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a stop, whereas cars with alternators don’t seem to have this problem. And why do modern cars generate AC first then rectify to DC rather than just generating DC? What are the pros and cons of both systems? I have read about digital video transmission systems where instead of transmitting complete new pictures they transmit only the difference between frames. This much I understand but what happens when you first turn on your TV; there is no picture to compare with for your first frame? I look forward to some inter­esting answers. (D. H., Kamerunga, Qld). • We know very little about the Cray computer except that it achieves its very high speeds by parallel processing. For these very powerful machines the clock speed is not the measure of speed but rather the number of instructions they can handle per second. Machines like the Cray are rated in “MIPS” which stands for “million instructions per second” and “MFLOPS” which stands for “million floating point operations per second”. Computers with touch screens have a row of infrared LEDs along the top and one side of the screen and matching rows of infrared detector diodes along the other edges. When your finger touches the screen, it breaks one of the horizontal and vertical infrared beams and the logic does the rest, just as in the rows and columns of a numeric keyboard. Note that your finger does not actually have to touch the screen for the system to work. We have little information on vacuum fluorescent displays. They are normally custom designs intended to to be directly driven by the microprocessor in the appliance. Junked displays from VCRs and other appliances are just that – junk. Modern cars use alternators because they are much more efficient than generators and they have no need for a commutator which wears out. When you think about it, a generator actually produces AC and this is “rectified” by the switching action of the commutator. Modern alternators also come with much better voltage regulators and so their voltage output is more constant, regardless of engine revs. The digital video transmission systems you refer to do take an appreciable time to transmit the first frame. After that, the compression algorithm Notes & Errata 40V/3A Adjustable Power Supply, January & February 1994: some readers have experienced difficulty with the wiring of switch S4 and potentiometer VR1. Unfortunately, with multi-turn pots, the pinouts are not necessarily the same for all brands. Usually, the pin arrangement is shown on the body and the correct wiring can be worked out from this. Basically, you only need to find the wiper and connect it to the PC board on terminal 21 as shown on the wiring diagram. Terminal 22 goes to one end of the pot. If the output voltage from the power supply is a maximum when the pot is turned fully anticlockwise and a minimum when rotated fully clockwise, connect the wire from terminal 22 to the other end of the pot. takes over and only transmits the video information which changes from frame to frame. Component substitutions I am building several SILICON CHIP projects at present and have some queries on them. The first concerns the 40V/3A Power Supply described in January and February 1994. What altera­tions would I need to make to use a 10kΩ 10-turn pot in place of the 50kΩ unit specified for VR1? My second query concerns the Nicad Cell Discharger de­ scribed in the May 1993 issue. This specified at BZX79C5V1 5.1V 500mW zener diode for ZD1. All I can get is a 5.1V 400mW type. It this OK? Finally, would the 25W amplifier module described in the December 1993 issue be suitable as a small guitar amplifier. If so, what would I need to add for guitar and microphone inputs. (A. M., Melrose Park, SA). • To replace the 50kΩ pot for VR1 with a 10-turn 10kΩ pot you will need to reduce the value of the 1.5kΩ resistor connected to pin 4 of IC1. The new value should be 300Ω. This can be made up with a 330Ω resistor paralleled by 3.3kΩ. For S4, the switch specified in the parts list is an Altron­ics S-1394 momentary pushbutton type which has the wipers of the double pole switch at one end rather than the centre as is cus­tomary with toggle switches. If a momentary pushbutton switch with the wipers in the centre of the switch is used, the wiring will have to be changed as mentioned on page 71 of the February 1994 issue. Finally, the orientation required for S4 on the wiring diagram is with the common terminals facing the mains switch S1. 12-240VAC 200W Inverter, February 1994: the 1kΩ resistor which connects to pin 6 of IC3 on the overlay should be 10kΩ as shown on the circuit. Use of a 1kΩ resistor will cause the inverter to shut down prematurely. In the Nicad Discharger, you can substitute a 400mW or 1W zener diode for the specified 500mW type. The 25W amplifier featured in the December 1993 issue would be suitable as a guitar amplifier. If you need a preamp, the best approach would be to use the 4-Channel Guitar Mixer featured in the January 1992 issue. This is available as a kit from Dick Smith Electronics, Jaycar Electronics or Altronic Distributors in Perth. TL496C IC is hard to come by As an electronics hobby buff, I decided to make the Induction Balance Metal Locator from the May 1994 issue. I like making my own PC boards and gathering the necessary compon­ ents. I asked Coffs Harbour Electronics to get the TL496C con­verter and went ahead with the construction. Some time later, I was informed that this IC is only available with a kit. Is there a substitute or perhaps a supplier not known to Coffs Harbour Electronics? (D. K., Coffs Harbour, NSW). • This IC (TL496C) is very hard to come by and is not cheap. We have not been able to locate a retail source. Your best ap­proach may be to buy the kit from Jaycar at $13.95 (Cat KC-5122). October 1994  93