ARDFrx80-7 Setup
pa0nhc 201711112

REM: "-B" is INSULATED from mass.
While measuring, be careful not to make an accidental short-circuit from +Bat to mass (0V), or D2 will burn out.

1. First check for a power-short-circuit : 
- Connect a 2 A dc meter in series with a 9V battery.
- While observing the meter, connect them for one tenth of a second to the receiver.

The battery current should be lower than 25 mA. If much higher, search for the fault.

2. Using a hiZ multi meter and oscilloscope, check the following voltages against mass (not to "bat-").

I (B+) max. 20 mAdc With gain varied from min to max, and I(B=) varies :
FETs 1&4 are working.
IC1p8 +5.0V VR1 is working.
R24 -B = ca. -0.75V D2 is OK.
R25 ca. +0.63V D1 is  OK.
At R21 slider in pos. "Max" abt. 0.1 Vdc Adjustable by R25 (82k)
At R21 slider in pos "Min" abt. - 0.51 Vdc Adjustable by R24 (27k)
R105 ca. +1.4V or 5V (When sense PB pressed / released).
R110 / 111 ca. +1.4V Buffers are working.
R22 ca. +2 to +3V Detector is working.
R203 0.05V HF / +0.2Vdc BFO is working.
IC2p7 1/2 B+ 
= +3.0 to +4.4Vdc
& max. audio is 3Vpp.

Monitor the frequencies of both oscillators by using a SW-receiver and a cable with a pick-up loop near the DF PCB.

4. BFO (Beat Frequency Oscillator) T1 / 10.7 MHz "biases" FET5, making this infinite impedance detector far more sensitive and linear.
IMPORTANT: if T1 does not oscillate, REPAIR this fault first.

- Check the BFO for oscillation at 10.700 MHz +- 1 kHz..  Monitored with an oscilloscope, on R22 should be ca. 700mVpp / 1.25 Vdc.

5.    Local oscillator IC1 / L2.
With tune potentiometer set to "Min. freq.", adjust C110 for 14.190MHz oscillator frequency.

Then check: With the tune potentiometer set to "max. freq", the oscillator freq. should be between 14.29 MHz and 14.32 MHz.

REM: To widen the tuning range, the value of C7 must be made smaller. After that, readjust C110.

          After finishing :
The local oscillator in IC1 can be made less temperature sensitive, by covering all oscillator components (except trimmer capacitor C110) with purified cosmetic bee wax (imkershop.nl) .   Do NOT use candle wax !
Add 3 - 5 droplets wax at a time, and melt it using a solder iron t = 170 C, which is grounded to a 3mm ground screw. .
Also heath the pins and the IC body, so the wax flows between the IC and the PCB.
Trimmer capacitor C110 should be left free from wax.


6.     Antenna.
    Use loop on a cable connected to a signal generator, couple a signal of 3.56 MHz into the ferrite antenna
    transmit low power into a dummy load or antenna. 
- Tune the ARDFrx to it, adjust gain, and adjust C105 (NOT L102) for max. audio.
- Check for reasonable constant sensitivity over the full tuning range (at 3.50 , 3.54 and 3.58 MHz).

7.    IF.
Adjust L3 and L6 for max audio.

If the core of L3 and/or L6 comes fully to the top, reduce the value of C25 and/or C26 a little.
If the core of L3 and/or L6 ends fully down, enlarge the value of C25 and/or C26 a little.

REM: without signal input, and "Gain" set to max; the mixers background noise should clearly be heard. Indicating good sensitivity.

Sensitivity test :  Listening during evening hours, outside city environment in a noise less area, you should notice a little lower noise background when turning the ferrite antenna from horizontal into vertical position.

8.    Gain regulation.
With correct adjusted of minimum and maximum sensitivity, the sensitivity potentiometer R21 works smoothest. 

With R21 in position "Min", a maximal strong antenna signal should cerate not more than 200 mVrms audio tone measured at R19.
With R21 in position "Max", the self noise of the receiver should create receiver audio noise of abt. 200 mVrms measured at R19.

Maximal sensitivity can be set by the value of R25 (82k).
Minimal sensitivity can be set by the value of R24 (27k).

9.    Sense.
On one side of the antenna pattern the signal should become minimal when the sense-button is pressed. The maximum signal should point towards the heard transmitter. The deepness of the minimum is influenced by the length of the sense antenna, and the coupling strength between ferrite rod and L101. 

You can adjust the deepness of the signal  minimum by adjusting R105. Adjust this sense action when abt 500m away from a low power transmitter connected to a vertical polarized antenna.The opposite pattern side should become 3dB louder with a pressed sense button. 

 If wires connected to SA1 and SA2 are interchanged, the maximum and minimum in antenna diagram interchange in direction too.

Cheap dB meters

10.    Ear protection (FB6, FB7).
Prevent damage to ears of users of your equipment.

The maximal safe sound pressure is 85 dBa when measured with a dB meter. Even one short loud click can do damage. When someone experiences ear sizzling or whistling, one should give his ears a few days rest.

a. Tune to a signal.
b. Hold a dB meter against the inside of the headphone to use, and find the place of maximal loudness.
c. Tune volume and tone pitch for maximal indication. Note the value.

d. Use the table below for lowering the max. sound level to 85 dBa.
e. After modification, check max. loudness again.

Ferrite beads FB6 en FB7 can be exchanged for resistors.
These resistors will, like ferrite beads, block possible received RF on the headphone cord,  AND can function as attenuator for to loud audio output

Resulting attenuation [dB]. Resistor values for FB7 and FB8  [Ohms]
17 1k5
12 680
6 220

REM : i take NO responsibility for these values and advises !     Max. loudness is YOUR OWN responsibility.