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voltage than I seem to remember. I’m not just talking about it moving about a bit with temperature; if I just tweak the 25-turn trimpot the tiniest bit, the voltage can jump 2mV up or down. After going back and forth every half hour or so today, I’ve fluked it and got it sitting at 4.6mV, which is OK, I guess. Could the trimpot be damaged by the DC component flowing through it? Seeing that this amp destroyed itself after a lot of use, and now this bias thing, I’m wondering if the two are connected. (T. B., Bumberrah, Vic.) ● It doesn’t seem common for SC200 amplifier modules to blow. From the feedback we’ve received, they are pretty reliable. There can be many causes of such a failure, including a faulty component or a solder joint that went bad (possibly due to thermal cycling). Having said that, the adjustment shouldn’t be that sensitive. We think the quiescent current adjustment Advertising Index Altronics.................................31-34 Blackmagic Design....................... 7 Dave Thompson........................ 103 DigiKey Electronics..................OBC Emona Instruments.................. IBC Jaycar............................. 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That certainly could explain why it blew up. If the bias was jumping around during operation, it could have gone into thermal runaway. We suggest you replace the trimpot. If you want to, you could make it a bit more robust by shunting the trimpot with a fixed resistor so that it’s operating near the bottom of its range. That way, even if the trimpot goes open circuit, the bias will only go up a little. Speed controller fails after motor stall I built two 230V/10A Speed Controllers for Universal Motors (Feb & Mar 2014; siliconchip.au/Series/195) from Jaycar KC5526 kits. I have been using them on my porting bench for quite some time. Unfortunately, both failed in the same way. The motor stalled at quite low RPM due to the cutter getting stuck, the fuse blew, but when I replaced it, the unit still didn’t work. Do you know which part most likely failed so I can replace it? I absolutely love the ability of these speed controllers to give good torque at low RPM. (S. G., Narre Warren, Vic) ● Most probably IGBT Q1 (STGW40N120KD) has gone open circuit. We can supply a small set of parts (siliconchip.com.au/Shop/20/2614) that includes the IGBT, diode, driver IC and NTC thermistor. It would be worth replacing the IGBT, diode and driver IC in case any of the others were also damaged. Jaycar has discontinued its kits for this project, but we can supply the PCB, programmed microcontroller and the set of parts mentioned above. They’re all listed at siliconchip.au/ Shop/?article=6120 Our latest mains motor speed controller is the Refined Full-Wave Motor Speed Controller (April 2021 issue; siliconchip.au/Article/14814). CDI with a wasted spark ignition system I’d like to use either the High-­ Performance CDI Ignition from September 1997 or December 2014 on my late 1970s/early 1980s Suzuki 4-­cylinder, 4-stroke motorbike engines. That vintage of Japanese engines had two coils for the four cylinders; one would fire cylinders 1 & 4 simultaneously, while the other fires cylinders 2 & 3 simultaneously, 180° of crankshaft rotation later. On the 1970s vintage machines, the 12V signal went direct from the crank sensor (points or hall effect) to the coils, whereas on the 1980s machines, a CDI ‘igniter’ unit was used (between the crank sensor and coils) to boost the spark energy. Can either of your CDI multi-spark units be adapted to work in this situation? It would require two inputs (from the Hall effect crank rotation sensors) and two outputs to each coil. (P. H., Seattle, WA, USA) ● You would need to build two of either version of the CDI unit, one to drive the coil for cylinders 1 & 4 and the other for cylinders 2 & 3. It may work if you just build a single high-voltage supply in one of the units and supply this voltage to the other unit as well. Then use the two separate trigger circuits for driving the SC individual coils. Errata and on-sale date for the next issue RGB LED Star Ornament, December 2025: in the circuit diagram, pin 2 of CON3 should connect to pin 11 of IC1 before the 330W resistor, rather than after. Power Electronics part 2, December 2025: in Fig.4 on page 32, the labels Zr and Zc are swapped in the high-pass filter circuit. Digital Preamplifier, October 2025: in Fig.5 on page 36, pin 3 of the ADAU1467CORE BOARD connector should be labelled ADC_BCLK to match pin 73 of IC18. Also, in the pinout for Q1, the pins should be labelled (left-toright) B, C & E and the tab is C. On p39, in Fig.8, pins 13 & 15 of IC15 go to IC17, not IC12. Finally, the designator CON8 for the 8×2-pin header that’s the alternative to CON19 is not to be confused for the RCA output connectors, CON8.1-CON8.4. Next Issue: the March 2026 issue is due on sale in newsagents by Tuesday, February 23rd. Expect postal delivery of subscription copies in Australia between February 20th and March 11th. Australia's electronics magazine siliconchip.com.au