Project 02

AM/FM RADIO RECEIVER WITH VARICAP TUNING

by TheAstro30


RADIOS are awesome to build; hearing broadcast stations coming out of something you’ve created for the first time is a really awesome feeling. This is why I have presented this “weekend” construction project; an AM/FM radio receiver utilising varicap tuning utilising a TA7642 and TDA7000 IC. The MVAM108 (BB-112) varicap diodes, TA7642, TDA7000 and the MV2105 varicap diode may be hard to source locally, but these can be found on eBay rather cheaply. The TA7642 is the newest replacement for the discontinued MK484 AM radio IC and shouldn’t present a problem to locate.

Circuit Details

Referring to the circuit diagram of fig. 1, RV1(a) controls the tuning of the receiver. This is achieved by applying a small amount of voltage (isolated by R1) to D1, a varactor (varicap) BB-112 diode. The specs on this diode are it only requires 0 to 8V for a full capacitance range of 22 to 760 pF. The way a varactor diode works is the higher the voltage applied to it is in a reverse-biased configuration, the lower the capacitance will be. This is to do with how the reverse break-down voltage affects the silicon substrate within the diode causing a variation in capacitance and, in essence, we now have a variable capacitor. This new “capacitor” is isolated from the inductor, L1, via varicap D2 (connected in reverse bias with D1) which now, in turn, forms a “tank” or tuned circuit.

Next, L2 forms an RF coil, isolated from L1 and is fed in to pin 2 of U1, an TA7642 Tuned Radio Frequency (TRF) IC which contains around ten transistors on board providing all the necessary RF and IF amplification and an AM detector. This IC requires only a maximum of 1.8V to operate, so this is biased at pin 3 via R3 and D3 – D5 to provide the necessary voltage drop (around 1.2V). Pin 3 is fed back into L2 to provide RF gain and also passes via R5. R5 and C6 provide a low pass filter arrangement to roll-off the highs to around 4kHz. After passing via C7 (a DC blocking capacitor) it is presented to transistor amplifier Q1. This is biased via R7 and R8 which provide a voltage divider and provides a gain of around 100. R9 and C9 allow the transistor to provide an AC output signal and also control the frequency response. C8 is another DC blocking capacitor which is fed to output point “A”.

Moving to U2, the TDA7000 FM radio IC, it’s a pretty typical circuit topology as described in the original Philips datasheet with the exception of varicap tuning provided by D6, an MV2105 diode. This diode operates on a maximum capacitance range of 39pF between 0 and 8V respectively. L3 is a 79nH air-core coil consisting of 6 and a half turns of 0.63mm wire around a 3.5mm former. This is used to adjust the center of the band by expanding and contracting the coil. Tuning is derived from RV1(b) which is wired in reverse to RV1(a) and applies a voltage to D6 via R10. Output is formed at pin 2 of U2 and fed into Q2, which is of a similar arrangement as Q1 and operating the same way. This boosts the 75mV output of the TDA7000 to a sufficient level. The output is formed at point “B”.

Figure 1 – Complete schematic diagram for the receiver using an TA7642 TRF IC (U1) and a TDA700 FM IC (U2) as the heart. Amplification to drive a loud speaker to sufficient levels is obtained by use of U3, an LM386N amplifier IC which in turn drives loudspeaker LS1.

Point “C” is the input to the main amplifier section which is taken from the center point of SW1. Points “A” and “B” connect to the upper and lower pins of the SPDT switch respectively and this switches the output of the AM and FM receivers into the main amplifier. R16 and C28 form a low-pass filter together with RV2, the “Tone” control. As the control is rotated counter-clockwise, capacitor C28 becomes closer and closer to ground and the low-pass effect becomes more prominent and the highs roll-off to around 1kHz. In the other direction, the capacitor is essentially doing nothing allowing full frequency response.

RV3 is the volume control which is fed into U3, an LM386 IC and its gain is set at its default 20 which is a voltage gain of around 40dB. The output is fed to DC blocking capacitor C32 and also has R18 and C33 forming a Zobel network. This provides an output power of around 360mW which is sufficient enough to drive any small loud speaker (say 76 mm from a clock-radio, for instance) to pretty loud levels.

The power section of figure 2 is based around an LM317T adjustable voltage regulator (U101) so we can precisely adjust the voltage to a given “maximum” voltage. BR1 is a standard W-01 bridge rectifier package which rectifies the incoming 12VAC voltage from the transformer and is then filtered by C101 and 102. RV101 adjusts the voltage of U101. This will be set later to around 9.6V at the output. The output is further filtered by C103 and also goes via R102, a dropper resistor to LED1 which is the power indicator.

 

 

Figure 2 – The power supply section for the AM/FM receiver based on an adjustable LM317T regulator IC which is adjusted to provide around 9.6V.

