Class D output measurement.

[Yamin]

Hi guys,
I was hoping to clear some doubts I've been having about troubleshooting a class D amplifier.
I understand that to properly measure the output of the amplifier a differential probe would be needed or two probes ground connected and subtracted signal viewed on the scope.
Since I didn't have one, I measured the output with a normal probe. I made sure that I didn't accidently connected to the common ground.

1. I was using a handheld scope so there wouldn't have been a path to mains earth ground which probably would connect to the common ground of the amplifier. Had I used a scope which connected to the mains would the I have faced the problem just mentioned.
2. What would happened if the output ground be connected to a common ground?
3.With the measurement I made- (with no input signal) I measured a 100kHz sine wave. Should I be measuring that? Shouldn't the LPF  take it out?When an input signal is applied the 100kHz sine wave modulates with the input signal.
4.Whats the purpose of R19 the 1k resistor between the two drivers?
Thanks

[BreakingOhmsLaw]

Please use a 1kHz test signal and repeat the measurement. Post results.

[Audiorepair]

Please use a 1kHz test signal and repeat the measurement. Post results.

I think the question is what should you see with no signal.

[EHT]

2. What would happened if the output ground be connected to a common ground?

What do you mean "output ground" and why do this anyway? As you said, the battery powered scope is ideal for this work since you can connect the earth of the scope input to something other than mains PE.

3.With the measurement I made- (with no input signal) I measured a 100kHz sine wave. Should I be measuring that? Shouldn't the LPF  take it out?When an input signal is applied the 100kHz sine wave modulates with the input signal.

The output measured at the IC pins should be HF squarewave with PWM. 100 kHz sounds OK.

Because its all on the IC, doesn't look like many options for what could have failed. What's wrong with it?

[Yamin]

Thanks guys for the responses. I have made a previous post about the unit and the problem I was having https://www.eevblog.com/forum/repair/top258-smps-ic-keeps-blowing-(bose-l1-compact-speaker-system)/new/?topicseen#new

The summary of it is that the SMPS IC (TOP258) kept blowing up after a week of moderate use. Looked for the common culprits and everything was in order. I had a suspicion that one of the High frequency driver might have shorted (DC coil resistance remains to be ok, but I had a previous experience where the DC resistance was good but during playback that driver caused the amp to go into protection) so I wanted to measure to see if the amplifier IC was going into protection mode.

I had some general doubts about Class D amplifier measurements and that's why I made this post. I have indicated in the attached photo where I place the scope probes in this case.

2. What would happened if the output ground be connected to a common ground?

This question was about what would happen if I placed the common lead of the probe to a common ground and placed the other lead on the output connectors. Also I'm finding it a bit hard to understand why the output ground (speakers negative) is isolated from the common ground of the amplifier.

Could I have blown the amplifier chip had I used a mains connected scope? Like I said earlier which may have caused the common ground to connect to the speaker negative lead.
 
The 100kHz wave that I measured with no input signal remains if I put an input of of 1kHz. The 100kHz rides on top of the 1kHz wave.

Also what the purpose of the 1k resistor?

Thanks again for the help.

[BreakingOhmsLaw]

I think the question is what should you see with no signal.

True, however my intention was to see if the amp is working at all or we are looking at the chopping frequency of the amp. Or OP maybe is seeing 100kHz millivolts because he is not aware of the scope settings. Hence my suggestion.

[EHT]

I had a suspicion that one of the High frequency driver might have shorted (DC coil resistance remains to be ok, but I had a previous experience where the DC resistance was good but during playback that driver caused the amp to go into protection) so I wanted to measure to see if the amplifier IC was going into protection mode.

You could measure the impedance using a frequency generator and scope. Do the woofer as well, using appropriate frequency for each, say 100Hz and 5kHz.

This question was about what would happen if I placed the common lead of the probe to a common ground and placed the other lead on the output connectors.
...
Could I have blown the amplifier chip had I used a mains connected scope? Like I said earlier which may have caused the common ground to connect to the speaker negative lead.

Yes, don't think of the black speaker connector as a ground. Also, the ground of the circuit may not be isolated depending on the PSU, often not. Stick to what you originally said, which is using a battery powered scope; you can't go wrong with that. Connecting either of these "grounds" to mains earth could cause you a problem, although I would have thought a fairly obvious one, indicated by smoke or a bang!

The 100kHz wave that I measured with no input signal remains if I put an input of of 1kHz. The 100kHz rides on top of the 1kHz wave.

This may be because of where you placed the common lead of your scope. I think the main circuit ground to the output pin on the IC should show you a PWM waveform, NOT AM (which is what you describe).

Also what the purpose of the 1k resistor?

Don't know but don't think it is very relevant here unless it has shorted!

[Yamin]

This may be because of where you placed the common lead of your scope. I think the main circuit ground to the output pin on the IC should show you a PWM waveform, NOT AM (which is what you describe).

I was measuring after the filter etc, that's where I saw the 100kHz sine wave. I haven't measured the output pin of the amplifier chip yet.

[EHT]

I was measuring after the filter etc, that's where I saw the 100kHz sine wave. I haven't measured the output pin of the amplifier chip yet.

Where exactly? I think you should see 100kHz PWM square-wave on the output pin of the IC (wrt circuit ground) and then the audio signal (e.g. 1kHz test tone) over C8, C9. If you're seeing the 100kHz at this point then I suspect maybe C8 / C9 have failed. I guess these are the main caps. If you replace them, note that you can't just use any old cap; it needs to be high performance so that it doesn't de-rate at this frequency. I think these caps get worked hard, so may be likely to fail. Decent ones are relatively expensive parts so the mfr could have skimped on them.