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There is now a seperate page for COMTECH mods. |
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CCIR 405 PRE-EMPHASIS / DE-EMPHASIS BOARD |
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The CCIR standard pre-emphasis is intended to raise the hf portion of the video so that it can be reduced at the receiving end by the same amount and in doing so drop the noise level. Inductors used were SMD with a Qmin of 50.
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| SURFACE MOUNT COMPONENTS OVERLAYS...... |
| Pre-emphasis |
De-emphasis |
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This is a smd layout , i.e. components mounted on the printed side, the same pcb being useable with either pre- or de- emphasis circuits. The optional filter Lf Cf Rf is to notch out any signals around 5-6Mhz on video to transmitter.When testing you can reduce the 75ohm in and output matching resistors as this can improve the picture sharpness by better matching lower impedance you may be coming from. Do not go below 18 ohms and ensure both in and out resistors are the same value.
I suppose if you really have to you could drill through from the top of the board and use axial resistors stood upright- but hey! this is 2009.
Farnell parts 10uH=3838780 : 33uH=3838845
Cf = 2% disc = 303513 (no 1206 smd version found)
All R's,L's and C's are 1206 smd. 000 is a link. |
Click on circuit for pdf version |
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AUDIO LIMITER/COMPRESSOR Specifically designed to use with very low-cost electret desk microphones. 1 IC& 2 transistors.
Overcomes the heavy attack/decay problems with diode rectifier VOGAD circuits. |
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A TL074 was used with a dual supply of +/-5v Note the lack of rectifier diodes in the feedback loop, instead the circuit uses high gain BC848(smd BC108) This uses the full wave rectifier effect to give a non-linear compression level avoiding the worst of the 'pumping' normally noticed with discrete diodes. Note Q2 is a P channel Junction FET eg J176 (not 2N3819). |
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| WORTHING- SOLENT MODULATOR IMPROVEMENTS |
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Despite a couple of improvements to the original design, these useful transmitters still suffered from poor syncs, colour crushing and colour beat pattern and a non-standard pre-emphasis circuit. The design shown was intended to overcome these shortcomings. Q2 (Tr6 in original circuit) needs an hfe of over 100 at 150mA which is beyond the BC557 capabilities this can cause crushing on peaks and can give chroma distortion. The pre-emphasis is nothing like the CCIR-405 especially after capacitors to' improve' the hf are added. Here an emitter-follower is added to allow a higher impedance feed to the audio/video balance preset enabling a notch filter at 6Mhz to be used to ensure that no harmonics can get through to the mixer and cause a beat pattern with the audio.The subcarrier is mixed with the video at R17(which can be fixed resistors once set ok) and joins the modulator at point B via screened wire on the original circuit at L3/R5 junction. C5 really should be a non-polarised capacitor, or perhaps two 100mfd back to back. Cx is the main decoupling cap, 470mfd or 1000 mfd in parallel with a 100nF. The supply of +8v comes from the regulated line on the transmitter and should not be exceeded. Another version of the Comtech video board with Black Level Clamp modified for the Worthing/Solent transmitter will be published later. DK.
Click on circuit diagram for pcb layout (pdf)
Farnell parts 10uH=3838780 :C4= 2% disc = 303513 (no 1206 smd found but 0805 will fit) |
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Isolating the Porty/SL55 DC lnb feed when using a yagi or filter rather than a pre-amp.
Cutting the leg of the LM317 regulator does remove the dc at the rear socket, but generates an error message on screen. This mod avoids that happening.
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Run a link wire from Pin11 of PL1 to the end of R203 nearest to front of unit.Cut adjacent track.
Photos and details click here for a pdf. |
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To prevent the on-screen message ' there is an lnb short-circuit' appearing when the dc line to the tuner and aerial socket is either open or short circuit, lift the collector(centre)lead of smd transistor QL3 (just in front of regulator heatsink) and cut the dc line to the tuner by slicing the track just above RT3 (SL55) or CT5 (Porty) with a knife. Dont worry about cutting the thin ground track alongsidethe edge of the board as it is not needed.
(SL55 layout shown)
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