
This helps to minimise installation problems in modern vehicles.
The transmitter output will automatically fold back in level if the heatsink
temperature rises to an unacceptable level.
The LED display will automatically dim to a low setting when ambient light
levels fall to a preset threshold.
Channel numbers 1 to 99 are selected by use of two simple ‘step forward/step
backward’ buttons.
It has an intelligent mute circuit which varies its response time proportionally
with input signal levels to help minimise squelch-chop on noisy signals.
There is a magnet built into the standard microphone bracket which will
enable a ‘scan mode’ (if programmed in) when the microphone is hung up.
There are two possible scan groups, each with a maximum of ten channels.
Scan mode is enabled by pressing the AUX button once or twice. Scan groups
are stored in the EPROM.
Normal receive standby current is about 600ma. This may be reduced to 250-300ma
if the Economiser mode is engaged in the EPROM. If enabled and the mic
is hung up on its proper magnet bracket, the yellow RX led will flicker
rapidly which lets you know that the rig has gone into a power saving (receiver
time slice) mode. If the mic is out of the cradle, the ECONO mode is halted.
Note that the Economiser mode, when engaged, will interfere with packet
radio reception.
Reports from a number of operators using this rig while mobile indicate
that it has good immunity to interference from paging services when compared
to many modern 2M transceivers.
Removing the screws in the lids on more than one unit at a time will guarantee
a case of RSI unless a cordless screwdriver is used.
This is the tricky bit. You will have to get an EPROM memory chip from
somewhere with the frequencies for the 2 metre band embedded within it.
Some of these have been for sale recently at the various hamfest markets
for $4-6. If you have EPROM burning facilities you may be able to copy
someone else’s chip for insertion into your own machine. Lastly, you may
be able to get a hold of the original Philips program which allows you
to create a small 8K computer file containing your own frequencies and
preferences, which in turn can be programmed into an EPROM chip. This is
a reasonably tricky program to use and could be a separate article on its
own, so I shan’t go into any detail on within this guide. .
The EPROM chip that comes with each unit is the 2764. It probably still
contains data for the original commercial frequencies. This chip can be
removed and placed under a strong UV light source to erase its contents.
After erasure it may be reprogrammed for the 2 metre band. Note that the
2764 device is a fairly old device that requires a 21V programming voltage.
My EPROM burner only supported the later 12V burning protocol, which meant
that I had to buy in some 27C64 versions of the chip. (this CMOS version
only requires a 12V programming voltage.)
The EPROM stores some general data about each set, such as the model number
and CTCSS tone squelch settings (if that option board is fitted). It also
contains data on each of the ninety nine channel positions. Each channel
number has a transmit frequency, a receive frequency, a TX power level
setting and scanning group data. Naturally repeater frequencies will have
different TX and RX frequencies programmed in.
When it came to choosing which frequencies to allocate to what channel,
I looked at some at the ‘standard’ lists that were in circulation and was
a little dismayed by the ‘dogs breakfast’ approach as seen on some of them.
As I do not own a dog I decided to contrive my own list.
In channels one to nine I placed my favourite simplex frequencies, a couple of
call frequencies and the SSTV liaison channel. Then came the
repeaters. Australian 2 metre repeaters are divided into two
groups, Group A and Group B. Group A has sixteen
consecutive allocations in increments of 25KHz, each with
a 600 KHzNegative offset. Group B has
fifteen allocations, each with a 600KHzPositive
offset. I programmed these into every Even numbered channel
from 10 to 70. On all the Odd numbered channels I added the
same repeater frequencies again with TX/RX frequencies transposed.
The idea is that if you are listening to a repeater and wish
to listen to a signal on the input frequency, you simply
step up one channel, have a listen and step back again.
After the repeater frequencies I inserted a bunch of consecutive
‘Packet’ frequencies, followed by a few oddball spots of lesser
significance. Finally these references were put onto a single
page ‘Excel’ spreadsheet which was laminated to be kept near
the transceiver.
The first step of the upgrade process is to fit the EPROM to the unit.
Place the transceiver on the bench, disconnect the 12V power and remove
all screws from the top and bottom of the unit. Store all the screws in
a jam jar for the duration of the alignment process. Note: Ensure that
there is no jam left in the jar, apart from the regrettable loss of good
screws, you may find some unwanted impurities in your toast the next morning.