 

Construction

The construction of the receiver is built on two PCB’s; one is for the power supply and the other is the main radio PCB. All you need do is copy the etching pattern of fig. 6 and either using photo etching or have someone make the PCB for you; you can then refer to fig. 3 for the component overlay.

Start with the wire links first (except the large one between all three pots) then the resistors and finally the capacitors taking care with their polarities.

For U2 and U3, I would install 18 pin and 8 pin DIL IC sockets respectively as this makes the job easier rather than soldering directly to the IC. This gives you the option of replacing the LM386N-1 and the TDA7000 in the case that something goes wrong and you damage the IC.

 

 

Figure 3 – The printed circuit board overlay diagram for the AM/FM receiver. Take note of the large link wire that is soldered between all three potentiometers and terminated at the PCB.

Next, D1 – D6, Q1 – Q2 and U1 can be installed taking care of their orientation. On U1, pin 1 is ground (or negative) and D1/D2/D6, the anode is on the left of the TO-92(18) casing.

It would be a good idea to verify this ahead of time before installing these components, especially the varicap diodes D1/D2/D6 by doing a diode test on it first to determine where the anode and cathodes are. Installed in reverse direction to how it should be will result in the circuit failing to tune anything at all.

The loop stick coil (L1/L2), is made up on a 180mm long ferrite rod using 60 turns of 0.63mm ECW for the main coil and 3 turns of 0.63mm ECW for the RF coil spaced around 30mm from L1. Refer to figure 4 for a guide.

This is achieved by first wrapping a piece of paper around 100mm wide just long enough to wrap around (loosely) no more than twice on itself. This is then cello taped to stop it for coming apart. You should be able to easily slide this up and down the length of the rod. If you can’t, un-tape it and re-tape it a little more loose.

 

 

Figure 4 – Loopstick antenna coil L1 and L2 showing the spacings required for satisfactory reception.

Next, you can begin by taping a length of ECW to the paper around 5mm from the edge of the paper leaving yourself around 10cm of wire “hanging-off” the end of the rod. Begin winding 60 turns of ECW for the main coil first, periodically checking that the paper “tube” still slides freely up and down the rod. Try to keep your windings as neat and as close together as possible. Once winding of this coil is complete, place a piece of tape over the windings sticking it to the paper to stop them moving but still allowing the paper to slide on the rod.

 

 

Figure 5 – L3 winding data. It is formed using 6.5 turns of 0.63mm ECW wound around a 3.5mm former.

Next, wind the 3 turns required for L2 coil around 30mm from L1 using the same method as above. The ends of the wires can now be scraped of their enamel, tinned and finally soldered in to the respective holes on the main board.

Finally, the two 16mm single-gang 10 k potentiometers and the 50 k dual-gang potentiometer can be installed and (except for the volume pot) it’s not overly critical as to whether they are of linear or logarithmic types. If you have used an IC socket for U2/U3, now would be a good time to insert the ICs taking note of their correct orientation. The notches both face towards the top edge of the board. Now would be a good time to install the large wire link across all three pots. This is achieved by gently filing a scratch on the top of the pot(s) casing(s) so solder will adhere to them. First, solder the length of wire into the hole provided on the PCB and then solder it to the top of each pot in turn until you’ve reached the volume control. Trim off the excess and this part of the construction is done. The idea of the wire is to earth the pots and if a metal front panel is used also acts as a grounding plane.

Testing

DO NOT connect the main board to the power supply board yet. Doing so and applying power if the supply isn’t correctly adjusted could result in damage to both the TDA7000 and LM386 ICs because of too much voltage.

 

 

Figure 6 – The etching pattern for the PCB at a resolution of 300dpi for easy printing.

Testing begins by adjusting the output voltage of the regulator IC101 after all necessary mains wiring has been done and SW2 is wired in. Apply power and measure the output voltage with a voltmeter. Adjust RV101 until the meter reads around 9.6VDC. This should be more than adequate to power the main board and also allow the 0 – 8V across the varicap diodes (D1/D2/D6) to give the desired frequency coverage. Once you are satisfied that the voltage output is correct, you can connect it the main board and also connect the speaker.

Connect power and listen for any unusual noises emanating from the speaker when switched between AM and FM respectively. If it starts squealing it’s indicative of the circuit oscillating. Disconnect the power and check for solder splashes across tracks or components not soldered in correctly.

Assuming no unusual noises are present, slowly rotate RV3 up and listen for “static”. You should be able to hear the normal AM noise (on AM) or white noise of FM (if switched to FM) of a radio not tuned to a station (or if you’re lucky, depending on the position of RV1, an actual station!). Next, rotate RV1 and see if you can tune anything in. If you hear nothing and after playing with it for a while, switch off and check you have soldered everything in the correct place and the correct way around.

