I built a Leach preamp around 1980, and it has worked well for the past 37 years. However, a few months ago it started sending a signal to the amp that produces a sound like wind blowing—first in one channel and then a few months later in the second channel (output stage, not phono stage).
I'm a geologist and not an electronics troubleshooter, so on a whim and not knowing what else to try, I bought the 4 transistors (new old stock) in the output stage, and replaced them. To my surprise, this solved the problem.
Much to my disappointment, however, the wind-blowing sound returned to one channel (much louder than before) after about 10-20 hours of use. I've ordered some new transistors in the hope that I received a defective one.
The alternative is that something else might have gone bad that is causing the transistors to fail. For example, one of the power supply electrolytic capacitors has a bulging end. So I've ordered a set of 4 new capacitors.
Does anyone have any advice for what else to try/check? In addition to being satisfied with the preamp's sound, I have a sentimental attachment to it since I did everything including etching my own circuit boards.
Thanks for any tips.
I'm a geologist and not an electronics troubleshooter, so on a whim and not knowing what else to try, I bought the 4 transistors (new old stock) in the output stage, and replaced them. To my surprise, this solved the problem.
Much to my disappointment, however, the wind-blowing sound returned to one channel (much louder than before) after about 10-20 hours of use. I've ordered some new transistors in the hope that I received a defective one.
The alternative is that something else might have gone bad that is causing the transistors to fail. For example, one of the power supply electrolytic capacitors has a bulging end. So I've ordered a set of 4 new capacitors.
Does anyone have any advice for what else to try/check? In addition to being satisfied with the preamp's sound, I have a sentimental attachment to it since I did everything including etching my own circuit boards.
Thanks for any tips.
short the inputs with zero ohm plugs.
Measure the output Hum+Noise using a 199.9mVac scale.
Measure the Output Offset using the 100.9mVdc scale.
Do this while it is still playing up.
Measure the ripple voltage on the +-ve PSU, again for the -ve PSU amd finally across both PSUs.
Report your findings (seven voltages in total).
Now switch off and let it cool down to ambient temperature.
Repeat all seven measurements immediately after power ON and while the amp is still cold.
Measure the output Hum+Noise using a 199.9mVac scale.
Measure the Output Offset using the 100.9mVdc scale.
Do this while it is still playing up.
Measure the ripple voltage on the +-ve PSU, again for the -ve PSU amd finally across both PSUs.
Report your findings (seven voltages in total).
Now switch off and let it cool down to ambient temperature.
Repeat all seven measurements immediately after power ON and while the amp is still cold.
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