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Beomaster 2400-2 input selection fault, symptoms include shut down or randomly changes inputs

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cozza
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cozza Posted: Wed, Aug 21 2013 12:34 AM

I have just finished this repair and thought I would share my findings.

Symptoms of the failure include random shutdown, random changes to preset and input selection.

In this case the fault was caused by corrosion and electrolytic metal migration between the pins of the SAS570S integrated circuits, creating intermittent short circuit paths between IC pins.

Replacement of both SAS570S devices cure the fault.

 This fault may also occur in Beomaster 1900 as it uses the same ICs.


Dillen
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Dillen replied on Wed, Aug 21 2013 6:37 AM

Thanks for sharing this.
Those IC pins surely look like they were eaten away by oxidation. Water-damage ?

It's actually very rare to see these early ICs fail.
The above symptoms are usually those of a bad low-voltage rectifier.

Martin

cozza
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cozza replied on Wed, Aug 21 2013 10:54 PM

Hi Martin!

This Beomaster  was very clean and original internally with no evidence of water damage to the rest of the board.

A similar type of failure can occur with small signal plastic packaged transistors. The metal migrates out across the plastic as a plating and shorts the pins. This has been discovered in 70's era vintage Sansui equipment (and other brands)

 I have seen one other instance of the IC failure in a Beomaster 2400, although the problem was not as severe. A bad batch of ICs? Who knows...

I'm not sure of the mechanism behind the failure, whether is caused by humidity or contamination in the encapsulation plastic.

johnmath
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johnmath replied on Thu, Jan 10 2019 11:40 PM

Thanks Cozza, I found the same fault in a BeoMaster 2400-2 which would shut down as soon a the finger was removed from the input selector button. I removed the two ICs 5 & 6, buffed the pins and between with a fine wire brush in a Dremel tool, then cleaned the IC cases with solvent and replaced. The unit now operates correctly.

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