Nano-amp leakage currents

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Joined Jan 27, 2015
Logitech x-540 Stereo repair adventures

I had a stereo, that had a volume level problem. So I finally took it apart, I discovered the volume was controlled by a cheap DAC and the volume level would osciolate between levels, it did it from day 1, and other people have the same problem. (it seemed to go away when I had my scope probe on the pin, but then I made it all worse)

It also had a bunch of cap's over their ESR limit.

In the process , I seemed to have fried 2 op-amps , 1 with hotair, the other IDK. I made some bridges to GND, removing cap's, and reflowing all the SMD parts. There's GND plane all over the top and bottom of the PCB, and it scratches off easy when soldering. I probably damaged a DAC too, before I got my needle tipped DMM probes.

Actually I thought I fried my BM869 at this too. Well better to practice and learn the hard way on my stuff than someone's else's.

I mapped 95% of the circuit, and found a few things that didn't make sense with the large caps removed, so I really got into the op-amps last week. And they didn't make sense at DC. So removed them and made a test circuit and found 2 were damaged. And then the resistor networks still didn't match simulation. There literally is NO resistor's to GND or Vcc in the op-amp networks, that I can find anywhere, except for internally in the op-amps and the 6V regulator that is the reference it all floats around. So without those chips on the PCB, I should not be getting mA or uA's to GND from anywhere in the op-amp networks. So I removed the analog switches too.

I was able to use my PSU and DMM as a current probe, and measure the current between places. I cut up the 6V rail into sections, and seemed to have found 3-4 places where the current was leaking to GND.

Now I have it down to where, at spots that had 100's of microA leaking, are now down to 10-20nanoA. At least 1 place is still 150nA when I think it should be zero. So I better keep going.

This stereo used quad SSOP-14 op-amps, so for now, I soldered up some very very similar SOP-8 dual op-amps, onto a breakout board. That's not going to be fun to attach, but it's under a shield so I guess I'll make the wires comfortabla-lly long.

Well it's plenty of practice soldering, reverse engineering, getting used to uV and nA with my new gear. The next thing I work on should go a lot better.

For the last 1-2 months, I kept wondering, where's GND, where's GND, there has to be a path to GND, now I'm 99% sure there's not supposed to be.
Here's some of the schematic, that one was more to keep track of the labels, and not to run as a sim.


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