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

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

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sas570.JPG

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.

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

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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

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

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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.

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

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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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zeus2000 replied on Sun, Dec 12 2021 5:26 PM

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Hi cozza. I know this is a very old thread but was hoping that I might get a response. My 2400-2 started acting up a couple of years ago. It would lock on P4 after and hour or so of use which slowly went down to a few minutes. I read your thread and changed IC5 and IC 6 after finding the pins were corroded exacly as in your post. After reassembly it now switches to P1 after 3 or so minutes instead of P4. I can see in the schematic that transistors TR1 to TR7 and TR15 to TR22 are also part of the touch panel but I would like to see if it would be possible to narrow things down a bit more. I am also thinking that one of the new IC´s that I installed might be also faulty or that I might have mishandled one during instalation. Namely IC5 seeing that it handles the P1 preset. I have also changed all of the electrolytics. Apart from this problem, it sounds fine and doesn´t seem to have any other fault. Any help would be greatly appreciated.

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Dillen replied on Sun, Dec 12 2021 5:44 PM

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Check (or replace) the D50 rectifier as the first thing. It's known to cause intermittent problems, often related to source selection.

Martin

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zeus2000 replied on Sun, Dec 12 2021 5:57 PM

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Thanks Dillen. I try that first.