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Currawong Valve Amplifier Is Motor-Boating I recently completed a Currawong Valve Amplifier (SILICON CHIP, October to December 2014). All appears to be well apart from the level of “motor-boating” noise which is easily audible. I have attached a scope screen shot which shows the output of the left and right amplifier channels. As can be seen, there are positive and negative spikes superimposed on 2mV or so of 50Hz. The stated signal-to-noise ratio is -70dB and what I’m seeing doesn’t seem to be within that limit. The ripple on the HT appears to be as expected. Any suggestions please? (D. H., via email). •  We have not had any other reports of motor-boating; yours is the first. We would be inclined to suspect defective capacitors or associated poor soldering. Check the power supply, the feedback network and the 100µF bypass capacitors for the 6L6 valves. Also, check that you have the correct 47kΩ and 6.8kΩ decoupling resistors installed in the HT rails. 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Problem with ultrasonic anti-fouling unit I have purchased a kit for the Ultra­sonic Anti-Fouling unit (SILICON CHIP, September & November 2010) and completed the assembly. When I connected the unit I got a brief red light and the fuse blew. I have run the test as stipulated in your article that came with the kit and have set a 5V output on the potentiometer but still nothing. I am attaching the pictures I have from the top. Kindly note that the blue wire coming from the battery is positive and the brown the negative. KEEP YOUR COPIES OF LEDsales...................................... 95 AS GOOD AS THE DAY THEY WERE BORN! Microchip Technology..................... 7 A superb-looking SILICON CHIP binder will keep your magazines in pristine condition. Radio & Hobbies DVD.................. 92 ONLY 95 $ 1P6LUS p&p * Holds up to 14 issues * Heavy duty vinyl * Easy wire inserts ORDER NOW AT www.siliconchip.com.au/shop Also the transformer arrived with a minor damage. The top left black plate was broken but we sealed it. (J. J., via email). •  From your pictures, the construction looks good on the top side. However, the transformer clip behind Mosfet Q1 is not actually holding the ferrite core in place and that might be the problem. The core seems to be fractured also on the side near the SC 2200µF capacitor. Master Instruments...................... 95 Ocean Controls............................ 10 Sesame Electronics..................... 95 Silicon Chip Binders..................... 89 Silicon Chip Online Shop........ 84-85 Silicon Chip Subscriptions........... 11 Silvertone Electronics.................... 5 Threadboard................................... 9 Tronixlabs..................................... 95 Next Issue The November 2015 issue of SILICON CHIP is due on sale in newsagents by Thursday 29th October. Expect postal delivery of subscription copies in Australia between October 28th and November 10th. WARNING! SILICON CHIP magazine regularly describes projects which employ a mains power supply or produce high voltage. All such projects should be considered dangerous or even lethal if not used safely. Readers are warned that high voltage wiring should be carried out according to the instructions in the articles. When working on these projects use extreme care to ensure that you do not accidentally come into contact with mains AC voltages or high voltage DC. If you are not confident about working with projects employing mains voltages or other high voltages, you are advised not to attempt work on them. Silicon Chip Publications Pty Ltd disclaims any liability for damages should anyone be killed or injured while working on a project or circuit described in any issue of SILICON CHIP magazine. Devices or circuits described in SILICON CHIP may be covered by patents. SILICON CHIP disclaims any liability for the infringement of such patents by the manufacturing or selling of any such equipment. SILICON CHIP also disclaims any liability for projects which are used in such a way as to infringe relevant government regulations and by-laws. Advertisers are warned that they are responsible for the content of all advertisements and that they must conform to the Competition & Consumer Act 2010 or as subsequently amended and to any governmental regulations which are applicable. 96  Silicon Chip siliconchip.com.au