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Note: I am a beginner trying to learn, so advice is preferred over admonition, thank you.
So I was working on a 90s Sony amp (TA-FB930R). The amp was working mostly correctly (beyond a scratchy volume pot), so I hooked up my laptop’s external USB soundcard (no output signal just yet) and my oscilloscope like this:
the amp is double-insulated (two nested boxes on the back panel), and only has a Europlug power lead, so it does not have any mains earth connection.
the laptop was not plugged into its power supply, so it did not have a mains earth connection, either
according to the schematics of the amp (and tested with a DMM), the signal shielding of the input I used (AUX) is directly connected to speaker negative
The oscilloscope (Rigol DS1054Z) does of course have mains earth and that is connected to the probe terminals/probe crocs
When I switched on the amp, one of the power MOSFETs blew up instantly (and violently, including flames).
That in turn makes me believe that “somehow” I shorted its output to ground — but I don’t understand how this happened. AIUI, any input source could have opted to short signal shield to earth, no? And that would mean that the GND path through the probes/osc would not be the reason this happened?
Now I am about 95% sure I didn’t do this, but it’s the only explanation I have:
MAYBE I made a mistake connecting the probes to the speaker terminal and put speaker pos on the osc GND, effectively shorting speaker pos and neg (i.e. via the connected GNDs on the osc, I connected speaker pos to signal shield, and thus speaker neg)
Naturally, I’d like to avoid doing this again, so any constructive advice is appreciated!
Without knowing the particular amp, would suggest that perhaps the speaker outputs are driven in bridge configuration by two power-amplifiers (one in anti-phase relative to another), i.e. each speaker terminal is wiggled by a PA respectively. Then shorting the “-”-terminal to ground will blow the amp driving that terminal.
Cheers, heke
At first that’s what I thought, too, but looking at the schematic (I have the service manual), Speaker negative is tied to signal shielding. I am pretty sure I read it right, since in this particular set of schematics, ground/shield is always bolded. I also checked with a DMM and it has continuity (I do realize that that alone would not be enough, since the MOSFET blowing up might have caused damage resulting in a short).
OK. That sounds reasonable. Is that amp a stereo? In that case measure also the other channel for “-”-terminal conductivity for reference. Also, it is usually safe to just remove the blown up device (and perhaps the complementary one) prior to conductivity measurement.
Cheers, heke
good call on testing both channels separately, that makes it less likely that a short caused by the exploding MOSFET is a red herring. So I checked for continuity between L/R (and both speaker sets) negative and the signal grounds (AUX etc), and they all are connected. One thing I have noticed is that the Phono signal shield is actually tied to chassis, but I suspect that due to how the phono preamp works (and the separate grounding wire turntables have).
I think your measurement setup is well done, thus there should not be any problem with that. Even the distant probability that a probe is bad, won’t explain it.
Perhaps the amp was faulty already? The scratchy volume pot may have caused the PA input to float and that made the amp oscillating (as there was no speaker load damping to output)?
If you dare, you could repeat the test setup with the other channel (have some load across the speaker terminals!).
