The unit is based on a PHILIPS FM880 link transceiver which was
supplied to a Telecom Australia specification for telephone
remote areas of Australia. The FM880 in turn is part of the family of
including the PHILIPS FM828/FM814 series of fixed and mobile
main receiver and transmitter exciter boards are identical in most
are surrounded with specialized boards and circuitry for the particular
The VK6WIA modifications are therefore simply another variant of the
PHILIPS design, although a fundamental change has been made in that the
of modulation has been changed to direct FM of the exciter reference
oscillator. The inbuilt phase modulator on the original FM880 exciter
Other audio, power supply and control facilities have been added to
WIA Broadcast Officer with a transceiver designed to provide the
2. FM880 RECEIVER and EXCITER/PA
Extracts from the FM880 handbook, detailing receiver and transmitter
description, alignment instructions, circuit diagrams and board layouts
attached at Appendix I.
All Telecom telephony related boards are discarded, although some
have been recovered and re-used; e.g. antenna changeover relay.
3. VK6WIA BROADCAST FACILITIES
A number of new modules have been constructed and added to the FM880
(a) AGC/Limiting Amplifier.
(b) 200-3800 Hz Filter and Microphone Amplifier
(c) CTCSS Sub-tone generator.
(d) Vu and Peak level meter drive.
(e) Audio power amplifier.
(f) -12V supply.
(g) 1 KHz tone generator.
(h) Auxiliary audio amplifier
(i) Deviation audio amplifier
(j) Twin FM crystal oscillator
(k) Tone keying relay
(l) Single control relay
(m) Dual control relay
The following panel controls have been provided:-
12 Volt power switch
High Level (tape) input gain
Microphone/High Level switch
1 KHz Audio Osci11ator switch
Audio Level Meter switch
CTCSS Sub-tone switch and LED indicator
Transmitter Monitor switch
Audio Out (Receiver) or Audio
Receiver Audio gain
On the front panel are located :-
Phone jack (Stereo wired as mono)
Audio out jack (Mono >5OKohm load)
VU/Peak level meter
Transmit RF level meter
On the rear of the chassis are:-
jack (8 Ohm)
High level input (Two RCA sockets in parallel)
Audio osci11ator key jack
13.8 Volt input (Barrier strip terminals or 2
Antenna connector (S0239)
DIN connector for External Auxiliary Unit
4. OPERATING INSTRUCTIONS:-
Volts to rear connector, connect antenna and attach microphone. Set all
controls to max anticlockwise and toqqle switches to OFF (up). The DTMF
/ HL /
Tone Switch should be on "HL".
4.2. Turn on POWER. The two meters should be illuminated.
Receiver AF gain by a small amount. Rotate Receiver SQL control until
heard. Set AF gain for a convenient level and adjust SQL for muting
4.4. Signals on CH2 Repeater should now be received if present.
4.5. Advance the MIC gain and speak into the microphone without
PTT or TRANSMIT key. The microphone audio level should indicate on the
meter. Set the MIC gain so that at the speaking level and distance to
the VU meter on frequent peaks deflects to reference level (0 VU)
4.6. Apply test audio on the HL input. Select HL on the MIC/HL switch.
audio should indicate on the VU meter as the HL gain is advanced. Set
level for reference deflection on the VU meter.
4.7. If it is desired to listen to the audio applied so far, without
transmitting, insert PHONES into the front panel jack, or use the
extension speaker. Select AF IN. The audio from MIC or HL can now be
depending on which is selected. (However the MIC audio is muted if the
or external SPEAKER is in use).
4.8. Restore the Audio select to AF OUT.
4.9. Select PK DEV on the VU/PK DEV meter switch. The relative peak
level of any received signal will be displayed on the level meter. +/-5KHz
approximately to 100% on the level meter. (This indicated level is
de-emphasis so is not a true deviation measurement on speech, but is
for a single tone of 1000Hz). Reselect VU.
4.10. To transmit, either press the locking TRANSMIT key or use the
PTT. The RF meter will deflect to about 70% of full scale, providing a
indication of transmitter RF output. (No Receive RF level indication is
provided in the prototype units). Actual transmitter power output is
watts, depending to some extent on the regulation of the power supply.
power drain on transmit is about 5 Amps. The transmit frequency is
with CH 2 selected. With AUX selected, transmit and receive frequencies
depend on chosen crystals.
4.11. With TRANSMIT activated, select HL on the MIC/HL switch and TONE
VU meter should deflect to reference level, providing a relatively low
distortion 1000 Hz continuous tone. The tone can be keyed by inserting
a key or
keying contact in the rear panel jack. The key operates a reed relay
carries a current of about 10mA from 13.8 volts. Simple contact closure
open collector NPN transistor switch is required to key.
