Franzis Do-it-yourself User Manual Download Page 18

18

Explanations on the wiring  diagram 

Most VHF superhet receivers use an intermediate frequency of 
10.7 MHz. The receiving frequency is first converted to the in-
termediate frequency and then filtered, amplified and demodu-
lated. The VHF retro radio is a superhet, too, which converts 
the received signal to an intermediate frequency. But its inter-
mediate frequency is much lower, at 70 kHz. For this reason the 
intermediate frequency filters are able to function without bal-
anced coils. The FM demodulator becomes simpler and better 
protected against distortion. All essential stages fit into a single 
SMD-IC, the TDA7088 with 16 pins. Instead of a variable ca-
pacitor, as used in older receivers, the radio works with the D1 
varactor diode. The higher the voltage at the diode, the lower 
is its capacity and the higher the receiving frequency. The only 
balance  point  is  the  L1  coil  which  can  be  used  to  adjust  the 
lower edge of the oscillator frequency. 

The low frequency power amplifier is a simple class-A ampli-
fier with the two T1 and T2 transistors. The quiescent current 
is approximately 20 mA. The circuit will deliver a good sound 
quality at an operating voltage as of 2.2V. 

Summary of Contents for Do-it-yourself

Page 1: ...Do it yourself VHF Retro Radio...

Page 2: ...are only liable in case of intent or gross negligence according to legal regulation Beyond that publis her and author are only liable according to the law on product liability concerning hazards to li...

Page 3: ...tyle receives FM stations in the 87 5 MHz to 108 MHz band with good reception per formance You will mainly hear the powerful local stations in high sound quality However the sensitivity of the receive...

Page 4: ...alve receivers and therefore could be operated with batteries Apart from this they technically still resembled the older tube radios Thanks to the highly integrated receiver IC TDA7088 building your o...

Page 5: ...assembled PCB with TDA7088 rod antenna speakers 8 0 5 W volume control 22 k with switch tuning control 22 k insulated wire battery compartment with connection wires T1 PNP transistor BC557B T2 NPN tr...

Page 6: ...nally with the two port on off switch If you turn the axis all the way to the left the switch opens Insert the volume con troller into the left mounting hole A small tab secures correct insertion Fix...

Page 7: ...speaker by pushing it into the corresponding slot The connectors should face upwards to make sure that the connec tions to the board will be short The speaker sits securely in its slot You can howeve...

Page 8: ...rom the inside through the housing opening and push the end into the flat holder which you have to secure correctly in the housing with the enclosed adhesive tape Now the antenna is mounted sufficient...

Page 9: ...5 condensers and one resistor You will only need to solder in some components with con necting wires This includes all components of the AF amplifier the coils and all components needed for the diode...

Page 10: ...ectrolytic capacitor 100 F as winding mandrel First wind three turns closely ad jacent to one another Then pull the turns apart to reach a total length of approximately 7 mm Absolute accuracy is not c...

Page 11: ...the coils Insert the resistors R1 4 7 k yellow violet red R2 220 k red red yellow close to the connections to potentiometer PT2 The resistors R3 1 k brown black red R5 330 k orange orange yellow and...

Page 12: ...nt Now bend the connecting wires so that they are suited for vertical assembly The resistors Solder the D1 varactor diode 1SV101 in the flat transistor housing with two connections imprint V101 follow...

Page 13: ...which is marked with a white line points towards the IC Afterwards install the ceramic disk capacitor C17 with 100 nF print 104 The mounting direction is not important The capacitors Finally you have...

Page 14: ...ND connection The red connector goes to the volume control switch The remaining wire is to be soldered to the antenna connection If this connecting wire is longer than is necessary it can still be use...

Page 15: ...15 The wiring diagram The complete wiring...

Page 16: ...stop You can shift the recep tion range by modifying the SP1 coil Push the turns tighter to gether to receive lower frequencies If there is still a large empty area at the left to the bottommost stat...

Page 17: ...he power amplifier If this happens turn down the volume control a little With its moderate volume level the FM retro radio is ideally suited for a relaxed listening experience in the evening If you ex...

Page 18: ...e FM demodulator becomes simpler and better protected against distortion All essential stages fit into a single SMD IC the TDA7088 with 16 pins Instead of a variable ca pacitor as used in older receiv...

Page 19: ...width of the AFC catch range If you want for instance to receive weak stations in the vicinity of stronger stations it may be useful to decrease R2 down to 100 k or less to get a smaller capture rang...

Page 20: ...20...

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