Expert: Cleggsan - 10/3/2009

Question
Hello Mr Cleggsan,
  I have a 1983 Bose Spatial Control receiver cum amplfier , The pre amplifier went dead due to the sensor bose switching the amp on and off due to high humidity, the main amplfier works fine, i tried repairing the pre amplifier but used equalents for some chips I.cs and transistors since the original are not available, the pre amplifier worked fine for quite some time and even for long periods but recently when i switch it on it works only for few seconds and the volume fades off .
Do advice on this problem.
Thanks
Mario

Answer
Hard to say.  Bose equipment is difficult to troubleshoot because they use strange circuitry.

However, the symptom you describe are very much common with gassy/bloated electrolytic caps in the power supply.  Have you checked them?  They often do not like high humidity and high temperature.  You can replace all the lytics in the power supply for very little money. It may be worth doing that to see if it  helps the crashing of the unit.

The fading out is what happens when the dc voltages into the audio amplification section diminishes. The bad lytic may be charging up for a few seconds but when the voltage gets to the full amount it breaks down the electrolyte and the capacitor shorts out.

So, give that a try and let me know how it goes.

C
PS: If you have a voltmeter you can easily watch what happens to the voltage to the preamplifier stages as the power comes on.  If you see the voltage drain down a few seconds after turn on then I am sure it is a bad lytic cap.