4.12. Still with TRANSMIT and TONE selected, select MON on. The
be activated on the TRANSMIT frequency and the transmitted tone will be
in the phones or speaker. That is, “OFF-AIR” monitoring is
the VU/PK DEV switch to PK DEV will now show also the transmitted
level which should be about 70-80%. Select TONE off, and with audio
from the HL input (e.g. tape), this audio will be heard in the speaker
phones and its transmitted deviation level displayed. On audio peaks
modulation level should reach 100%. and occasionally beyond. Select MIC
speak into the microphone. The microphone audio can be heard in the
is muted from the loudspeaker outputs, to prevent on-air feedback.
should also reach 100% or so. Advancing the MIC or HL gain, perhaps
unintentionally at this stage will increase compression, but will not
Peak deviation significantly: the peak level being held by the
4.13. Release TRANSMIT. Signals on the repeater output will be heard as
and traffic can be exchanged with deviation levels being displayed on
meter for receive and transmit as the contact proceeds.
4.14. If a WIA Broadcast is being initiated, select SUB to insert the
controlled CTCSS tone. This is required to inhibit repeater time-out
and set up
repeater links. The sub tone level is pre-set by an internal trim-pot
should not require re-adjustment. Its level is not affected by other
4.15. The AUDIO jack on the front panel provides a sample of the audio
the audio PA but is of a fixed level independent of the AF gain. The
be used for example for:-
(a) Access to the 1000 Hz tone.
(b) For relay of the received signal
(c) For recording of one or both sides of contacts.
This output requires a load of >50,000 Ohms. Output level is
If the output is being used for relay purposes, MIC announcements can
inserted as required by switching from AF OUT to AF IN with MIC
The PTT should NOT of course be activated for this application. The MIC
will appear at the AUDIO output and on the HF relay. Restore AF IN at
conclusion of the ID.
4.16. The RX MODE switch is usually set to NORMAL. When set to STBY the
receiver local oscillator is disabled. When set to SX, transmit AND
occur on the repeater INPUT, providing an emergency simplex system in
of repeater failure, or an alternative "unrepeated" source for relay
4.17. The CHANNEL switch is usually set to CH.2. Selecting AUX (if
will provide either operation on a selected simplex with full
facilities, or an
alternative emergency operation WITHOUT the MON and transmitter
capacity. A pair of crystals for receive and transmit frequencies can
plugged in to vacant sockets to provide this backup facility when
tuning tone is provided by selecting HL and TONE, activating the 1 KHz
distortion audio oscillator in the FM880.
of a key
into the rear panel jack allows keying of the oscillator for
Morse practice, either on air or if AF IN is selected, for local
example a practice Morse recording could be prepared by recording from
AUDIO socket without activating PTT or TRANSMIT.
also the following
section for Automatic Morse messages (and DTMF control tones).
generation for control of remote facilities has been provided for in
To send DTMF
tones proceed as follows:
box plugged in to the FM880 rear panel , on the FM880 Front Panel
on the “MIC/HL” audio switch. Select “DTMF” on
the adjacent three position
toggle switch. While transmitting, key in the desired DTMF code on the
the input frequency of VK6RAP proceed as follows:
normally is on 146.100 MHz. To change the receiver frequency the
DTMF code must be keyed in manually while transmitting
on the frequency of 146.100 MHz.
Then all operation and control of VK6RAP must be on the new frequency.
change the repeater input back to 146.100 MHz transmit the correct code
alternate frequency. The repeater is now back on its normal frequency
Note that all DTMF tones on VK6RAP are not re-transmitted through the
Every DTMF tone is detected at the repeater and within 50 milliseconds
repeater audio is muted for one second. Hence a string of DTMF
not heard on the repeater's output except for the first 50
the first tone.
code generation has also been provided for in the External Auxiliary
5.2.1 The Morse groups are as follows:
(1) CT (Commence transmission)
(2) 73 DE VK6WIA AR (Concluding signoff)
(3) BT (Interval or break signal)
(4) E E E E E (15 Dots - use at commencement to cue relays)
(5) TEST DE VK6WIA (Include in test transmission)
(7) BT VK6WIA
BT (Relay ident pause)
(8) AR (End of transmission, short version)
5.2.2 Operation is as follows;
(a) On the FM880 Front panel set the MIC/HL switch to "HL". Set the
adjacent three way-toggle switch to "TONE".
(b) On the external keypad box, select the required message number on
(c) Briefly press the red button on the Auxiliary Unit and the message
sent once by keying the tone oscillator in Morse.
