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different preamps. Interestingly, with the RCA input plugged in, there is no hum and the amp is dead quiet. I measured 4.7V DC and 14mV AC across the speaker output (without the speaker connected). I’m assuming that is not good! The good amplifier measures only 3mV DC and 0V AC across the output. Since I have two of these amps operating independently (left channel and right channel), it’s easy to compare good and bad. I checked the DC voltage across many of the resistors, comparing them with my good working side. The voltages match on both apart from one resistor, the 10W resistor from the negative of the input connector to ground. On the good amp, this has 0V DC across it, but on the bad side, it has 4.7V DC across it. Hopefully, that can narrow down the root cause. (Murphy, via email) ● That 10W resistor has gone high-­ resistance or open-circuit. Replace it, Advertising Index Altronics.................................37-40 Dave Thompson........................ 111 Digi-Key Electronics...................... 3 Emona Instruments.................. IBC Hare & Forbes............................. 11 Jaycar........................ IFC, 9, 13, 15, ............................26-27, 53, 83, 107 Keith Rippon Kit Assembly....... 111 LD Electronics........................... 111 LEDsales................................... 111 Microchip Technology.................. 5 and we suspect the amplifier will be working again. As for the cause, we think when you were plugging and unplugging the preamps, somehow they must have applied a significant potential above or below Earth to the shield that delivered quite a bit of current through that resistor, burning it out. Perhaps due to an Earth loop. Modifying Capacitor Discharge Ignition I have some questions about the CDI system (September 1997; siliconchip. com.au/Article/4837): 1. Could I replace Q6 and Q7 with IGBTs to handle larger currents/additional leeway? 2. Could I use a 1MW bleeder resistor instead of the varistor and 680kW resistors? 3. How did cars run with this system? 4. My car (1979 MGB) has points. Would the points trigger circuit and the +12V signal from the points make the tachometer signal unnecessary? I think I could just connect the tachometer wire to the points terminal on the PCB. 5. The transformer can produce 400V. Do you see any problems with stepping up the voltage? I added an extra 33kW resistor, thinking that would keep the current flow similar to the original version. (J. M., New Haven, CT, USA) ● 1. There is no need to replace the Mosfets with IGBTs, but you could do so if you prefer. 2. The varistor is required. The two 680kW resistors are connected in series to obtain a sufficient voltage rating. Mouser Electronics..................OBC Ocean Controls........................... 12 Silicon Chip Binders................ 111 Silicon Chip PDFs on USB......... 75 Silicon Chip Shop.................98-99 Silicon Chip Subscriptions........ 52 Silicon Chip VGA PicoMite...... 109 The Loudspeaker Kit.com............ 8 Tronixlabs.................................. 111 Wagner Electronics..................... 14 112 Silicon Chip Errata and Next Issue Rohde & Schwarz.......................... 7 You could replace both with a single 1MW VR37 type high-voltage resistor. 3. Cars ran very well with this ignition system, especially during cold starts. 4. Your tachometer might not work with the CDI since it is more likely an impulse tachometer that relies on the high voltage produced as the points open on a standard Kettering ignition. You probably need to use the tachometer circuit shown in Fig.13 of that article that uses a transformer to step up the voltage for the tachometer. 5. Increasing the voltage from 300V to 400V could cause the CDI capacitor to fail as well as many of the other components. The transformer may also arc over internally. We did not design the circuit for 400V. The biggest challenge in making it work reliably at 400V DC would be obtaining a suitable CDI capacitor. Note that we have published several CDI systems since 1997, including the popular High-Energy Multi-Spark CDI for Performance Cars (December 2014 & January 2015; siliconchip.com.au/ Series/279). Finding past articles Some time ago, you published a feature on replacing the sacrificial anode in hot water systems. Could you tell me what issue this feature was in? (J. H., Nathan, Qld) ● You can search our article database here on our website at siliconchip.au/ Articles/ContentsSearch Entering “anode” in the Name field and pressing the Search button gives the following result: November 2012: Feature: Sacrifice Your Sacrificial Anode by Leo Simpson (siliconchip.au/Article/417). SC History of Op Amps, August 2021: in Figs.13 & 14 on p43, the 2π factors should be in front of the square root symbols, not within them. AVO Valve Testers, August 2022: on page 92 the text refers to potentiometer VR2 as applying the specified grid voltage, this should instead read VR5 to match Fig.4. Similarly, in the paragraph above, RLY1 should be RLYA. iSoundbar with Built-in Woofer, August 2022: the 1.2m lengths of DAR pine in the parts list should be 1.24m long to match the width of the sound bar. Also, the woofers are shown wired incorrectly in Fig.7; the two woofers should be wired negative-to-negative with the negative amp output and external subwoofer terminal going to the positive terminal of the left-hand woofer so they are phased correctly. Next Issue: the November 2022 issue is due on sale in newsagents by Thursday, October 27th. Expect postal delivery of subscription copies in Australia between October 25th and November 14th. Australia's electronics magazine siliconchip.com.au