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ranges, I only get 5mA or so test current for both. It should be 6.3mA for 63V and 10mA for 100V. What should I do? (A. J., via email) ● The test voltages at those settings could be incorrect, or the step-up circuit isn’t able to supply enough power. Check the open-circuit voltage with a multimeter to see if 63V and 100V are being generated. If not, check the divider resistors at pin 5 of IC1 for correct values. If the voltages are correct, then with the multimeter connected to the battery ground and the positive test terminal, short the test terminals. The voltage shouldn’t drop. If it does, perhaps the windings on transformer T1 are incorrect or the insulation on the Advertising Index Altronics.................................23-26 Beware! The Loop......................... 8 Blackmagic Design....................... 5 Dave Thompson........................ 103 Emona Instruments.................. IBC Jaycar............................. IFC, 51-54 Keith Rippon Kit Assembly....... 103 Lazer Security........................... 103 LD Electronics........................... 103 LEDsales................................... 103 Microchip Technology.............OBC Mouser Electronics....................... 3 OurPCB Australia........................ 11 PCBWay......................................... 7 PMD Way................................... 103 SC Bridge Rectifiers.................... 83 Silicon Chip PDFs on USB......... 50 Silicon Chip Shop................ 67, 77 Silicon Chip Subscriptions........ 59 The Loudspeaker Kit.com............ 9 Used Gear.................................. 103 wires has not been properly scraped off where it is soldered to the PCB. How to add a volume control to any amplifier My question is around fitting a potentiometer for volume control on the Hummingbird Amplifier (December 2021; siliconchip.au/Article/ 15126). I bought a single kit of the Hummingbird Amplifier from Altronics (Cat K5158). I have been through the instructions and read the last two pages about setting it up. I just want to set it up and run it with one channel of audio from ±15V DC. I bought a 30V centre-tapped transformer (Jaycar MM2005). I have built the Universal Power Supply board and I’m going to run it with the ±15V DC configuration. Where do I fit a potentiometer to control volume? I couldn’t see any mention of this in the instructions. Do I need to buy a separate volume control module? (E. M., Hawthorn, Vic) ● Referring to the accompanying circuit: 1. Connect the input signal ground to the ground of CON2. 2. Connect the input signal conductor to the clockwise end of the potentiometer track (‘B’) via a series capacitor. With the pot shaft facing you and the pins down, this will be the righthand pin. 3. Connect the pot wiper (middle pin, ‘W’) to the signal input terminal on CON2. 4. Connect the remaining pot pin (‘A’) to the signal ground (either at CON2 or the input connector, whichever is convenient). The value required for the series capacitor depends on the potentiometer value. For example, if using a 10kW potentiometer, use a minimum of 2.2µF (we’ve shown it as 10µF in the circuit, which allows a non-polarised electrolytic to be used). You could use a 2.2µF greencap or similar. You can get away with lower values Next Issue: the February 2025 issue is due on sale in newsagents by Thursday, January 30th. Expect postal delivery of subscription copies in Australia between January 24th and February 7th. Silicon Chip SMD markings are inconsistent I bought the SC6988 SMD kit for the Discrete Ideal Bridge Rectifier (September 2024 issue; siliconchip.au/ Article/16580). I got a bit stuck trying to identify the numerous SOT-23 parts. The confusing aspect is that if you search for a BC856, for example, you might find a package code of “3D”, depending on the variant, not “9AC”. By searching on the package codes, I think I have correctly identified the parts – it would be useful for others to have these codes – providing you continue to provide the same variants in the kit. Qty Code Device 2 1D MMBTA42 2 2D MMBTA92 4 9AC BC856CMTF 4 1C BC847C 4 Y2 BZT52C12/BZX84C12 Is that correct? (I. T., Duncraig, WA) ● The codes you list look correct. It is unfortunate that the same part from different manufacturers can have different markings. Each part should ideally have a standard marking for a given package. The BC84x we supply is actually a BC846C and it is indeed marked 1C. We have BC856Cs marked both 15S and 9AC; obviously, the latter is what we used for your kit. Y2 is the code listed in the Fairchild data sheet for the BZX84C12. Our supplier lists the MMBTA42 as 1D (the ST Micro data sheet says it can also be A42) and MMBTA92 can be 2D or A92. SC You can add a volume control to most amplifier modules like this. Wagner Electronics..................... 10 104 if the pot resistance is higher, but don’t make it too high, or it will introduce noise. 5-20kW is ideal for a low-noise amplifier, although value at the lower end of that range will load the signal source more. Most modern signal sources shouldn’t be bothered by that. If any of this requires significant cable runs, use shielded cable. You can use the shield to make the ground connections. Australia's electronics magazine siliconchip.com.au