(d) Reset the three way toggle to the centre "HL" position when
6.1 Receiver and Exciter/PA:-
Before commissioning the facilities external to the FM880 boards, both
receiver and exciter/PA boards should be aligned to 146.7/146.1 MHz in
accordance with the original Philips handbook. Sufficient extracts from
handbook to do this have been provided at Appendix I.
Note that the RF Level meter trimpot mounted on the meter should be set
minimum sensitivity during the recommended alignment for the exciter/PA.
6.2 Additional Faci1ities:-
6.2.1 -12 Volt Supply.
Apply +13.8 volts. Adjust TP1 for maximum -ve volts at the input to the
volt regulator. A voltage >15 volts should be available with a load
6.2.2. Limiting Amplifier.
Set all trimpots to maximum. Apply a tone to the input of about 50mV
Adjust TP2 until V out is about 60mV r.m.s. Vout can be increased by
TP3 until Vout is approx. 4 times Vin. The limiter will now
voltages in excess of 50mV in. TP1 and TP4 now become
In and Limiter Out gain controls for final system alignment.
6.2.3. Peak Meter adjustment.
Apply a signal modulated with a 1 KHz tone to 5KHz deviation to the
(Alternatively use average speech modulated traffic). Switch meter to
Adjust TP2 on the metering board so that the panel meter reads 100%.
The meter amplifier balance trimpot TP3 may be adjusted at this time,
it appears to be very uncritical in its operation.
6.2.4. Input and VU Meter Alignment.
Set MIC gain to about 50%. Switch to MIC and VU. Speak into microphone
adjust VU metering TP1 so that voice peaks reach OVU on the VU meter
(N.B. VU meter gain is on the low side. It may be necessary to
MIC gain above 50% to achieve reference
deflection, depending on the microphone in use)
Switch to HL and TONE and set the 1KHz oscillator level trimpot so that
meter again deflects to 0VU.
6.2.5. Deviation alignment.
Switch to TRANSMIT (into dummy load). Switch to HL and TONE. Switch to
PK DEV. Adjust Limiter IN trimpot to maximum. Reduce Limiter OUT
Deviation shown on the meter is 100%. Reduce Limiter IN trimpot until
deviation is about 80%.. Switch to MIC and speak into microphone.
should approximate 100%. deviation.
When finally testing with speech traffic, it may be necessary to make
adjustments to Limiter IN and Limiter OUT, but avoid any indication of
deviation as metered or onset- of excessive distortion as monitored on
6.2.6. RF Level.
The RF level metering indication may be adjusted to a convenient
indication by adjusting the trimpot mounted on the meter itself.
6.2.7. Audio monitoring balance.
Switch to HL and TONE, plus MON. Activate TRANSMIT. Measure audio level
AUDIO jack (with CRO or use the loudspeaker and calibrated ear). Switch
AF IN and AF OUT and adjust trimpot TP1 at Auxiliary audio amp output
level is equal on either position.
6.2.8. CTCSS Sub-tone.
The level of CTCSS sub-tone should be set to produce an FM deviation
of +/-500Hz. This ideally should be
with the aid of a suitable deviation meter or communications monitor.
select SUB, when the warning LED should light. Set the CTCSS level
about 25%. travel. Activate TRANSMIT and release. The subtone return
should be heard on the repeater "tail". If not, increase subtone
level and try again until just sufficient level is attained to reliably
the subtone return.
7.1. Power Supply
The transceiver is intended to operate from a regulated 13.8 Volt
supply. The FM880 power control circuitry provides +10 volt regulated
ancillary circuitry, switched +10 volt regulated for receiver and
respectively, and a variable voltage for the PA driver stage
part of an RF power regulating system. The transceiver is also
against accidental power supply polarity reversal. Note that the
+10 volt lines are NOT short circuit protected.
It was decided to provide in addition a negative 12 volt rail so that
requiring a dual rail could be used with the added advantage of a
greater voltage swing being available for direct FM for example.
The negative supply uses a 555 rectangular wave generator driving a
quadrupler arrangement via switches BC337/BC327 with final regulation
by a 3 terminal 12 volt regulator. A degree of pre-regulation is
feedback to pin 5 of the 555, which varies the mark-space ratio of the
As with any switching power supply, noise spikes must be eliminated, as
switching frequency is in the audio range. The inductance L1 and
pass capacitors remove noise from the supply rail, while additional
by RFC and associated l00uF capacitor keep undesirable spikes out of
output. With these precautions noise has not been a problem. The supply
deliver in excess of 50 mA.
