pa0nhc ARDFrx80-7 20140909

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 40mA. 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+) ca. 17 to ca. 32mA With gain varied from min to max :
FETs 1&4 are working.
IC1p8 +5.0V VR1 is working.
R24 -B = ca. -0.75V D2 is in order.
R25 ca. +0.63V D1 is  in order.
R105 ca. +1.4V or 5V (With 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 PCB.

4.    BFO (Beat Frequency Oscillator) T1 "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.  If monitored with an oscilloscope, on R22 should be ca. 700mVpp / 1.25 Vdc.

REM: If you prefer LSB reception, change C202 for 18pF to shift the BFO frequency ca. 2kHz up.
          If you only want AM detection, change the frequency of the BFO crystal to well outside the IF band pass (12MHz or so).

5.    Local oscillator.
With tune potmeter set to "Min. freq.", adjust C110 for 14.190MHz oscillator frequency.

Then check: With the tune potmeter set to "max. freq", the oscillator freq. should be between 14.31 MHz and 14.35 MHz.

REM: When C7 is made smaller, the tuning range becomes wider. If needed, adapt the value of C7 a little and re-adjust C110.

6.     Antenna.
Using a loop connected to a signal generator, couple a signal of 3.56 MHz into the ferrite antenna.
Tune the ARDFrx to it and adjust C105 (NOT L102) for max. audio.
Check for equal sensitivity over the full tuning range (at 3.5, 3.56 and 3.63 MHz).

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

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

8.    Gain regulation.
Less maximal sensitivity can be set bij R25.
Less attenuation (higher minimal sensitivity) can be set by R24.

9.    Sense.
On one side of the antenna pattern the signal should become minimal when the sense-button is pressed. You can adjust this to minimum by adjusting R105. The opposite side should become 3dB louder with a pressed sense button.

When pressing the sense button, the side of the box with the maximum signal should point towards the heard transmitter.  If you want the maximum reception to be at the opposite side of the box, then interchange the wires connected to SA1 and SA2.

10.    Ear protection.
If possible, measure the max. sound level in the headphone. Correct it by changing the values of R20 and C39.

If a dBmeter is not available, correct it according to the manufacturers stated headphone sensitivity. See the table below
Please check it with a dBmeter later on.

Manuractures spec. of
headphone sensitiviy
(dBspl @ 1mW)
90 150 470
100 470 100
110 1k5 33
120 6k8 12

REM: i take NO responsibility for these values and advises ! Max. loudness is YOUR OWN responsibility. I just want to make you aware of the dangers of to high power levels into a headphone. LIMIT IT. Especially when you give or lend receivers to others (kids !) for a foxhunt.