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For want of a shoulder washer...
For want of a shoulder washer...
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Old 10th March 2004, 02:09 AM   #1
OddOne is offline OddOne
diyAudio Member
Join Date: Nov 2002
Location: Earth
Default For want of a shoulder washer...

Finally got around to finishing a portable 100W x 2 power amp based on TDA7293s - a project that had been gathering dust for the better part of a year.

Built a basic LED power meter right into it. Made its chassis frame from welded 1/2" square steel tubing, bevel-cut and MiG-welded (even though I suck at welding). Carefully hand-carved smoked-acrylic front panel, so it's lookin' sweet. Packed tons of capacitance into the supply rails, wired everything sweetly, tested for AC coupling problems and any abnormal DC levels before trying to power a real load and all was well.

Powered up with some small speakers, and fed off the audio-out from my computer's integrated audio, and it worked fine! A little crackle action here and there, which I attributed to transients from my not having installed any output filtering and stray RFI/EMI that the exposed ciruits could pick up from wherever.

Then, a sudden buzz and the sound died. However, the speakers themselves were STILL WORKING.

And Winamp suddenly locks up, but the rest of the computer chugs along unassumingly.

A quick reboot, no POST report (my motherboard audibly reads out the POST results), and in Windows, no startup sound. Winamp starts and complains that there's no sound hardware installed.

Uh oh.

Wiring check time!

Everything checks out as near as I can tell, with no abberant signals or voltages on the outputs, so I probe the inputs.

45 volts from input ground to chassis ground. Chassis ground is ONLY connected to the ground lug of the AC power jack - nothing else ties to there. Well, nothing WAS SUPPOSED to, anyway...


Quick voltage check at the supply - chassis ground's at the negative supply rail voltage?! Unhook the amp from the power supply and chassis ground floats with respect to supply, as intended.

Check the amp - short circuit between the tab of the left TDA7293 and the chassis. Waitjustaminit here... those jokers are insulated and the screws are nylon-shoulder-washered.

Take out the screw, notice black ring around its shank.

Crap squared!

Pick out the shoulder washer - entire shoulder's scorched and part of it's missing.

Hang head, sigh, check other one and it's fine.

One of my nylon shoulder washers apparently didn't get the memo re: not conducting electricity, and let the TDA7293's negative tab, which is tied to the negative supply rail internally, bypass to ground. As a result of that the input ground immediately jumped to -44VDC and the computer's onboard audio chip, providing audio for testing at the time, took the voltage and sizzled in roughly a fifth of a second I'd say.


Welp, time to spend about $50 for a new soundcard...

Funny thing is that I suspect the amp is otherwise FINE. Everything checks out with the TDA7293s unbolted from the heatsink. So, tomorrow I'll fab up some lever-action style clamps and wrap the damn things with Kapton MT film to make sure this won't happen again.

At least it didn't wipe out the whole damn computer.


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