The board should be made up into its box with all the front panel components.
It is recommended that the first test be performed with a power supply
Adjustment of the circuit board.
12 - 15V with the current limited to 1A. Turn the volume button to minimum.
Power up the set. After a short transient when the current may rise
to about 0.5A, the consumed current should fall again to about 60mA. Turn
up the volume button - hissing should be heard from the loadspeaker.
Measure with an oscilloscope the trace on pins 1 and pin 7 of U1 -
OPA2604 (with respect to 0V). There should be an audio noise waveform (look
with a time base of about 1ms/div) with an amplitude of about 40mV pk-pk.
The adjustment of the various inductors is very straightforward and
should take only about 5 minutes.
First connect an oscilloscope to the point A marked in the diagram
below and adjust the Toko coil screw until the signal is maximised (should
be a sinusoidal
signal around 500kHz with about 1-2 volts of signal level pk-pk).
Connect the probe to point B and adjust the adjacent Toko coil in the
Connect two wires each about 5m long to the two aerial terminals and
connect each end to an earth connection.
Adjust the surface mount potentiometer at C until the red LED does
not light with incoming noise transients (this can be re-adjusted in the
future in case it is still too sensitive). The red LED will be the indication
of an incoming 87kHz signal.
With the wires still connected, connect a short (about 10cm aerial)
to a signal generator about 1m away from the radio aerial wires. Adjust
the generator to
give 88kHz sinewave with a level of about 1V. Adjust the radio volume
so that the background noise is quite audible and then adjust the generator
signal amplitude until the 1kHz tone heard on the radio is just discernible
above the background noise.
Now adjust the two Toko coils marked D in the diagram below so that
the tone heard is maximised. Repeat the adjustment until no improvement
Place a 2W 600ohm resistor in the aerial terminals. Turn the transformer
adaptation switch to the middle position. Activate the microphone switch
blow into the micro for about 10-20seconds. The orange LED should flash
when blowing harder. Measure the voltage across the resistor with an oscilloscope
probe (the output is floating so polarity is not important - either side
can be grounded). The waveform should be a modulated sinewave with an amplitude
of about 150V pk-pk (when blowing into the microphone). Turn the selector switch
to position 1 and measure again. There should be about 70V pk-pk. Replace
the resistor with a 2k ohm resistor and test on position 3. The output
should increase to about 300V pk-pk. When the microphone is activated (press
the PTT button), the oscilloscope trace should pulse 2 or 3 times (beeps
at the start of the transmission) and again 3 or 4 times when the PTT is
released and then drop in amplitude back to zero.
View of the surface mount side of the circuit board (the Toko coil adjustments
are on the other side!)