Logitech X-540 speakers - DAC or ADC oscillating ?

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Joined Jan 27, 2015
I had this 5.1ch system for years and years, and its had a 2.13Hz volume pulsing the whole time (not kHz, just Hz), at certain volume knob settings. It's on all speakers and headphone outputs, and the I/P audio freq doesn't seem to matter.

I think I found the problem, the volume knob, which is a normal 3 terminal pot, only seemed to change the volume smoothly, but right now, I can clearly see the volume only changes in fairly big discrete steps. So there's a couple of degree's of knob turn, before the volume jumps up to the next step. In the past I just never had the volume high enough to see on the scope, so the audio was in the noise floor, and at 2Hz, it looked impossible to see.

So I finally took it apart, it uses an always on
- stepdown trans, for a 18V rail
-a ~ 10v5 zener for the wired remote, w/ soft power for the outputs, and the volume knob, etc
-and a 6V regulator for 2 or 3 op-amps.

And powering from the bench supply @18V, if I input hundreds of mVAC to the audio I/Ps, and play with the volume knob, I find the condition, where the output signal is oscillating, and not just at 2.13Hz

With 900mVpp sinewave I/P , the O/P goes from 424mVpp to 528mVpp , stays for about 1/3sec , then back to 424mVpp, total period about 1.4sec, I saw about 4s period earlier.

But so far it only seems under 2-3Hz, certainly not faster when powered normally. So it's when the volume knob is on the threshold, that it oscillates between the 2 output levels.

The green/orange/black I/P from the PC, each split channel goes to what must be some DAC or ADC type thing, (doesn't the SC output from a DAC ?) It's a DIP-22pin, so far I haven't found a datasheet. I guess if it's off to an R2R network, and then the opamps' and then to the main O/P drive chips, that's about all there is, besides a pile of cap's.

There's 1x DIP-22 for green=front/L/R and 1x for Rear/L/R, and it seemed center/woofer was on it as well.
Powered from 10.5Vcc, most pins are around 5.3V, some GND.

So Center-L/R is on pin 2,3, and with the audio O/P volume oscillating at ~0.5Hz, when I probe pin 6 (on 1x probe AC or DC coupled), which is at 2.36VDC, the output settles at the lower 424mVpp state.

So, that's real progress on that. I should see what happens when I change volume knob now, and also what a 1Mohm resistor or a few pF cap does on that pin, and if any oscillation happens now.

The main output chips are STA540
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