Observe that one side of the transceiver has a smaller circuit board
with a metal plate that covers the PA. stage and the other side has a large
circuit board that takes up the entire side.
Undo the screws that hold down the PCB. These are the ones with little
arrows pointing at them. Once loosened, the PCB may be swung up vertically
as it hinges up at the rear end of the transceiver.
There are two metal covers on the underside of this board. (only one
if the set is an FM900 MK1) Undo the four screws that hold the cover which
was positioned furthermost away from the hinged end of the PCB.
Two ic’s in their sockets will be evident. The one to remove is the
28 pin device labelled ‘2764’. Place the flat bladed screwdriver between
the socket and the underside of the EPROM and gently ease it out. Take
care here as one false move will drive a few IC pins right up into the
tender spot between your fingernail and your fingertips. It can really
bring a tear to the eye!
Replace it with the new ‘2M’ EPROM, ensuring that the notch in the end
of the chip is facing away from the hinged end of the PCB. Double check
the EPROM socket to make sure that no legs are hanging out. This is a microprocessor
circuit, not a puffing billy ride.
Screw the cover back on and lower the PCB back into the chassis. Don’t
screw down the external aluminium cover yet as the mic gain and deviation
controls are here. Just flip it over and get ready for the next step.
This is probably the trickiest part of the tuning process. The aim of
this stage is to keep the VCO reference oscillator stable for the entire
operating range of the transceiver. If it is not set correctly, then the
rig will not transmit or receive on part or all of the band. Note that
the decimal point LED on the two digit channel display will glow if the
Voltage Controlled Oscillator ever does go out of lock. Once tripped, this
LED will tend to stay on until the set is powered down and powered back
up again. The original service manual has a rather complex hierarchy of
alignment steps depending on the exact portion of the VHF or UHF spectrum
slice that the set is to be programmed for. The procedure described here
has worked well on all the FM900’s that I have put on 2 metres to date.
Here you will need an inductor alignment tool. You can spend a lot of
money on tools and still not get the right one. Find an old plastic knitting
needle and with an knife or file shape the end so that it neatly fits into
the ferrite slug. Don’t make it pointy as it will stress the neck of the
slug and it may crack, then you will be in big trouble. Another good hint;
never ever use your cordless screwdriver as a slug adjusting tool,
I don’t care how good your clutch setting is.
Follow these steps…
Locate the receiver board. It is next to the transmitter PA. (This is on
the side opposite to where the EPROM was fitted) Undo the six screws with
the arrows pointing at them and swing the pcb to a vertical position
Locate the audio amp chip in the corner of the pcb near one of the hinge
pins. It is the ic with nine pins on one edge and a heatsink flag along
the other edge. Between this ic and the adjacent aluminium shielded box
is a PCB pin. This is Test Point 201. Stick the positive leg of
your voltmeter on here (with an alligator clip) and put the negative lead
to chassis ground.
Power up the set. Using the Up/Down channel buttons on the control head
select the Highest available frequency on the dial. (say 147.800
-148.000 MHz) Just above Test Point 201 you will see the aluminium
VCO can with two slugged inductors. The one CLOSEST to the test point is
the Offset VCO tune which affects the VCO while transmitting. The
one next to this is the Main VCO tune which affects the VCO while
receiving. While in the receive mode, carefully rotate the Main VCO
tune slug until the meter reads within half a volt of 12V DC
Now go to the Lowest frequency on the 2M band and check that the
VCO voltage is still above 5V. (Note that the FM900 MK2 set has holes in
the PCB so that these two slugs may be adjusted from either side, while
the earlier MK1 unit must be adjusted from the component side of the board.)
Plug the dummy load and wattmeter into the antenna socket on the rig. Return
to the channel on the control head to one that is at the High end
of the 2 metre band. ) Press the transmit button. Adjust the Offset
VCO tune slug so that once again 12VDC is achieved. Return to the Low
end channel on the 2M band and key the PTT. Check once more that the VCO
voltage is above 5V. (7-9V is typical)
Return to the high end channel and confirm that the VCO
is still stable at around 12V on both transmit and receive.
