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seems to work OK except with AA cells. When I run the calibration (Screen 6), I get a “scan failed check battery” error. It also does not show EXT on the top line. I am using a brand new Energiser Max battery; calibration works fine with a 12V battery and shows EXT. Running a test on the AA cell gives very erratic and unexpected results; sometimes I get a reading of 1.3W, but I also get occasional “I too high” or “V too low” errors. I have checked the cell, and it can deliver 7A when short-­ circuited. With an old 12V 18Ah SLA set to 20A, I get 17A and 11% drop, which seems about right. Thanks for a great magazine and great articles. I am 79, so this project was a real test of eyesight. I certainly would not want to try anything with much smaller components, but I guess I can put off a new eyesight test for a bit longer. (T. O., Ngaruawahia, NZ) ● We’re happy to have a look over Advertising Index Altronics.................................45-48 Blackmagic Design....................... 5 Dave Thompson........................ 103 Emona Instruments.................. IBC Jaycar.................. IFC, 10-11, 24-25 Keith Rippon Kit Assembly....... 103 Lazer Security........................... 103 LD Electronics........................... 103 LEDsales................................... 103 Microchip Technology.............OBC Mouser Electronics....................... 3 OurPCB Australia.......................... 8 PCBWay......................................... 7 PMD Way................................... 103 SC Dual Mini LED Dice.............. 103 SC Ideal Bridge Rectifiers........... 77 Silicon Chip Kits........................ 75 Silicon Chip PDFs on USB......... 34 Silicon Chip Subs...................... 61 Silicon Chip Shop.................86-87 The Loudspeaker Kit.com............ 9 Wagner Electronics..................... 93 104 Silicon Chip your construction if you can email a close-up, in-focus digital photo. The EXT message simply means that the Checker is powered from the battery under test (BUT). If EXT is not showing, we expect this to be showing 8-9V to indicate the health of the 9V battery that is powering the unit. The 1.3W reading seems quite high even for a AA cell. If it can deliver 7A, its internal resistance must be no more than around 200mW. So we think you have extra resistance in the circuit when you are testing AA cells and that is affecting the readings. Consider what is different between when you check the AA cell and the 12V SLA battery. The reader followed up with a find­ ing that the cell holder was presenting a high resistance that interfered with calibration. He was able to complete the calibration by holding the leads from the Battery Checker directly against the cell’s terminals while run­ ning the calibration sequence. 500W Amplifier design question Thank you for the fine publication that I look forward to reading each month! I am thinking of building the 500W Amplifier Module (April & May 2022; siliconchip.au/Series/380). Looking at the circuit diagram on page 30 of the April 2022 issue, I see that a 33kW resistor is connected to the wiper of trimpot VR1. Having built many Silicon Chip amplifiers in the past, this value looks too high to me, and I’m not sure if much/enough current is flowing through transistors Q1 & Q2, despite the fact that the power supply rails are ±80V. I can’t see any errata regarding an error. Is this value definitely correct? What is the design value of the collector current in Q1 and Q2? (J. D., Endeavour Hills, Vic) ● The current through Q1 and Q2 is set by the voltage across the 470W resistor at the emitter of Q5 to around 1.5mA, assuming a 0.7V base-to-­ emitter voltage for transistors Q5 and Q7. The 33kW resistor does not set the current; it is present merely to reduce the dissipation in Q5 by sharing some of the voltage drop that would otherwise be between the collector and emitter of Q5. If you multiply the 1.5mA current from Q5 by the 33kW resistor value, you will see that there will be around 49.5V across the 33kW resistor, giving a dissipation of 74.25mW (49.5V2 ÷ 33,000W) in that resistor. Since Q5’s emitter is at around +79V and the emitters of Q1/Q2 only swing a few volts above ground, you can see that there will be around 25V between Q5’s emitter and collector. That’s plenty of headroom, so the 33kW resistor won’t have any effect on the circuit’s operation except to let Q5 run a bit cooler. VHF aircraft radio receiver circuit wanted Do you have any articles or kits for receiving radio transmissions on aircraft frequencies? (A. L., Saratoga, CA, USA) ● The only article we have for an aircraft receiver is in the Circuit Notebook section of the December 2008 issue, titled “VHF Aircraft Receiver With Squelch” by Dayle Edwards SC (siliconchip.au/Article/2029). Errata and on-sale date for the next issue Four-colour e-paper display, Circuit Notebook, November 2025: diode D1 is shown backwards in the circuit diagram. Its cathode should be on the right. RP2350B Computer, November 2025: the pin numbering for CON8 in the circuit diagram, Fig.1, is mirrored compared to Table 1 and what’s shown on the back panel. The function of each pair of pins (1 & 2, 3 & 4 etc) on CON8 in Fig.1 should be swapped to be consistent. The pin numbering in Fig.2 also needs to change as it was based on Fig.1. Active Mains Soft Starter, February & March 2023: the 15V zener diodes should have been specified as 1N4744 types, not 1N4742 (which is the 12V equivalent). Next Issue: the February 2026 issue is due on sale in newsagents by Tuesday, January 27th. Expect postal delivery of subscription copies in Australia between January 26th and February 13th. Australia's electronics magazine siliconchip.com.au