Before you begin, read the directions below completely. It can prevent problems.
In a professionally made PCB all holes are moralized and do interconnect top and bottom of tracks and surfaces. At your home made PCB, all holes are insulated between top and bottom.
Therefore, solder as much as possible mass-connections on both top surface and bottom surface. To make ground interconnections between top and bottom mass planes, making all groundings low-z in favor for stability.
Measuring and setup:
For pre-adjusting of coils a dip meter is needed. For trimming is a signal generator needed (10-145MHz, AM 80%, output -120 to -10 dBm @ 50 Ohm), and a plastic trimming tool 1x1mm square. Voltages and currents can be measured with a (cheap) digital meter which has a 2Adc range.
To protect the ears of the user, the max. loudness of the headphone should be measured with a cheap sound pressure meter (dB meter) and be corrected accordingly
Winding and testing coils.
In this design NO hard to find TOKO coils are used. Instead the coil forms are from Conrad, but must be wound.
In the schematic and on the PCB top copper, the coil pin configuration is seen FROM THE TOP.
>> See "start winding" <<.
Adjust the ferrite cores of L1 and L2 only using a well fitting non-magnetic 1x1mm square trimming tool.
The PCB top surface under the coils has no copper to prevent a short circuit.
The coil forms are thermoplastic (filled with glass fiber). Use a electronically regulated adjustable soldering iron and know the irons temperature.
How to wind L3, L4 and L6 :
See "XRAY.png" and the schematic for the pin positions.
The coil pin configuration is seen FROM THE TOP.
The 10,7MHz IF transformers L3,4,5,6 are wound on FM5.1 coil forms from Conrad.
Primary and secondary 25t.
Prevent melting of te plastic coil form. Soldering a wire to a pin should be quickly done with a tip-temperature of 300C.
Hold the solder tip very shortly against the wire (not against the pin), and remove it immediately when the solder has flown.
Use lacquered wire of 0.15mm dia, a very fine soldering bit, and tin solder 0.8mm dia with resin core.
1. First heat the wire at a tip temperature of 375C, remove the burned lacquer, and tin the place at 300C for 2 to 3mm length.
2. Fold the tinned wire part around the coil pin 4 in a hairpin U-shape. The wire end should be at the inner side of the coil form to prevent a short circuit to the can. This is the start of the secondary winding.
3. Clip off exes wire. Check for short circuit to screening.
4. Temp. 300C. Clean and tin the soldering tip. Hold the tip SHORTLY against the wire (NOT against the pin) so the tin flows between pin an wire.
5. Remove the tip as soon as the tin between wire and pin is flown.
6. Let it cool down for 2 min. to prevent skewing of the pin.
7. Wind the wire 25x around the coil form. REMEMBER in which direction you wind and which coil side is between your fingers !
8. Fold the wire in a U-shape around pin5. This is the end of the secondary winding.
9. Remove the wire from pin5. At 375C remove the lacquer from the wire in the U shape, and tin it over 1mm at both sides of the U.
10. Replace the wire around the pin (with the end on the inside to prevent a short circuit with the can later on) and trim it.
11. Solder the wire to the pin (see [4 - 6]).
12. Likewise the primary winding.
13. Start at pin3.
14. Wind 25 turns in the same direction as .
15. End of winding is pin 1.
16. Glue the windings to the coil form (use transparent glue or nail lacquer). LET DRY.
NOW make it easy to trim the IF amplifier late ron:
17. Place the completed coil in its copper can. Don't forget to put a little glue onto the ferrite saddle core
18. Solder a test capacitor (39pF +- 1pF) with LONG legs to pins 4 and 5. The legs act as a coupling coil for the dip meter.
19. Adjust the screw core for a dip at 11MHz on a dip meter. The top of the screw should be 1 - 2 mm above the copper can.
20. Remove the capacitor.
L1 en L2
must be wound in a similar way, but on smaller coil forms FM5.2, which have non-insulated screw cores woth a square tuning hole of 1x1mm.
L1: 4+3/4 t.
See pin numbering! (TOP).
Start is pin1.
Glue the winding of L2 to the coilform, to make the oscillator more stable. Let dry.
Use a soldering iron wit regulated temperature (300-375C), very thin bit, and tin solder 0.8mm with resin core.
Connect the mass of the iron to the copper of the PCB.
Solder FETs and ICs as LAST.
Wind the coils and pre-tune IF coils to 11MHz with a 39pF +-1pF capacitor connected between pins1 and 3.
