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Overload problem in Mosfet amplifier I have built a stereo power amplifier using Hitachi Mosfet devices (2SK1058 and 2SJ162) in the output stage. With the 2SJ162s, I am having trouble with severe distortion developing after a few seconds at maximum power into a 4Ω resistive load (I assume the devices are having “severe hernias” as they go into overload). This is occurring with each device supplying about 5A (peak current). This is below the specifications which state that the device should be able to supply 7A max. As stated above, this is occurring only on the negative signal swings. I was wondering if you know of any suspect “clones” that are circulating in the guise of Hitachi devices? They are TO3-P type. (G. G., via email). • Without knowing anything about the circuit you have built, it is possible that the output stage is checked components, etc. Could you please advise me what I should be seeing on the scope at U3 pin5 and U2d pin11? Is it a pulse that reaches 5V and what it is its timing duration? (R. F., via email). • You should be seeing a negative-going pulse at pin 5 of the LM1881 (U3), with an amplitude of nearly 5V and a duration of about 4µs. You should also be seeing a positive version of the same pulse at pin 11 of U2d, to operate switches U1a/b and trig­ger U5. Finally, if U5 is triggering properly from this pulse (which corresponds to the colour burst and horizontal ‘back oscillating at very high frequencies, possibly at 100MHz or more (yep, way up in the FM band). You need to check this with an oscilloscope. If you don’t have one, or your scope does not have a 100MHz capability, a check with a communications receiver or even an FM radio may indicate a high level of radiation. If your scope has low bandwidth, you may observe apparent clipping or distortion of the waveform. When you use a scope with sufficiently wide bandwidth, this will be revealed as a large burst of oscillation superimposed on the waveform. We think it is unlikely that you have suspect Mosfets. It is not commonly known that Mosfet amplifiers can be susceptible to supersonic oscillation and it can be quite difficult to “tame” them. The problem may be due to the PC board and general wiring layout. You may also need stopper resistors (try 100Ω) in series with each gate to cure the problem. porch’ of each line), you should see pulses about 50µs long at TP2 and pin 3 of U5, again of about 5V peak amplitude. From your description it’s hard to suggest what may be going wrong if none of these pulses is present, and you’ve alrea­dy tried replacing U3, U2 and many of the components around them. You could try reducing the value of U3’s input series resistor R5, to 560Ω or 470Ω. We understand that in one or two cases in the past, transistor Q1 has had a high enough output impedance to add significantly to R5 and caused excessive atten­uation of the higher frequency video and sync components reaching U3. In turn, this seems to have prevented U3 from producing the colour burst pulses at pin 5. Perhaps this is also the case with your unit, so it’s worth reducing the value of R5 a little to see if that cures the prob­lem. Turbo timer playing up I have recently purchased a Turbo Timer kit from Jaycar Electronics. All is installed (on a 1993 Mitsubishi Lancer GSR) but I have a problem – if the engine is running and then switched off, it stops. Investigations show that the timer does not even attempt to operate. If I just turn the ignition on but do not start the engine, when the key is turned off the Turbo Timer responds as it should and it leaves the ignition on for the required time. I have left the thermal circuit open such that the circuit thinks the engine is hot. The reset circuit works fine. It seems that the engine (either ignition spark or alternator?) are reset­ting the timer at switch off. Can you please help? (L. C., via email). • Try changing the 2.2µF capacitor at pin 2 of IC1 to 100µF or if this still gives problems, to 470µF. This will provide a timing trigger when the ignition voltage falls slowly. Notes & Errata Sunset Switch, June 2003: the 10µF capacitor connected from the +12V rail to the emitter of Q1 has been omitted from the circuit diagram on page 36. The wiring diagram (Fig.3) is cor­rect. Also, four 10µF capacitors have been SC omitted from the Parts List. 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 www.siliconchip.com.au