At this point, it may be necessary (if you haven’t already done so) to check that D1/D2 and D6 anodes are definitely on the left of the device. This can be verified with a multi-meter set to the ohms or diode test scales and should only show a reading one way around. The pin that has the positive probe on it is the anode. If it is installed the incorrect way around in the circuit (i.e.: forward-biased), the variable capacitance effect of the diode will not be present.

If you are switched to FM and can successfully tune in stations, it may be necessary to adjust the center of the band by expanding and squeezing L3. Take note, only a movement of a fraction of a millimetre is enough to change the inductance of the coil.

Next, you can now adjust the lower end of the AM tuning range which will require another AM receiver to verify what station is at the lowest end of the tuning range (531 kHz). Slide the paper “tube” slowly up and down the rod and you will notice that the tuning will “shift”. Re-adjust tuning pot RV1 to re-tune the station in. Repeat this until RV1 is tuning that station as far counter-clockwise as possible.

It’s now time to move on to the final hardware construction phase.

Final assembly

Final assembly begins by fitting out the plastic housing by drilling its front and rear panels and the necessary holes on the lid for the speaker. The enclosure (Jaycar catalogue number HB5912) measures 200 mm wide by 160 mm deep and 70 mm high and each panel measures 196 mm wide by 57 mm high. Fig. 7 shows the artwork for both the front and rear panels which can be printed on to A4 sticker paper (purchasable at most office supply stores), cutting them out with a pair of scissors and sticking them carefully to each panel.

 

 

 

Figure 7 – Front and rear panel artwork of the receiver produced at 300dpi for easy printing.

You could do one of two things; use the original plastic panels that come with the enclosure, or you could cut them out of a sheet of 1 mm thick (18 gauge) aluminium. The latter would probably be the better choice (even if it involves more effort) for two reasons. One, the metal will give the pots something to earth to acting as a ground plane and two, its less messy to drill. Plastic has the tendency, together with the paper sticker, to melt and burr.

After deciding what type of material to make the panels out of, you can use the stickers that you have affixed to each to assist in drilling. Start with the smallest drill bit first (and if using aluminium, centre-punch each hole) and then step it up to the final size. The LED hole on the front panel should be drilled out to 4.5 mm and you can use an LED clip to mount the LED to the panel. The pots are drilled out to around 7 mm and take note that there are no pilot holes drilled to accept the dowels on the pots. These will have to be broken off the pots prior to marrying the PCB to the panel.

The band and power switches are of a miniature panel mounted toggle variety and the hole can be drilled out to around 6.5 mm. The antenna hole on the rear panel is of an BNC type connector and these usually have a 9 mm mounting hole.

The AC socket is an IEC/C7 figure 8 variety and is drilled out to around using two 9 mm holes and finished with a file so the part fits and the screw holes are 3 mm. It is placed on the inside of the panel and using two short screws with 2.5 mm threads attached to the panel snuggly. Basically, everything is drilled out in steps until they fit snuggly in their respective holes but don’t need any extra force to insert them.


Figure 8 – The drilling pattern for the speaker grill produced here at 300dpi which can be printed on paper and sticky-taped to the top cover as a template prior to drilling.

Next, you can move on to the drilling of the speaker holes and I have provided a rough sketch of the drilling plan in fig. 8. Use this as a guide and this is solely dependent on what size speaker you’re using. In my case, I’m using an 100 mm 2W 4 ohm speaker (Jaycar catalogue number AS3008).

Again, start with the smallest drill bit first which will assist in drilling roughly central in the plastic and move up to the final size. A size of around 4 mm should be adequate for this purpose. Once cleaned up and de-burred, the speaker can be glued in to its new home on the inside of the lid using some Quik-grip or contact adhesive cement and left to dry.

Keep all wires short as possible to prevent binding inside the case and also from picking up stray noise. The power switch is wired back to the power supply board’s SW2 position and a small 150 mA 6.3/0/6.3V transformer is wired between the AC socket and the power supply board. The LED is wired to its respective location on the PCB via two lengths of wire and with heatshrink around the anode and cathode leads of the LED to keep them from touching and also to keep them neat.

NOTE: If you are not confident with doing mains wiring, do not do it. Seek advice from someone with more experience with this type of wiring and, as it is at 240 VAC mains potential, can be lethal.

The DC output should be wired to the main board (if not all ready) with short wires as well. It’s a good idea to twist the negative and positive wires around each other to prevent noise pickup/injection.

 

Wire a very short, as possible, wire between the antenna connection and the associated BNC socket connector. Finally, you can wire the speaker with reasonably short wiring to keep things neat to its respective location on the main board. Now would be a good time for a final power-up test to make sure the device still works.

Conclusion

In conclusion, this project should only take around four to five hours to complete which includes all of the drilling of the hardware. I hope you enjoy building this project as much as I have presenting it here and I also hope that the construction details presented here are detailed enough. Enjoy! – TheAstro30