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Building the induction balance metal locator I am building an induction balance metal detector to locate metal marker pegs in rough terrain. I am using the circuit pub­ lished in the May 1994 issue of SILICON CHIP but I am having difficulty obtaining the TL496C (8pin DIL) voltage converter. Could you please tell me where I can obtain one from, or an equivalent? (G. C., Christ­ church, NZ). •  The TL496 can be obtained in Australia from Farnell Elec­ t ronic Components, provided you have an account or a credit card. Their phone number is 612 9645 8888. Failing that, you could try obtaining it from the Motor­ ola or Texas Instruments distributors in New Zealand. Finally, you may consider building the metal locator without the TL496 and just use a 9V battery supply instead, comprising six AA cells. Background hum in Dolby Decoder I have just finished the Dolby Pro Logic Surround Sound Decoder Mk.2 featured in the October & November 1997 issues. During the soak test, I am happy to report that every­thing works just as outlined in the article. The only problem I have is that there is a lot of background hum. I have used earth shielded cable in every place outlined in the article. The case earth from the mains plug is present and most important, the 0.47µF cap was soldered between the signal earth and mains earth. It doesn’t matter what setting you have any of the switches in, the hum gets worse as you increase the main volume con­trol (VR1). With the volume at zero, the hum has all but disap­peared. I built up six kits of the first Pro Logic Decoder you presented and teamed these up with 50W power amplifiers which I also mounted in the same case and did not experience any noise or hum at all. They were very quiet. What do you think is the prob­lem? Could it be radiation between the transformer and the pro­cessor board IC6? Would it help if I use a piece of metal to screen the two from each other? I have carefully checked all earth shielded connections from the pots and other various points on the main PC board. I would like to comment by saying that having all the audio cable connections in the centre of the main PC board is very untidy and hard to achieve. Would it help if I remove the large transformer and the three power amplifiers from the case altogether and mount them in a separate case and refit a much smaller transformer in the decoder case? If so, what size transformer should I use? (K. S., Morphett Vale, SA). •  Your hum problem is almost certainly an earthing problem. We do not recommend completely rebuilding the unit with the power amplifiers and transformer in another case. This would be com­ pletely unnecessary since our prototype unit was as quiet as the first version of the Pro Logic Decoder. First, check the isolation between the heatsink tab on the power amplifiers to case, using a multimeter set on “ohms”. There should not be any connection and the meter should show a high resistance or open circuit. If the resistance is low, check the insulating washer between case and the amplifier tab and also check the insulating bush. Next, check that the signal earth (the shields on the audio leads) do not connect to mains ground by checking for resistance to the case TOROIDAL POWER TRANSFORMERS Manufactured in Australia Comprehensive data available Harbuch Electronics Pty Ltd 9/40 Leighton Pl. HORNSBY 2077 Ph (02) 9476-5854 Fx (02) 9476-3231 of the decoder. Finally, check the earthing to the power amplifiers. We have shown shielded wire connecting from the decoder board to the power amplifier board. Make sure that the shields connect the signal earth to the amplifier board as shown on the wiring dia­gram. Notes & Errata Nicad Zapper, August 1994: experience has demonstrated that this circuit does not dump the capacitors’ charge reliably if the supply rail is less than about 12V. Also, the test procedure involving a 2.2Ω dummy load should be changed to 0.22Ω. Some variants of the MTP3055 have also proved to be unreliable. We recommend the MTP3055E, made by Motorola. Less well-known brands can be suspect. 5-Digit Tachometer, October 1997: the PC component diagram on page 25 has link LK1 incorrectly labelled. In fact, the unla­belled link next to it, connecting to pin 6 of IC5, is LK1. SC WARNING! SILICON CHIP magazine regularly describes projects which employ a mains power supply or produce high voltage. All such projects should be considered dangerous or even lethal if not used safely. Readers are warned that high voltage wiring should be carried out according to the instructions in the articles. When working on these projects use extreme care to ensure that you do not accidentally come into contact with mains AC voltages or high voltage DC. If you are not confident about working with projects employing mains voltages or other high voltages, you are advised not to attempt work on them. Silicon Chip Publications Pty Ltd disclaims any liability for damages should anyone be killed or injured while working on a project or circuit described in any issue of SILICON CHIP magazine. Devices or circuits described in SILICON CHIP may be covered by patents. SILICON CHIP disclaims any liability for the infringement of such patents by the manufacturing or selling of any such equipment. SILICON CHIP also disclaims any liability for projects which are used in such a way as to infringe relevant government regulations and by-laws. Advertisers are warned that they are responsible for the content of all advertisements and that they must conform to the Trade Practices Act 1974 or as subsequently amended and to any governmental regulations which are applicable. April 1998  93