You should now have reliable operation at both ends of the
band without the Out Of Lock LED ever tripping. Remember
that the LED tends to latch on when triggered. It can be
reset by turning the Pwr button off and on again.
While we are here with the dummy load and wattmeter attached we may as
well do the transmitter next. Here we go…
Select a channel somewhere around the centre of the 2 metre band. (say
146.00 - 146.200 MHz) If you have a FM900 MK2 unit then remove the metal
cover to the PA module. (The MK1 has no cover)
Locate the two adjustable components on the board. There is a trimmer capacitor
near the antenna socket and a trimpot in the diagonally opposite corner.
The trimpot is the power output adjuster. Carefully rotate this trimpot
hard clockwise with a non conductive alignment screwdriver. (don’t use
your good slug adjuster, whittle down the other knitting needle.)
This is done so that the PA can be aligned without the automatic-power-level
adjusting circuit trying to compensate for your fiddling.
Hit the PTT button and adjust the trimmer capacitor for maximum watts into
the dummy load. The PA stage is pretty broad so that the peak is only a
little one. In fact you should get twin peaks (no relation to the TV drama)
as resonance is achieved either side of full mesh.
Now re-adjust the power level trimpot so that the wattmeter reads about
20-25W into the load. Note, if the set has its power setting too high,
the deviation symmetry can be upset and the transmitted audio quality may
be degraded as a result.
That is the easiest transmitter you will ever tune. This
is a good time now to fill the kettle and make yourself
a nice hot coffee fix. You see, the caffine in the bloodstream
helps shield the effects of electromagnetic radiation
on the body. (Or so I read once in a fortune cookie.)
The FM900 series transceiver has typical Philips audio quality, which is
to say it will sound a bit tinny and you usually have to talk loud. The
audio is consistent with being very readable under high background noise
conditions such as experienced by drivers of large fire engines and Volkswagon
Beetles. If it is to be used as a shack rig, you may wish to consider adding
a better quality dynamic mic or electret insert. If mounting it in a car,
then leave the mic as it is. Either way, my experience has been that the
mic levels are down a bit compared to popular amateur rigs and will need
to be adjusted. On the solder side of the synthesiser pcb (the same PCB
that holds the EPROM) there are three holes in a row with trimpots below
them near the hinged end of the pcb. They are labelled in small tinned
copper text as SENS, DEV and CAL.
The ideal way to set up the mic levels is to couple a peak deviation meter
to the set and measure dev. In KHz while trimming the mic gain and deviation,
but this may be a little hard to get a hold of. You can get it to sound
pretty good just by using another regular 2 metre rig as a receiver. Preferably
one with a Busy lamp and a signal strength meter. Try the following:..
Tune your other 2 metre rig to the operating frequency of the FM900 so
that when you transmit into the dummy load you can hear yourself coming
out of the second rig. (yes, that strange voice really is yours)
Rotate the SENS and the DEV trimpots ANTICLOCKWISE
about one half of a turn. You should notice that the received audio is
a little louder when you talk.
Now talk really really loudly into the microphone. You should notice on
the second rig that on voice peaks the busy light goes out, the voice gets
chopped off and the signal strength meter bounces around a lot. This means
that you are over-deviating and dropping out of the passband of your receiver.
Now that we have made a fine mess of our transmitted audio, slowly rotate
the DEV control CLOCKWISE until the Busy lamp no longer winks
out when you yell into the mic.
Note that the CAL control will have an effect on the transmitted audio,
but it should probably be left alone as it is used to adjust audio bandwidth
to null out the effects of CTCSS tones on speech audio.
This setting should be pretty close to what you need. Connect an antenna
later on and get some reports from another station, with an emphasis on
getting this station to compare your audio to that of a third station
for quality and legibility.
The final adjustment! This is exciting stuff, I put it somewhere between
removing mildew from a shower recess and the final vote tally at a federal
election. (both of which I hasten to add, are of immense interest to some
people.)
