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MARKET CENTRE Cash in your surplus gear. Advertise it here in SILICON CHIP FOR SALE KIT ASSEMBLY & REPAIR LEDs, BRAND NAME and generic LEDs. Heatsinks, fans, LED drivers, power supplies, LED ribbon, kits, components, hardware, EL wire. www.ledsales.com.au tronixlabs.com.au – Australia’s best value for supported hobbyist electronics from Adafruit, SparkFun, Arduino, Freetronics, Raspberry Pi – along with kits, components and much more – with same-day shipping. PCB PRODUCTION PCB MANUFACTURE: single to multi­ layer. Bare board tested. One-offs to any quantity. 48 hour service. Artwork design. Excellent prices. Check out our specials: www.ldelectronics.com.au NEED A NEW PCB DESIGNED? Or need to update an old board? We do PCB layouts from circuits, drawings, photocopies or sample boards. Contact Steve at sgobrien8<at>gmail.com or phone 0401 157 285. Get a new PCB and keep production going! KEITH RIPPON KIT ASSEMBLY & REPAIR: * Australia & New Zealand; * Small production runs. Phone Keith: 0409 662 794 keith.rippon<at>gmail.com DAVE THOMPSON (the Serviceman from SILICON CHIP) is available to help you with kit assembly, project troubleshooting, general electronics and custom design work. No job too small. Based in Christchurch, NZ but service available Australia/NZ wide. Email dave<at>davethompson.co.nz VINTAGE RADIO REPAIRS: electrical mechanical fitter with 36 years ex­ perience and extensive knowledge of valve and transistor radios. Professional and reliable repairs. All workmanship guaranteed. $17 inspection fee plus charges for parts and labour as required. Labour fees $38 p/h. Pensioner discounts available on application. Contact Alan, VK2FALW on 0425 122 415 or email bigalradioshack<at>gmail. com MISCELLANEOUS RADIO TV & HOBBIES MAGAZINES 1945: January, July, August, September, November, December – $8 each (includes price of postage worldwide) 1946 all issues – $40 (+ postage) 1947 all issues – $37.50 (+ postage) 1948 all issues – $37.50 (+ postage) 1949 all issues (except March) – $32.50 1950 all issues – $40 (+ postage) November 1956 – $8 (includes postage) Email silicon<at>siliconchip.com.au for details on conditions and pricing. See: siliconchip.com.au/Shop/3 Where do you get those HARD-TO-GET PARTS? Where possible, the SILICON CHIP On-Line Shop stocks hard-to-get project parts, along with PCBs, programmed micros, panels and all the other bits and pieces to enable you to complete your SILICON CHIP project. SILICON CHIP On-Line SHOP www.siliconchip.com.au/shop ADVERTISING IN MARKET CENTRE Classified Ad Rates: $32.00 for up to 20 words (punctuation not charged) plus $1.20 for each additional word. Display ads in Market Centre (minimum 2cm deep, maximum 10cm deep): $82.50 per column centimetre per insertion. All prices include GST. Closing date: 5 weeks prior to month of sale. To book, email the text to silicon<at>siliconchip.com.au and include your name, address & credit card details, or phone Glyn (02) 9939 3295 or 0431 792 293. er a speaker is attached. The heatsink temperature climbs steadily to 70°C at around 3°C per minute. The thermal protection then activates and the amplifier cycles on and off about once per minute as the heatsink cools and heats again. This occurs with the lid off the enclosure and even when the amplifier circuit board is totally removed from the enclosure. My basic understanding is that Class-D amps are supposed to run cool, so this behaviour seems odd to me. siliconchip.com.au I have checked the component placement and orientation, and checked all the PCB voltages against the instructions. They are within the expected ranges. The supply rails are ±47V DC. I measured the amplifier gain and it is around 15 times, as expected. Q1 and Q2 appear to be the source of the heat as they are hotter than the heatsink and Q3. Do you know why it is doing this and how I can solve it? (J. C., via email) • The most likely scenario is that the amplifier switching frequency is too Australia’s electronics magazine high and/or the dead time is incorrect, resulting in much greater switching losses than normal. Check the switching frequency and the components at pins 1, 2 and 4 (which set the frequency) and the dead time setting resistors at pin 9. Presumably there is no significant DC offset at the output, since you have the speaker protector connected and it would switch the output off if that were the case. So the frequency and dead time are the leading suspects for this overheating problem. SC November 2018  103