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one of the output power transistors, an NEC D587. Unfortunately, this has stumped the locals, who don’t require a lot of stumping, if I may say so. If you can help me in sourcing a supplier I would be most grateful. The only reason the speakers go is because my other half is getting hard of hearing. A hearing aid would be cheaper but this is a no-go area. Hope you can help. (H. F., Perth, WA). • We suggest that you consider in­ stalling PTC polyswitch thermistors in series with the speakers. These go temporarily open circuit if their current rating is exceeded. We suggest you try installing a type RXE090 or RXE110 PTC thermistor. These are available from Jaycar Electronics in Perth. Phone (09) 328 8252. As far as the output transistors are concerned, why not contact your local Mitsubishi service agent? They are at 329 Collyer Road, Bassandean WA 6054. Phone (09) 377 3400. Troubleshooting the Insulation Tester The Insulation Tester described in the May 1996 issue of SILICON CHIP is a very useful piece of equipment but I have found that the voltages produced are somewhat lower than expected. The 100V, 250V and 500V ranges all produce about 150V. The 600V and 1000V ranges both produce about 450V. This is an improvement upon the 19V, 148V, 465V, 540V and 290V being produced from the respective (lowest to highest) voltage ranges after first completing the project but still not optimal. I effected this improvement by replacing the CMOS oscillator chip as I had noticed that the CRO traces from the oscillator when the 100V range was selected were almost nonexistent. Could the replacement IC be faulty as well or is there a problem with the error amplifier? Keep those great projects coming. (N. P., Seven Hills, NSW). • Your problem is possibly due to incorrect transformer wind­ings. Check that the secondary is wound in the same direction as the primary wind­ ing as shown in the diagram of Fig.4 (page 35, May 1996). Try winding the secondary in the opposite direction to the way it was. In addition, check that the range Dud micro in Dolby decoder I have recently purchased the Dolby Pro-Logic Surround De­ coder, Mk 2, as described in the November & December 1995 issues. My problem is that, upon powering up the unit, the dis­play does not flash “—” at all and the relays do not change state. In addition, the noise LED does not light when the noise sequencer button is pressed. I have measured the voltages on the power supply module and all are ±5%. I have also measured the voltages on ICs 1-9 as indicated on page 78 of your magazine article (bearing in mind the errata previ­ ously advertised). I have identified that when the unit is powered up, the measured voltage at PC0 is 0V and remains at 0V; ie, it does not go high. Hence Q1 does not switch on to energise the relays. I have re­moved IC6 and applied 5V to each of the 7-segment LEDs and ascertained that the 7-segment displays are not burnt out but are in good working order. resistors on switch S2b are arranged on your PC board in the correct order. Flatpack transistor washers I am looking to use the new flatpack MJL21194/21193 tran­ sistors as fea­ tured in the April 1996 issue but am having trouble finding the isolating mica/silicon washers. The magazine article shows a washer bigger all round than the transistor. Where did you get yours? (R. G., Chapel Hill, Qld). • You can obtain these washers from Altronics in Perth. They have two types: silicone/fibreglass Cat. H-7220 x 4 or mica type Cat. H-7120 x 4. You can phone Altronics on 1 800 999 007. Trigger happy laser pistol user I am writing in regards to the laser pistol and electronic target described in the December 1996 issue of SILI­ CON CHIP. Could you please suggest a way to alter the circuit so that the Each of the switches S5, S6 and S7 produce 5V at pins 17, 18 and 19 of IC6 respectively when not pressed and 0V at the same pins when the respective switches are pressed. Pin 2 of IC6 remains at 5V all the time. I have noted that the voltage sig­ nals for “B, A, E, R, S, D” are 5.4V, 1.0V, 2V, 2V, 2V respectively and that they do not change under any condition. I have noticed, though, that IC6 was supplied as an MC­ 68HC705C8ACP, not an MC68HC­ 705C8P. I assume that this is just another variation of the MC68HC­ 705C8P microprocessor chip and is a valid substitute. Could you please help me identify what is the problem with this kit? (C. C., Leem­ing, WA). • It appears that the microproces­ sor (IC6) is either not programmed or faulty. If it is programmed it will be marked accordingly. Either way, the microprocessor should be replaced. The ACP version is slightly different to the P version but we have programmed them to accommodate this difference. laser will remain on constantly when­ ever the trigger is pressed down? It occurred to me to short the 1.5kΩ resistor but I figured this would still create a pulse because of the 100µF capacitor. Your help in this matter would be much appreciated. (J. N., Greenacre, NSW). • As you suggest, shorting the 1.5kΩ resistor will allow the laser to stay on while ever the trigger is pulled. The 100µF capacitor can then be omitted. Notes & Errata MultiMedia Loudspeakers, November 1996: the perspective diagram on page 61 shows the wrong enclosure depth; it should be 224mm. Control Panel For Multiple Smoke Alarms, December 1996: a 47kΩ re­ sistor should be added to the circuit between pin 11 of IC5f and the +9V rail, while one of the 100µF bypass capacitors on the +9V rail should be 10µF. Note also that the parts list should show four 1kΩ resistors (not SC three). February 1997  93