You are going to need a signal source for doing your receiver that
you can make progressively ‘noisy’ so that the slugs in the receiver front
end can be optimised. The ideal tool is an RF signal generator that can
produce a 1KHz audio tone with about 5KHz of deviation. Another alternative
is to use a DIP meter set up in the next room, though these tend to be
pretty touchy to tune to a spot frequency. Lastly you can connect another
2 Metre rig to the dummy load on the end of some coax and progressively
move it away from the FM900 as the receiver sensitivity increases.
Try this:
Flip the transceiver over so that receiver board (the same as the VCO board)
is facing up. Locate the six holes in a row on the solder side of the pcb.
These are L1 to L6 and will all have to be tuned in turn to optimise the
receiver performance.
If you have a signal generator, connect it now, if you don’t, then get
half a metre of solder and jam it into the BNC antenna socket of the FM900
as an antenna and use the aforementioned technique of moving another rig
progressively further away. Either way you will want to be hearing a noisy
signal on your receiver about midway through the 2 metre band.
Using your you-beaut knitting needle slug alignment tool, carefully adjust
L6 through to L1 (in that order) to maximise the signal from the noise
Whenever the signal becomes close to full quieting, attenuate your RF source a
little more. You will have to make two or three passes over
these slugs as there is a bit of interaction between adjacent
coils. At the end of this process the receiver should be resolving
signals down to around a third of a microvolt. If there is
any doubt about this, get another station to transmit a carrier
while rotating his beam antenna away from you so as to produce
a really noisy signal for you to tune up on.
The diagram below shows the rear of the FM900 with its two accessory
DIN sockets. A couple of items of note… The Speaker Audio Out is
high level audio that is adjustable via the main volume control, while
the Low Level Audio Out is fixed at around 2V peak to peak and is
not
affected by the volume control. This makes it perfect as a packet or SSTV
audio source. The Busy Status signal is normally at 0V when the mute is
opened, either manually or by a received signal, but switches to +5V when
the speaker is muted.
PIN
FUNCTION
1
Cradle Input (simulates mic hung up)
2
Chassis
Ground
3
N/C
4
Speaker Audio Out
5
N/C
The Cradle Input signal goes straight into the FM900 microprocessor
via a 10K resistor with a 4K7 pullup resistor to the +5V rail. If this
input is grounded then it will simulate the effect of the microphone
being hung up in the ‘magnetic’ microphone hook position. As with most
normal transceivers, if the PTT Input is grounded, the unit will
go into ‘transmit’ mode as if the Push To Talk button had been pressed.
While some experimentation will be needed to work out connections for
specific applications, there should be enough information here to allow
the appropriate patch leads to be made.
The FM900 is a fairly sophisticated transceiver with lots of neat little
secrets lurking here and there. This guide will surely not be the last
you see of how the set may be used and maintained, but a few service hints
have been given to me which may be the difference between repairing and
discarding a faulty set.
The symptom is poor stability of the VCO which allows the unit to drop
out of lock or drift. The VCO relies on a Voltage Doubler stage on the
synthesiser board as a voltage reference for its operation. ( this is on
the same board as where the EPROM lives) Around the VCO stage is IC306,
a TL072 device, near this ic are two ten microfarad electrolytic
capacitors designated as C342 and C349. Apparently these
two capacitors are notorious for drying out over time and becoming unstable,
so if you are experiencing VCO problems, seek out and replace these two
components before wasting too much time on conventional diagnosis. This
modification appears to be valid for both MK1 and MK2 model transceivers.
Occasionally the operator will press the PTT button and instead of squirting
a bucket of RF into the antenna socket, it puts a little squawk into the
speaker and sits there lamely while you poor out your life story into a
dead microphone. On the receiver board there is a voltage regulator ic
designated as IC201, which is an OM815 device. For reasons
best known to itself, this chip can go into self oscillation from time
to time which will in turn scramble the +10V supply that feeds the transmitter
PA. Should the symptoms occur , locate this ic and add a 0.1 microfarad
greencap capacitor between pins 6 and 16. (pin 16 being
gnd) Like the previous tip , this change is valid on both the MK1 and MK2
transceivers.
That is it! Now, get on the air and tell the world all about the wonderful
bargain that you have astutely acquired and remind all who missed out purchasing
one of how bad they must be feeling right now. Then perhaps, pour yourself
a small glass of rum.
Please
make any suggestions or comments on any of this by email:-