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If the Earth is solid, then it would be worthwhile trying a bigger mains filter. This could be as simple as winding the mains wires through a toroidal ferrite core. Speaker Protector not sensing AC I have built the October 2011 Loudspeaker Protector module (siliconchip. com.au/Article/1178) from an Altronics kit (Cat K5167). I configured it for my supply rail voltage and tested it with the prescribed method, and it seemed fine. However, as soon as I connect the AC sense lines from my transformer, it refuses to work. Connecting AC sense to the positive rail as detailed in the instructions and performing all other tests shows the module to be working. Can you enlighten me as to the likely cause of this problem? (D. J., Mandurah, WA) • The first thing to do is check that you have wired up the transformer to the AC sense terminals correctly. Typically, you would have a centre-tapped winding with the centre tap Earthed and the other connections going to both your bridge rectifier AC terminals, and the terminals of CON2 on the Speaker Protector. Assuming the connections are correct, verify that the base voltage of Q2 is low (below 0.2V) when the AC voltage is present at the AC sense input. If that is incorrect, then check diodes D2 and D3 and the transistor Q1 to ensure they are orientated correctly, have good solder joints and are not faulty. That it worked with a DC test suggests transistor Q1 is working OK and that at least one of diodes D2 & D3 is functioning correctly. It could be a problem with the resistor or capacitor values around Q1. If the 470nF capacitor is not soldered correctly or has the wrong value, the circuit will work with DC voltage applied but not AC. Similarly, if the resistor values are wrong, the circuit might not hold up through dips in the AC voltage. Flexitimer with higher supply voltage Many years ago, I built the PICBased Flexitimer Mk.4 (June 2008; siliconchip.com.au/Article/1847) from 112  Silicon Chip a Jaycar kit (Cat KC5464). Can it be modified to run from a 24V supply? (J. S., via email) • You need to change the relay to a compatible type with a 24V DC coil, the 470μF capacitor to a 35V rated type, and the 1kW LED current-limiting resistor to 2.2kW. AEE ElectroneX.......................... 7 Incorrect component in low ohms meter Control Devices..................... OBC Advertising Index Altronics...............................83-86 Ampec Technologies................. 25 I have built the Low Ohms Tester by John Clarke from the June 1996 issue (siliconchip.com.au/Article/4987), but I can’t get it working. The text says that the voltage at pin 2 of IC1 should be the same as pin 3. Adjusting VR1, I can get 2.4V on pin 3, so it appears REF1 is working OK. But the voltage at pin 2 is 1.64V. I have replaced IC1 and Q1 to no avail. (N. L., Christchurch, NZ) • Since you have replaced IC1 and Q1, that seems to rule out either component being faulty (which would explain what you are seeing). However, if Q1 is the wrong type or orientated incorrectly, that would cause this sort of fault. The only other possibilities are a lack of continuity or incorrect value with the 2.4kW resistor, trimpot VR2 or the 200W resistor. Try changing range switch S2 to see if that has any effect; if it does, it is likely one of the latter three components at fault. Note that if this part of the circuit is operating normally, pin 6 of IC1 should be around one diode drop (approximately 0.6V) below the voltage at pins 2 & 3. Dave Thompson...................... 111 Pulse generator circuit wanted Vintage Radio Repairs............ 111 I’m interested in building a pulse generator. I found a pulse generator design in Practical Electronics, February 1979. Can you suggest a circuit as simple as that one, but up to date with similar specifications and features? (R. M., Melville, WA) • We published a pulse generator circuit in the Circuit Notebook section of the November 1997 issue, which has similar features to the one you refer to (siliconchip.com.au/Article/5833). However, there was no PCB design to accompany that circuit. You could also build up the Practical Electronics design as there is nothing wrong with it. All the parts used in that circuit are still available. SC Australia’s electronics magazine Digi-Key Electronics.................... 3 Emona Instruments................. IBC Hare & Forbes............................. 9 Jaycar............................ IFC,53-60 Keith Rippon Kit Assembly...... 111 Lazer Security......................... 111 LD Electronics......................... 111 LEDsales................................. 111 Microchip Technology.................. 5 Ocean Controls......................... 11 PMD Way................................ 111 Silicon Chip Shop...............96-97 Switchmode Power Supplies....... 6 The Loudspeaker Kit.com......... 10 Tronixlabs................................ 111 Wagner Electronics................... 63 Notes & Errata Ultra-LD Mk.4 Amplifier, July-August 2015: the circuit diagram (Fig.1) incorrectly specifies 135mV across the 68W emitter resistors of Q2a & Q2b, the correct value should be around 68mV. The September 2021 issue is due on sale in newsagents by Thursday, August 26th. Expect postal delivery of subscription copies in Australia between August 25th and September 10th. siliconchip.com.au