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Notes & Errata Use Your old PC Power Supply For High Current Outputs, December 1998: the circuit diagram on page 75 incorrectly shows the nega­ tive terminal of the bridge rectifier as being connected to earth. It should go to negative side of the bottom 220µF electro­­ lytic capacitor instead. Fig.1 (above right) shows the correct circuit arrangement. Thermocouple Adaptor for DMMs, December 1998: the 4.7kΩ resistor from ZD1 to the 2kΩ trimpot VR2 is incorrectly shown as 47kΩ on the wiring diagram of page 34. A 15kΩ resistor has been omitted from the parts list. The current limiting and current limit LED don’t agree too well. Pulling 4A say and winding the current limit back would result in the current dropping appreciably (say to 2A) before the current limit LED would light. The power supply would growl even worse during current limiting. Aren’t the main filter caps a bit underrated voltage wise? My supply measures 52V on the caps. I’m fearful of a bang – or five! (A. W., Grange, SA). • An audible squealing from the power supply is normal when in current overload or when current draw exceeds the output setting. This was mentioned on page 57 of the April 1998 issue. The 470Ω resistor at the input to The accuracy of the current limit LED can be adjusted by altering the bias voltage on pin 10 of IC5b. You may wish to use a trimpot (100kΩ) in place of the 220kΩ and 100Ω resistors. Adjust the trimpot so that the Overcurrent LED lights when the supply just begins to current limit. The 50V rating for the filter capacitors is satisfactory. Troubleshooting an amplifier Fig.1: the corrected power supply circuit. Improvements To AM Broadcast Band Reception (Vintage Radio), December 1998: the diagram on page 67 shows the two twin flex leads as being joined where they connect to the antenna loop. This is incorrect – there should be no connection between the leads at this point. REG1 should be sufficient to reduce its input voltage to below 35V. This is because the combined regulator and IC standby currents will total more than 35mA to produce a 16.5V drop from the 50V supply. However, it will do no harm to add the extra 470Ω resistor in series to reduce the voltage even further. You will be able to reduce the output ripple from the power supply by adding a small amount of capacitance between pin 1 of IC1 and ground. This will filter the feedback voltage from the output of the supply before it is applied to the pulse width modulation circuitry. Try 100pF or a larger value until the squeal­ing noise and excess ripple disappears. I have built an audio amplifier and it was working OK but now isn’t. It uses two MJL21194 and MJL21193 Mosfet transistors and I am afraid that these might have blown but I am not sure. Do you have any extra information you could send me about these transistors, such as how to test them? I have checked the voltages that were given in the instruction manual and the negative side is fine, but the positive side reads as 0.2V instead of 55.8V! The posi­tive (NPN) transistors get extremely hot and the PNP transistors stay cold. (S. E., St. Ives, NSW). • Just a small point, the two transistors you mention are bipolar types, not Mosfets. You also did not mention when the circuit was published so we can’t be too specific in suggestions. From your description, it appears that you might have blown the positive rail fuse. If you are lucky, this might be all you have damaged. If the transistors are damaged they will usually have a direct short between collector and emitter and you can check this with your multimeter (switch to a low ohms range). You should check the other transistors in the circuit to make sure that SC they have not blown too. 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 Trade Practices Act 1974 or as subsequently amended and to any governmental regulations which are applicable. 92  Silicon Chip