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TV transmitter for VHFVCRs This simple add-on board will allow you to use the TV Transmitter described in December 1991 with VCRs that have a VHF output. The project can now be used to transmit signals from any VCR to a remote TV set without the need for cables and splitters. By JOHN CLARKE In the December 1991 issue, we published a TV Transmitter for use with VCRs that had a modulated UHF output at about channel 3 7. This channel frequency was dictated by the size of the two 3-element Yagi antennas that are so crucial to the performance of the unit. Unfortunately, VCRs with modulated UHF outputs are relatively far and few between - about 3% of all VCRs, we gather. Instead, most VCRs have a modulated output in the VHF band, generally switchable between channel O and channel 1. We can't use these channels with our TV transmitter however, since the resulting Yagi antennas would be far too big for indoor use. Basic concept The original TV Transmitter operated by amplifying the modulated UHF output signal from the VCR and then coupling it to the transmitting antenna. This antenna then radiated the signal to a second identical antenna connected to the remote TV set. Fortunately, it's quite easy to modify the circuit to work with VCRs that have a VHF output. All we have to do is add a UHF modulator (tuned to channel 37) in front of the original transmitter. The video and audio outputs from the VCR are now fed to the modulator and the resulting RF output (at channel 3 7) fed to the RF input of the transmitter which then operates exactly as before. Fig.1 shows the general arrangement of the modified transmitter. Note that it is the video output that's now used to drive the TV transmitter;. not the modulated VHF output. The VHF output connects instead to the main TV receiver in the conventional manner. Because the video and audio outputs of the VCR are connected to the TV Transmitter, these outputs have been duplicated on the TV Transmitter so that they can also be fed to a hifi amplifier or to a video monitor. We have also made provision for VCRs with stereo sound, although readers should note that the left and right audio channels are mixed together to form a mono signal before being fed to the transmitter. If a mono VCR is used, the audio output from the VCR can connect to either the left or right audio input on the TV Transmitter. Circuit details The new TV Transmitter can be used with all VCRs and has a useful range of about 15 metres, provided the correct antennas are used. Note that the TO TV output (designated on the front panel) is no longer used. 16 SILICON CHIP Refer now to Fig.2 for the complete circuit details. The power amplifier section is virtually identical to the December 1991 transmitter, except that it no longer includes a splitter to drive the main TV set. This splitter is no longer necessary, since the VHF output from the VCR now drives the main TV set - see Fig.1. As before, the power amplifier uses two OM350 hybrid wideband amplifier ICs (ICl & IC2). These each have a nominal gain of 18dB over the frequency range from 40-860MHz. The ANTENNA TV ANTENNA ANTENNA Warning! VIDEO OUT VCR ANTENNA IN L AUDIO OUT R VHF OUT * * VIDEO IN L UOIO IN AUDIO R OUT R L VIDEO OUT 75n TVNIOEO CABLE TRANSMIT OFFC.ON RF OUT TO HI-FI AMPLIFIER UHF CHANNEL 37 SIGNAL i! TV RECEIVER Although this is a low-power device with limited range, it may interfere with neighbouring TV receivers if used incorrectly. The onus is on the user of this equipment to ensure that such interference does not occur. In particular, do not point the transmitting antenna toward neighbouring TV antennas. lation level. A separate RCA socket is also connected in parallel with the video input socket to provide the video output signal. This video signal can be fed to your TV set (if it has a separate video input) or to a video monitor. The RF signal from the modulator is at UHF channel 3 7 and this signal is coupled to pin 1 of amplifier IC1 via a 220pF capacitor. Power for the modulator is derived directly from the 12V supply rail and is bypassed using a .01µF ceramic capacitor. Note that the 47Q resistor used to supply zener diode ZD1 in the original circuit has been reduced to 33Q. This change ensures that ZD1 remains in regulation despite the increased current now drawn by the circuit due to the UHF modulator. REMOTE TV RECEIVER TO VIDEO MONITOR *FOR MONO VCRS • CONNECT AUDIO OUT OF VCR TO EITHER L OR R AUDIO INPUT ON TV TRANSMITTER Fig.1: this block diagram shows the general arrangement of the TV Transmitter. It is now driven by the audio & video outputs from the VCR (not by the RF output). The RF output drives the main TV set is the usual manner. via 2.ZkQ resistors to give a mono signal. This is then AC-coupled to the audio input of the modulator via a ZZµF bipolar electrolytic capacitor. The left & right audio input sockets are also paralleled to provide the left & right audio outputs. These outputs may be connected to a hifi amplifier or to a stereo monitor, as mentioned previously. The video input signal is fed to the modulator via a 22µF bipolar capacitor and level control VR1. This level control is set to give the correct modu- two amplifiers