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• While the MJL21193/94 transistors from ON Semiconductor are recommended, they are now obsolete and difficult to get. We recommend the ON Semiconductor NJW21193/94 transistors instead. You could use MJL1302/MJL3281. As you mention, they have a higher cut off frequency. This might or might not be a problem. They may give better performance, but it’s also possible that the amplifiers could oscillate. If you find the DC fuses are blowing for no reason, try increasing the value of the 100pF 500V compensation capacitor. However, as that value is quite high, it should be OK. According to the circuit, you need seven of each type of transistor, regardless of which types you use. Power supply cable stripe polarity I use figure-8 speaker cable (eg, Jaycar WB1703) in many projects. Should the black stripe for polarity identification be used for positive or negative? I originally thought black indicates negative, but then I noticed that most plugpacks have the striped side of the wire indicating positive. Now I’m thinking that as positive is the wire I want to be identified, perhaps I should always use the black striped wire for positive. I use the wire for motors and Arduino projects, but if anyone was connecting speaker wire to the black and red terminals on a speaker box, surely the wire with the black stripe would go to the black terminal. Is there a standard for this? (J. B., Benalla, Vic) • If you have red and black wires, then usually red would be positive and black would be negative. But it’s a bit more tricky when you have a stripe. Sometimes you have a white stripe, sometimes a red stripe and sometimes a black stripe (and possibly other colours). Usually, the stripe is used to indicate positive, but that certainly is confusing when the stripe is black. Ultimately, it doesn’t matter as long as you are consistent so that there is no confusion. As you say, plugpacks tend to use the stripe for positive (usually a white stripe, though), so it would make sense to follow that convention. Probably the best solution would be to use Jaycar Cat WH3057, WH3087 or similar cable which has red and black insulation for the two wires in the cable. Flashing lights wanted for model railway I am trying to find a railway crossing flashing light kit, or at least a PCB for it. I am sure I have seen something like it in past magazine issues. I have searched your site without success. Could someone point me in the right direction? (P. C., via email) • We published a two-lamp flasher circuit (January 1998; siliconchip. com.au/Article/4748). You can download its PCB pattern from the following page: siliconchip.com.au/ Shop/10/2362 Jaycar also sells a kit for that project, Cat KJ8070. This design runs from 12V and so is suitable for 12V SC lamps. Advertising Index Altronics...............................89-92 Ampec Technologies................. 19 Dave Thompson...................... 111 Digi-Key Electronics.................... 3 Emona Instruments................. 101 Jaycar............................ IFC,53-60 Keith Rippon Kit Assembly...... 111 LD Electronics......................... 111 LEDsales................................. 111 Microchip Technology.............. IBC Mouser Electronics...................... 7 Ocean Controls......................... 39 Rohde & Schwarz.................. OBC SC Micromite BackPack............ 47 Silicon Chip Binders............... 106 Silicon Chip PDFs on USB..... 111 Silicon Chip Shop.................. 100 Silicon Chip Subscriptions....... 88 The Loudspeaker Kit.com......... 99 Tronixlabs................................ 111 Vintage Radio Repairs............ 111 Wagner Electronics..................... 5 Notes & Errata Balanced Input Attenuator for the USB SuperCodec, November-December 2020: the photo shown halfway down the lefthand column on page 71 of the December 2020 issue, showing the wiring to the power connector, is incorrect. The positive (red) wire should be shown going to the bottom-most pin in the socket, with the black (negative) wire to the top. Also, in the circuit diagram on pages 50 & 51 of the November 2020 issue, the centre (ground) pin of CON3 at upper right should only be connected to the junction of the two zener diodes, the negative end of the 100µF capacitor next to switch S1 and the negative ends of all relay coils. The junction between this ground and the other grounds in the circuit is on the main SuperCodec board. Two LED Christmas Stars, November 2020: in the parts lists on page 41, there is no such part as a 75HC595. It should read 74HC595 instead. D1 Mini LCD BackPack with WiFi, October 2020: in the circuit diagram (Fig.1), the connections to pins 7 & 8 on the LCD module via CON1 are swapped. The drain of Q1 should go to pin 8 (LED) while pin 7 is the display SCK line and also connects to pin 10 on the LCD module and on to the D5 pin of MOD1. The February 2021 issue is due on sale in newsagents by Thursday, January 28th. Expect postal delivery of subscription copies in Australia between January 27th and February 12th. 112 Silicon Chip Australia’s electronics magazine siliconchip.com.au