Notes to Check chart 1, 2 and 3

Check chart 1. Aerial and feeder

Check chart 2. Receiver

Check chart 3. Receiver direct pick-up

1. The visual check of the aerial should include the following: a. whether aerial of correct group or type. See Appendix 7. b. Siting/elevation in respect of screening by buildings, trees or other aerials. c. Orientation. d. Polarization. e. Physical condflion - corrosion, loose elements, etc. f. Adequate gain for the location.
2. Checks of the feeder should include: a. whether of low-loss type. b. Condition - corrosion of braid - water penetration. c. Terminations and soldering. d. If pre-amp in use check pre-amp is working satisfactorfly and is the correct type for the channel in use. e. Check any splitter boxes, etc.
3. . Aerial Terminal Voltage. The television broadcasting companies specify minimum field strength levels for the areas served. At the boundary of the service area these field strengths at the vision frequency will, assuming typical aerial gain and feeder loss, equate to a receiver input level of approx. 50 dBuV (Band IV) to 55 dBuV (Band V)

   Howeven within the service area a good signal of nominal 60 dBuV should be expected.

4. . If Aerial Terminal Voltage is low. a. Change aerial position. or b. Fit larger aerial (higher gain), or c. Fit aerial pre-amp.Notes to Check chart 2

   Notes to Check chart 2

5. . The attenuator is to terminate the receiver input with 75 ohms. Use the highest dB value available.
6. . Except in very strong signal areas, interference will probably be entering the receiver from the aerial. Fit a high pass filter or braid-breaker (which prevents unwanted signals flowing down the outer braid of the co-ax feeder) or a filter combining both high pass and braid-breaker characteristics. See Appendices 2,3 and 5 for types.
7. . If it is impracticable to remove the lead from the tuner to the IF. place a short circufl across the input to the IF strip by soldering in a 10 nF disc ceramic capacitor The capacitor leads should be as short as possible.
8. . Toroidal ferrite on mains lead. Remove the mains plug and wind the mains lead on to the toroid as Appendix 2, as close to the receiver as possible.
9. . Mains borne interference. Check that the RF bypass capacitor on the mains input is fitted and is not open-circufl. If the capacitor is satisfactory and the toroid on the mains lead clears the problem. re-wind the toroid so that fl lies inside the receiver between the chassis and the cable strain relief cleat. If only a partial cure is effected, it will be necessary to fit a choke/capacflor input filter

   Notes to Check chart 3

10. Direct pick-up in teed from aerial co-axial socket to tuner.

    The best arrangement is where no teed cable is required since the socket is mounted directly on the metal case of the tuner

    If the socket is not mounted on the tuner then it is important that the qualiW of the lead is good. ie, one with a closely woven screen or a double screened lead. The lead should be as short as practicable.

    The length of the braid between the lead and the socket should be as short as possible, as shown below.

    Tuner

    Direct pick-up in the tuner is usually due either to poor screening of the tuner section or inadequate RF de-coupling of power supply and control leads connected to the tuner.

    Ideally the tuner should be housed in a metal case to provide adequate screening. The majority of modern receivers incorporate tuners that are screened by this method but the tuner lid should be checked to ensure that it is making contact at all points.

    If any of the various leads to the tuner is suspected it may be necessary either to replace it with a screened lead or to fit a feed-through which incorporates an L-C filter, which is more effective than a capacitor feed-through.

    IF section

    Direct pick-up in the IF section usually occurs by the same means as given for the tunen and the same remedies should be applied.

11. . In audio stages using discrete transistors, this is due to rectification at base/emitter junctions of transistor(s) after the volume control. Fit I nF capacitors as in Appendix 4, In extreme cases of AF interference, it may be necessary to break the signal path to the transistor and insert one or more ferrite beads.

    Where an audio IC is used and the volume control is a variable resistor which varies the DC conditions of the IC. check its decoupling capacitor at the IC end of the leads, and if necessary fit a lOnF disc ceramic capacitor.

    A remote possibility is RF picked up on the loud speaker leads. and these should be fitted with ferrite beads at the chassis end. Howeven this sort of pick-up is usually encountered only in hi-fl systems where the speaker leads are of considerable length. Surplus lead lengths may be cut or bundled to reduce pick-up.

12. . This is to check the audio circuitry between the discriminator and the volume control.
13. . This may be due to pick-up at the discriminator IC. Check earthing of screening leads. In extreme cases it may be necessary to make and fit metal screens from tinpiate or metal foil.
14. . Treat as for ll.
15. . Usually due to pick-up at 28 MHz or higher frequencies in the common IF, due either to cross-modulation of the early stages. or to the interfering signal being within the skirts of the passband.

    If will be necessary to fit tuned filters in the IF

16. . Treat each transistor in turn as for 11 until the fault is cleared.