operate in cascade, with IC1 driven by the RF signal from the UHF modulator. Apart from that, only a few other parts are required to complete the unit. The left & right channel audio inputs are fed in via RCA sockets and mixed Fig.2 (below): the signals from the VCR are first fed to a UHF modulator. THs generates an output at channel 37 & drives an output stage consisting of two OM350 hybrid amplifier I Cs. Construction There are two PC boards associated ----• TRANSMIT ON/OFF 33n S1 0.5W 12VOCl+ " ' ! " ' - - ¥ N ~ + - - - - - . - - . . . . . - - - ' INPUT-~ .00111 NOT USED &DOSTER SUPPLY LEFT AUDIO INPUT (FROM VCR) RIGHT .,. CHANNEL 37 UHF OUTPUT (TO RF INPUT) L1 II II .001•1. II 11 II L2 " 75n RF INPUT (FROM UHF MODULATOR) ~------~OpF 1 22opF ~ >"-+---tl--:-0 . LEFT AUDIO OUTPUT RIGHT oJf~i~~o TRANSMITTER ANTENNA UHF MODULATOR HL-RMBT-E36H L1,L2: PHILIPS 4312 020 36700 sovw VIDEO INPUT (FROM VCR) VIDEO OUTPUT BP y ~BOOSTER OUTPUT (NOTUSEO) VR1 ,_ _ _--13 VIDEO 2.2k IN TV MODULATOR/TRANSMITTER CASE MARCH 1992 17 S1 • IC2 OM350 ·••!>1 C• + - • I TOP -~ tl <at> OM350 •• ! i) 1 • --- -- ~ l2 - VIEW • -;s ·11111V !Cl .001"0" blNPUl ~ 220pF L1 • BASE VIEW Fig.3: install the parts on the two sides of the amplifier PC board as shown here & don't forget to install the "pin-throughs" where indicated. Be careful when installing the two !Cs, as they can be damaged if overheated. RF chokes Lt & L2 are supplied ready wound. on the base side. There are also several "pin-through" points , where a short piece of tinned copper wire is passed through the hole and soldered on both sides of the board. Start the board assembly by installing the 33Q resistor, ZDl and the two ICs. ZDl's anode lead must be soldered to both sides of the board, while its cathode lead should be looped to provide thermal stress relief. The two with this project. The first board, code SC02212911, carries the two amplifier ICs and is identical to the board described in December 1991. The second board, code SC02204921, carries the UHF modulator and its associated resistors and capacitors. Fig.3 shows the assembly details for the amplifier board. Note that some parts are mounted on the top side of the board while others are mounted AMPLIFIER PCB IC1 IN IC2 OUT VIDEO~ 22uF . L---mu-. AUOI L ~-'---~ R 22uF 0 O S v Re-f:TID-,-------------------. • IN • • HL-RMBH36H OUT RF OUT 18 ICs should be pushed down as far as possible and their pins soldered on the underside of the board only. Note that the two ICs are hybrid devices and can be damaged if overheated. In fact, it's best to tin the copper pads on the board before installing these ICs to assist in fast soldering times . The remaining parts are mounted on the base side of the PC board. Note that some component leads do not pass through holes in the PC board but solder directly to the pads. Keep all component leads as short as possible. Do not mount the two sockets at this stage; that step comes later. Work can now begin on the SC02204921 PC board. Begin by installing PC stakes at the seven external wiring points, then install all the minor components. Finally, install the modulator with the sockets facing the edge of the PC board. With the PC boards complete, the three Dynamark labels can be carefully affixed to the plastic case and holes drilled to accept the switch and the various sockets. The cutout for the switch can be made by first drilling a hole at the position indicated and then carefully filing it to the required shape. The mounting positions for the two 75Q PAL sockets are marked using the amplifier PC board as a template. To do this , place the board against the side of the case with the base side facing outwards. Check that the socket holes line up with the labelling on the front panel, then carefully mark the holes with a pencil and lightly centre punch them. The socket holes can now be drilled and reamed to size, as can the holes for the modulator sockets and test switch. This done, mark out and drill the two mounting SILICON CHIP • Fig.4: the UHF modulator is mounted on a separate small PC board, together with a few minor components. Install PC stakes at the external wiring points & use shielded cable for the video & audio input leads. PARTS LIST Only four components are mounted on the top side of the amplifier PC board the two OM350 ICs & the 47Q resistor. Note that "pin-throughs" must also be installed at three locations. 1 double-sided PC board, code SC02212911, 30 x 97mm 1 PC board coded SC02204921, 39x101mm 1 plastic case, 130 x 67 x 43mm 1 UHF modulator, HL-RMBTE36H, preset to channel 37 1 Dynamark front panel label, 125 x 64mm 1 Dynamark label, 60 x 38mm 1 Dynamark label, 121 x 35mm 6 RCA panel sockets 1 2.1 mm DC panel socket 1 2.1 mm DC line plug 2 panel-mount 75Q PAL sockets 1 DPDT slider switch 2 Philips 4312 020 36700 chokes (L 1,L2) 1 2.2kQ miniature horizontal trimpot 2 Yagi antennas (see December 1991) 2 3mm x 6mm screws 1 3mm nuts Wire & cable The remaining parts in the power amplifier stage are installed on the base side of the PC board. Keep all component leads as short as possible. 