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pickup on a kart)? Any help would be highly appreciated. (W. M., Newcastle, NSW). • The voltage output produced by the LM2917 is directly pro­portional to the product of the resistance and capacitance at pin 3 so if you want to obtain 1V at 167Hz, you need to reduce the resistance or capacitance by a factor of 75/167. For example, you could use 47kΩ instead of 100kΩ and then use a trimpot on the output for final calibration. Current drain for interface card I bought the “Flexible Interface Card For PCs” kit from Jaycar electronics and have a question about it. Could you tell me what current should it draw on the 5V line? It seems to be drawing an average of half an amp and is burning out the power supply we have. (J. A., via email). • The current drain from the 5V rail should be quite modest; no more than 50 to 100mA at a guess; nothing like 0.5A. You have a fault there somewhere. Current sharing in amplifier output stage I have constructed several of the 125W amplifiers based on the MJL21193/4 output transistors (April 1996) to use in my home theatre system. All went find for a while but I am currently having a problem with two of the channels blowing pairs of output transistors. I have checked for dry joints shorts etc and can‘t find anything obvious. When I power the amplifier up all goes well until it blows the fuses, which could be half an hour later or even a couple of days. After replacing the transistors one time I ran the amplifi­er again, resetting the bias as described in the instructions and let it run. A while later the fuses popped again and on removing the amplifier from the heatsink I noticed one each of the MJL21193 and MJL21194 transistors were extremely hot (and blown) while the other pair was quite cool. Is this thermal runaway? Could it be the gain of the output transistors are mis­matched? I don‘t think I am overloading the amplifier as I have run it very hard into low impedances with no problems before with the heatsink getting very hot. Unfortunately I have no oscillo­scope or any other test equipment other than a DMM. Can you please help me? (G. W., Auckland, NZ). • If one pair of transistors is getting hot while the other pair is cool, it suggests that the second pair are not connected at all. You can verify this by checking the voltage drops across the 0.47Ω resistors. They should all be roughly the same. It sounds to us as though one pair of transistors is doing all the work and yes, they are ultimately suffering from thermal runaway or just straight-out overload. Check that the bases of all the transistors are connected to the relevant points on the circuit. You could possibly have open circuits on the copper tracks of the PC board. Confusion with Low Ohms Tester I am currently building the Low Ohms Tester as described in the June 1996 issue. I have checked your Notes and Errata file, but have not come across this problem. On the last page of the above article, under Test & Calibration, there is an incorrect statement under paragraph two. It states that pin 2 of IC1 should be at the same voltage as pin 3. I can understand how you can draw this conclusion, if this is an op amp voltage follower circuit. However it is not, because the feedback loop is also in parallel with the RANGE switch S2b. Only with the S2b disconnected, can you get identical vol­ tages appearing at pins 2 & 3 (the BE emitter junction of Q1 was bypassed under test. Otherwise, there is a voltage differential of 1.2 volts if switch S2b is left in, let’s say on position 4 (see schematic diagram). Therefore does this outcome in anyway affect the calibration procedure? (P. B., Canterbury, NZ). • IC1 simply buffers the reference voltage which is applied to its input at pin 3. The pin 6 output drives transistor Q1 so that its emitter, which is connected to pin 2, is at the same voltage as pin 3. Thus the statement in our article that the pin 2 vol­tage will be equal to the pin 3 voltage is correct. If you are measuring a 1.2V difference between pins 2 & 3, then this will be due to the lack of a collector load for Q1. Connect up a low value of resistance across the Rx terminals and then check the voltages at pin 2 and pin 3. Assuming that IC1 is operating correctly, there will be no problems with the calibra­tion procedure. Notes & Errata Command Control Decoder, May 1998: the circuit on page 62 shows a 100kΩ resistor connected to pin 1 of IC3 whereas the component overlay on page 65 shows it as 3.3kΩ. It should be 100kΩ. 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