First solder on the top copper all low profile components :
- IC-fittings, resistors, ferrite pearls, ceramic Cs. REM: Solder where possible mass-connections on BOTH top copper and bottom copper.
- Connector pins, elcos, film capacitors, crystal filter. Use some (thermal) glue to mechanically bond elcos and ferrite pearls to the PCB surface.
- Solder the IF coils in place. REM: the top copper is removed under the coils to prvent short circuits. Solder coil cans only at the bottom copper.
Solder chokes L7-12.
Solder L1 and L2 in place.
FETs and IC2 are static sensitive. The gain connection is sensitive too, when no pot meter is connected.
To prevent damage to semiconductors, connect the soldering iron and you body to the PCB mass.
- Solder Vr1 en FET1-5. REM: NB: IC1 and IC2 will be placed later on.
Testing the PCB:
- Connect gain pot meter R21.
- Connect a 9V battery in series with a DC current meter range 2A.
- Connect this combination briefly to the PCB, while observing the current meter. Current flown is maximal 40mA. If much more, find the fault, and replace D2.
- Insert IC1 and IC2. Connect the tune potmeter.R5, headphone (it can be to loud !), and a signal generator.
- C9: 5.00V.
- D1a: ca. 0.60V.
- D2k: ca. -0.71V (gain pot meter R21 to minimum).
- C15: +0.36V (gain pot meter to maximum) tot -0.58V (gain pot meter to minimum).
Max. en min. gain are adjustable by changing R25 and R24.
- IC2-p7: ca. 4.35V.
- R22: 2.0 - 3.0 V. Non-critical, depends on de spread in characteristics of FET5.
- Compare the voltage drop over L7, L8 and L10 with gain at max. They should be equal (15mV). A defective FET shows abnormal voltage drop. REM: a short circuit to mass damages D2.
- Battery current is 10mA to 27mA (depending the setting of the gain potmeter.
Setup and adjustment.
- Feed a very strong IF signal (-10dBm, 10.700MHz) in the antenna connection. Adjust L3,4 and 6 for max output.
REM: the core screws should be abt. 1mm above the copper coil can.
Be aware: L1 and L2 should only be adjusted using a non-metallic trim tool of 1mm x 1mm square. A broken core means replacement of the coil.
- Feed a strong signal (-35dBm, 100 - 150MHz) in the antenna connection. If you hear a signal when changing the generator frequency between 100MHz and 150MHz,
it shows that the oscillator is operating correctly.
- With the tune potmeter in low position, and the generator tuned to 143.950MHz, adjust L2 for reception of the signal.
- With the tune potmeter in high position, check that the highest frequency is satisfying (ca. 144.825MHz).
You can change the tuning range, by changing C7 (NP0 !), and readjusting L2
- Tune to 144.4MHz and adjust L1 for max. signal.
REM: the top of the cores of L1 and L2 should be approx at the same height as the top of the copper screening.
Check the oscillator stability.
- Let the receiver "warm up" for 15 min.
- Tune generator and receiver to 144.4 MHz.
- Monitor the generator frequency with a counter.
- After 15min. more, the receiver should not need to be retuned for good reception.
Check the sensitivity.
A very weak signal of -120dBm (m=80% 1kHz) should be clearly heard.
Check the gain regulation range.
A signal of -35dBm should be just be heard with the gain potmeter at minimum. This can be adjusted by changing the value of R24.
- Feed the receiver with a weak signal of -110dBm.
- With gain at max. with an oscilloscope at R19 should be measured 1Vrms 1kHz sine tone without distortion. This can be corrected by changing the value of R25.
Protecting the ears of the user. Adjustment of the acoustical output of the headphone.
If you do not know the sensitivity of the used headphone:
- Feed the receiver with a signal modulated with 1kHz that saturates (overdrives) the audio output of the receiver.
- Use a (cheap) dB-meter (sound pressure meter) and measure the maximal sound pressure at the surface of the headphone speakers.
- Change the value of R20 until you measure not more than 85dBspl & 1 kHz (A or C curve).
REM: the -3dB cut-off frequency of the combination R20/C39 should stay at about 2.5 kHz.
Example: if you enlarge R20 from 470 Ohms to 1kOhm, C39 should be changed from 100nF to 47nF and vice-versa.
If you do know the sensitivity of the used headphone (by the makers data): Use the theoretical values of the table below.
I still recommend to check the max. loudness afterwards using a sound pressure meter (dB meter).
REM: once a headphone and receiver are matched for max. loudness, they are a pair. Do not use another headphone with another sensitivity wit that receiver.
(dBspl @ 1mW)