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application would be the March 1990 design which featured a 15-second sloshing delay. All the circuits referred to were published in the Circuit Notebook pages and so no PC board designs are available. Query on variable ignition timing My query is related to John Clarke’s Knock Indicator for leaded-petrol engines, as featured in the April 1996 issue. I own a 1980 Volvo 264 GLE with the Bosch K-Jetronic mechanical fuel injection system and the Bosch contactless electronic ignition. This is the V6 engine used in Volvo, Renault & Peugeot cars and in some of the later Volvos in an EFI version. As was usual in those days, the engine was designed to run on 97RON leaded petrol and had a reasonably high compression ratio. I had to do a recent valve regrind and decoke on the engine and having restored it to full compression again, it now “pings” at a specific rev range on a very light throttle, exactly as John Clarke has described. I define a “ping” as more a “breaking glass” type sound as opposed to a “knock” or heavy “pinking”. Fortunately, it does not knock or ping at any other point except at about 3000 revs with a high manifold vacuum, which equates to just over 80km/h. If the vacuum advance is disconnected, there is no ping at all, confirming that somewhere along the extra 10 degrees of advance provided by the vacuum unit, it is too advanced for the lower lead petrols available today. However, the vacuum unit is necessary for good performance through­out the rev range. This engine/ignition has a very aggressive advance curve with very high advance figures in the mid to higher rev range. I have just purchased the Knock Sensor kit from Jaycar electronics after seeing just such a kit in operation on another car and am impressed by its performance. Before I put this kit together, may I ask for the possibility of an extension to the design? The circuit provides 10 points of reference when pinging occurs, to trigger the LED displays. Could one or all of these points be utilised to trigger a time-delay circuit inserted between the distributor and the electronic ignition amplifier unit to effectively Multimedia power worries I am interested in making the Multimedia Amplifier featured in the October & November 1996 issues of SILICON CHIP. However, I am concerned because the circuit has been described to me as a most irresponsible design as it takes absolutely no account of the limitations imposed by most computer power supplies. Is this a reasonable comment? What would be the maximum additional load that the power supply would be expected to deliver during normal use of the amplifier? (L. S., Kenthurst, NSW). As far as power supply capacity is concerned, we would not have published the project if there was any risk of running computer power supplies into overload. The typical computer power supply these days is rated at 250 watts and this usually includes 5V 20A (100W) and 12V 12A (144W) supply rails. A typical 1.6GB drive consumes 0.4A at 5V and 0.27A at 12V DC. A typical CD-ROM • retard the spark timing by a specific factor when pinging occurs? Once the pinging ceases, the delay unit would pass the distributor impulse straight through the amplifier as normal. This would have the effect of dynamically modulating the impulse signal only if and when required and would alleviate the need to start tampering with the advance weight springs in the distributor. Please believe me when I tell you (through experi­ence) that tampering with the advance curve in this manner on an engine like this is a horrendous task, even with access to the appropriate distributor timing machine levels. Besides, I only have the problem at a specific rev range. I appreciate that I could fit the entire electronic kit described in a previous edition of SILICON CHIP and do away with the existing ignition control unit but arriving at the desired advance curve with this new unit is a very daunting task. Bosch Australia is a very helpful organisation but they just drive might pull 1.8A at 5V and 1.5A at 12V. Unless your computer is loaded up with lots of accessory cards and has perhaps two or three hard disc drives, CD-ROM, etc your computer’s power supply will have lots of current capacity to spare. By way of comparison, we would expect the total current drain of the multimedia amplifier card to be less than 250mA for virtually all of the time. Even if all amplifiers were driven into serious clipping simultaneously, which should never happen, the total current drain would be no more than about 2A at the maximum. If you did manage to drive the multimedia amplifiers into serious overload, their own protection circuitry would quickly shut them down. In any case, your computer’s power supply has over­load protection and in the event of a serious overload, which is very unlikely to be due to the multimedia amplifier card, the worst that might happen is that your hard disc might slow down momentarily. do not have all this data for the older cars (I have tried). (G. D., Berowra Heights, NSW). Your suggested concept of using the Knock Indicator to modulate the ignition advance curve is certainly feasible and is the same principle as used in cars with full engine management systems. However, designing such a system with hardware would be quite complex. Nor could we necessarily produce a circuit which would be compatible with the many ignition systems available. • Notes & Errata Multimedia Amplifier, October 1996: There is an error with the test procedure for the PC board. Without the power link in­stalled, pins 1 and 9 of IC3, IC4 and IC5 are at about +0.5V and pin 7 is at +12V. No voltage is present on the other pins. With the link in, pins 1 and 9 are at about +2.2V; pins 3, 4 & 6 are at +5.6V; and pin 7 SC measures +12V. July 1997  93