I am exploring the mainboard of an Epson 3010 projector. This particular projector has internal speakers but does not have an audio out and - as I have built my own external audio amplifier - I wanted to modify it and extract audio from the projector.
At this moment, I am NOT considering an HDMI audio splitter. I am aware of that solution but I do not wish to go that way.
I opened up the projector and backtracked the integrated speakers wires to a chip that is a MAX9736A, a class D audio amplifier, conveniently located on top of the mainboard. Note: I wasn't able to find a schematic of this board, so I have to try to guess how it works.
From the datasheet it looks like this chip takes a line level input and outputs an amplified audio signal. I connected my oscilloscope (ground to chassis) and (for example) I can see the "PWM" speaker output. With the probe on pins 23..26 I see this wave that changes according to the input, and I can hear it while being played.
However, I expected to see a signal when putting the scope probe on pin 18 - instead I get a (noisy?) fixed 2VDC. Kind of surprisingly, I get some sort of noisy waveform that matches the input on pin 19, that is the feedback channel, connected to the input through at least a resistor and a capacitor (in the picture, see pins 5 and 6 for the left channel). Note I did not use the analog ground (pins 13,14), as my scope is not floating and I want to exercise caution. Now, this is just an audio signal, I'd expect levels and bandwidth to be fairly simple to see on a scope, I'm not exactly debugging DDR memory chips. I even tried to mess with AC coupling and bandwidth limiter, I gave a 440Hz audio input to the projector - I can hear it well in output - but I cannot detect it on pin 18.
My questions are:
- Why cannot I "see" a good audio wave input on the oscilloscope? I suspect I'm missing something obvious here
- If I manage to solder a line audio cable to pins 6 and 18 and ground to 13,14, what trouble can I expect? Please note I have an OK lab equipment with stereo microscope and I'd be technically able to perform the rework, in fact this chip is not even that small. But I typically work mostly on fully digital boards and this audio stuff escapes me.
EDIT: I have soldered an audio cable to the entry point of the input network of the left channel of the MAX9736A. I cannot hear anything on that input, not even a faint signal. Difficult to make a good picture, but I soldered it with the aid of a stereo microscope and I have made sure they are good. I checked with a DMM that all grounds, AGND, PGND, main ground plane are all shorted with the chassis and the ground prong in the AC connector. (and yes, I even floated my fully-plastic cased oscilloscope to see if it made a difference)
Note: before and after this modification, the projector's integrated audio works perfectly with no issues.
I have probed the circuit again with the oscilloscope; the probe point is the entry point of the input network as explained by the answer below:
The result is this, an extremely noisy waveform that in fact, might be a 440Hz audio input:
The extremely noisy signal persists even if I solder a short lead to the AGND of the chip, I float the scope (yes, I know bad practice) and ground my probe there: