Pioneer SX-750 problems

Yesterday was working good, there was not negative power on the right channel output. I've checked the power transistors with a PEAK ATLAS analyzer and there was not leakage current. I resoldered the transistor board conectors and then was working good without oscilation on the output. Now I was making the video and when I went to check other problem I had again oscilation on the right channel. What can I do...?

Category:

Education

Tags:

• Pioneer
• SX-750
• amplifier
• audio
• fix

[jpdylon]

Do you have an oscilloscope? Can you post a video of what the noise looks like in the bad channel?

[Atilanos]

@jpdylon yes, I will post the video. Thank you!

[jpdylon]

Also something important to note is if the machine has been near the ocean or a body of salt water in its past. As the sun goes down and the dew point settles in, the dew contains salt water which settles on parts like transistors. This will create a leakage path that can cause the same symptom you're experiencing. Often times, scraping between the leads of the transistors will solve the problem. The saline deposits can be seen under magnification on the bottoms of plastic transistors.

[Atilanos]

@jpdylon I'll tell you the history of this amplifier. Weeks ago I got it on the street, those days it was raining and probably rain penetrate into the amplifier. The place where I live is near the ocean. A 2SC1649 was not on the board and I used a BF422 as replacement. I bought new capacitors but I didn't change the 330uF caps of the difdrential amplifier because somebody must changed it, they were not the original. I'll also replace it.

[Atilanos]

@Atilanos Sorry, I'm wrong. The 330uF of the differential amplifier were replaced by me. There are another two 330uF 50V caps that were not replaced by me.

[jpdylon]

If you are getting intermittent voltage fluctuations accompanied by noise then likely a transistor is either leaky, or has been contaminated with saline deposits. The fact that the offset voltage isn't that big means that the problem is probably not in the output or driver stage. Likely the problem is in the differential amplifier stage or class A driver stage.

As far as the protector problem, that is a separate issue. I'll have to take a closer look at that one.