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Old 7th March 2012, 02:47 PM   #1
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Default Amplifier digital input select problems

This isn't a problem relating to a specific amplifier, but rather one I have come across a few times and am not sure of the cause. Twice now I have been working on a home theater/AV amplifier with a digital front end with no sound output.
Both amps were Sony, and the only problem with them was no sound output. So, I fed a sine wave into the unit and began tracking the signal with my scope to see what was going on, turns out it was the input selector IC, as there was no signal out of it. Having found that, I put the probe onto the data input pin to check it was getting a signal when the input select buttons were pressed, and as soon as I touched the pin the amp came back into life and has worked fine since then. Input select chip was an LC series analogue switch.

Second amp was similar, it was a much later model amp etc. After following the schematic and comparing it to the pcb I started to check the signal in a similar manner. Once I started probing to check the digital control signals the amp suddenly came back and has also worked fine since then. This chip was an LC7822.

What would likely be the problem here? Could it be as simple as static buildup that prevented it functioning properly? Also, is it likely that it could reoccur? On both amps I did not disconnect anything in the signal chain or change any parts. All it took was the touch of a metal probe it seems and everything is okay.
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Old 7th March 2012, 03:24 PM   #2
LAJ is offline LAJ  United States
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Default Looking at the data sheet...

looks like it might be CMOS (uses + & - 12V).
Most likely you generated a "high" on one of the logic pins causing it to select that input.
I would check the data select logic and make sure its true for the desired selected input on the chip.
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Old 7th March 2012, 05:32 PM   #3
Bone is offline Bone  United Kingdom
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Might have been a poor solder joint on that pin; probing it moved it a fraction, enough to make contact again.
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Old 8th March 2012, 06:26 AM   #4
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A VERY common failure is grease ...

grease is applied inside the selector switch that obviously controls data ... the factory approach is to put a blurp of grease inside at the "factory " and when the product finds its way to the costumer and while using the selector then its assumed that grease will "travel" all around the switch .

Not always working and grease might become solid or dry ...problem is that there is no actual contact issue at least audible but if the processor looses touch with the selector system fails like you describe or "jumps" from a source to another ....

kind regards
sakis
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