1 75Q RF cable terminated with male & female PAL plugs 1 mono or stereo audio cable, RCA to RCA 1 video cable to suit (see test) 1 100mm-length twin shielded audio cable 1 100mm-length single shielded audio cable 1 100mm-length red hookup wire 1 100mm-length black hookup wire Semiconductors 2 Philips OM350 wideband amplifier ICs (IC1 ,IC2) 1 12V 1W zener diode (ZD1) Capacitors 2 22µF S0VW bipolar electrolytics 1 .01 µF ceramic 2 .001 µF ceramic capacitors 3 220pF capacitors Resistors 2 2.2kQ 0.25W 133n o·.sw Th e UHF modulator is supplied pre-tuned to channel 37. It also contains the necessary circuitry to generate a test pattern (a white bar down a black screen). Note: constructional details for the Yagi antennas used with this project were given in the December 1991 issue. Do not use different antennas - the correct antennas are vital to ensure adequate range. MARCH 1992 19 Fig.5: this is the full-size artwork for the modulator PC board. Full-size patterns for the amplifier PC board & for the front panel were published in the December 1991 issue. Two small Yagi antennas, one at each end of the transmission path, are required to ensure adequate range. These antennas must be pointed directly at each other. Full constructional details are in the December 1991 issue. holes for the modulator board. Note that this board is mounted directly on the base of the case without standoffs, to allow sufficient clearance between the modulator and the case lid. At this stage, the various hardware items can be mounted in the case. Secure the six RCA sockets to the end of the case first, followed by the two 75Q PAL input sockets, the power input socket, the on/off switch and, OUT + + + OUT finally, the modulator board. Before mounting the amplifier board, it is first necessary to install the four power supply leads. These leads are then run to the DC input socket and on/off switch. Make sure that the socket polarity is correct, with the centre pin positive as shown on the front panel label. At this stage, it's also a good idea to check that the centre contact of the DC plug is also positive with respect to the outer contact. If it isn't, swap one set of leads (either to the plug or to the socket) around now. The amplifier board can now be fitted to the PAL sockets and the centre pins and earth lugs soldered (see Fig.3). Note that the latter are soldered to the groundplane on the base side of the PC board. Complete the assembly by wiring up the RCA sockets and by running supply leads to the modulator PC board. Testing Before testing the unit, you must first tune your remote TV set to the TEST + VIDEO IN + + + VIDEO LEFT RIGHT IN + RF OUT TO INPUT Fig.6: here are actual size artworks for the RCA input socket panel (left) & the modulator panel. 20 SILICON CHIP + a II QUALITY NO BRAND DISKETTES 5.25" 5.25" 3.5" 3.5" DSDD DSHD DSDD DSHD $4.70 $8.95 $8.50 $16.95 *************** JAPANESE BRAND ._;: •, t i / / f,, _ FLOPPY DRIVES ~~"- SUPER . ',:;,..3.5" 1.44M $99.00 ?z..,~PECIAL ~ 5.25" 1.2M $125.00 -~~~ ----------EPROMS 2716 ............. $9.95 2732A ........... $7.50 27C32 ......... $11.50 2764 ............. $6.50 27C64 .......... . $7.00 27128 .... .. .... . $7.50 27C128 ......... $8.00 27256 .... .. .. ... $8.50 27C256 .... ..... $9.00 27512 .. .. .... . $12.50 27C512 ...... . $13.50 27C 1001 ..... $23.50 RAMS 6116 ...... .. .... . $4.50 6264 ............ . $9.50 62256 ...... .. . $17.50 628128 ....... $60.00 SERIAL PORT CHIPS 8250 ............. $7.50 82450 ......... $17.50 16C450 ....... $18.00 16C550 ....... $29.50 ** ** ** ** *** * *** ~ _{.~~~~!N~~~r~~ ~ nr I\""~··/~ The modulator PC board is secured directly to the base of the case while the amplifier board is installed by fitting it over the three PAL sockets & then soldering the centre pins. Take care with supply polarity. signal from the UHF modulator. This is done as follows: (1) connect a lead from the RF OUT of the TV Transmitter to the antenna socket of the remote TV set. (2) Set the modulator TEST switch to the up position and apply power to the TV Transmitter. (3) Tune the TV set to channel 37 to receive the test pattern (a white bar down a black screen). To test on transmission, you will need a couple of Yagi antennas as described in the December 1991 is- The RCA input sockets are paralleled to preserve the original video & audio output signals from the VCR. sue. Connect the TV Modulator/Transmitter as shown in Fig. l. The left and right audio channels will require leads fitted with RCA plugs at both ends, while the video connection will probably need a BNC to RCA lead. You also have to connect a 75Q coax cable between the modulator RF OUT terminal and the transmitter INPUT terminal (see Fig.1). This lead must be fitted with a PAL plug at one end and a PAL socket at the other. Finally, connect the antennas and switch the modulator TEST switch down for video modulation. Apply power and play a tape through the VCR. Provided the transmit switch is set to ON, you should now receive the VCR program on the remote TV set. Note that the two antennas should be aimed directly at each other for maximum effect. If it doesn't work, check your connections and the supply voltage to the OM350 ICs and the modulator by measuring the voltage across ZDl. You should get a reading of 12V. If not, check the orientation of ZDl. 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