7.2. Microphone Preamp.
The microphone preamp is a standard design, capable of lifting the
600 ohm or 50000 ohm microphone inserts to a compatible level with the
mV expected from the HL input.
7.3. 1KHz Oscillator.
The oscillator is based on the Wien Bridge principle, with the
oscillation determined by the phase shifting components R1C1, R2C2. The
of feedback to maintain oscillation is controlled by the AGC system
the 1N414S, 7.5 Volt zener diode and the 2N3819 FET. The AGC network
the effective resistance to ground from the inverting input to the 741
hence maintains its gain as just sufficient to maintain oscillation.
is thus held to a 1ow 1evel. Oscillation is inhibited by applying a
bias to the 2N3819 gate. Keying is provided by a reed relay in the
Direct keying of the oscillator was not provided as the time constants
necessary in the circuit produced an unsatisfactory keying
Inhibition of the oscillator has been used to eliminate the possibility
level audio cross talk into the microphone or HL channels when the
is not required.
7.4. CTCSS Generator.
The requirements of this generator are stability and repeatability, so
has been based on a crystal controlled source, using a cheaply
MHz crystal and a clock IC type MM5369AA.
The MM5369AA produces a high amplitude square wave which is "brute
force" filtered by a two stage ladder filter to produce a somewhat
distorted but useable sine wave. The level of distortion does not
appear to be
sufficient to provide intrusive harmonic content in the audible range,
doubt an active filter would provide a cleaner wave form. Ample level
available to provide direct injection to the frequency modulated
7.5. Audio Filter.
An audio band pass filter has been provided to limit the bandwidth of
presented to the limiting amplifier and of course for transmission. The
is actually a low pass followed by a high pass section. Gain of the
1 with little distortion contribution at the level of operation. The 3
bandwidth is about 200-3800 Hz. The 200 Hz cutoff also helps to keep
undesirable audio out of the sub-tone band.
7.6. Limiting Amplifier.
The limiting amplifier has been included rather than clipping to keep
distortion contribution down. Clipping can produce greater "talk
power" but also can produce distortion products which are magnified in
relay system on which the wide coverage of the W1A broadcast depends.
The limiter uses a specialised control 1C type MC3340 for which the
manufacturer specifies low distortion in the limiting application. In
standard circuit used, a control voltage is developed in a 741 1C with
wave detection and filtering applied to a 2N3819 FET, which in turn
the gain of the MC3340. Some extravagant claims are made for this
while it does the job could be improved by the inclusion of a full-wave
detector, and a reduction in attack time. To avoid delay it has been
A number of adjustments are provided by trimpots. TP3 sets the gain of
feedback amplifier while TP2 sets the limiting threshold. TP1 and TP2
conventional input and output level controls for overall system
It was found necessary to operate the limiter from closely regulated
supply rails to avoid an unsatisfactory lock-up and slow release
switching to transmit. The effect was due to about 0.5 volt shift in
Volt rail originally used to supply the positive feed. The present
"unbalanced" power supply rails are an undesirable consequence, but
don't appear to have introduced any uncontrollable penalty.
to VK6WIA FM880 - 25th March, 1994
7.7.1 Facilities have been added to generate DTMF control tones.
Control is via
an External Auxiliary Unit which plugs into an 8 pin DIN connector on
panel of the FM880.
example, to allow
VK6RAP operation on an alternate frequency to provide some degree of
from accidental interference when running news broadcasts through Perth
repeater VK6RAP, proceed as follows:-
switch to the
nonstandard input frequency:
Commence transmission with
the bandswitch set to "CH. 2."
(b) Select "HL" on the "MIC/HL" audio switch. Select
"DTMF" on the adjacent three position toggle switch.
(c) Send the appropriate DTMF code on the external keypad to instruct
change its input frequency.
(d) Select "AUX" on the rotary channel select switch to change the
FM880 to transmit using the alternate frequency
(e) Centre the three position toggle switch to "HL"
(f) Select "SUB" to transmit the CTCSS link control tone.
(g) Cease transmission and note if the subtone return signal is heard,
signifying a successful changeover.
All other operations of the FM880 should operate as normal.
DO NOT FORGET TO RESTORE THE INPUT FREQUENCY TO 146.100 MHZ AT THE END
SESSION by reversing the procedure!
7.7.2 An additional facility has been included in the External Unit,
the automatic morse keying of the FM880 tone oscillator. Eight
are stored in a ROM, each group being selected by the 8 position rotary
switch. The keyed tone can be used in the preparation of tapes,
audio via the "AUDIO" jack, "PHONES" jack or external speaker
jack on the rear panel) or during "live" transmission.
also under Section 5 “External