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Old 17th February 2015, 12:08 AM   #1
mpitogo is offline mpitogo  United States
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Default AU-6900 repair and on going mystery

AU-6900 repair and on going mystery.

Single family owned Sansui AU-6900. Was working great until recently, it would not come out of protection or if it did, it would play for a minute then the sound would crackle on the right channel then cut out.

I downloaded the manual and searched the internet for fixes. Here's what I've done so far. Replaced all of the VD1212 with series pair of 1N4148 diodes and replace the 220ohm fuze resistors on the AMP board with 220ohm metal film resistors as recommended by the repair community. I also started replacing most of the capacitors with new parts (40yo AMP). Took out the old relay and cleaned it.

It now comes out of protection from a cold start but after maybe around 8 minutes the right channel would start to get noisy and the relay would quit. It is either going into protection or the relay is bad and goes out after warming up. It would then not start up until its cold.

I have more parts on order to complete the full re-capacitor of the AMP board and the main board. Replacement for the relay. I also found one of the pair of 3.3K ohm 2W resistors (reading 4K) on the power supplies was out of value and will be replaced.

I'm hoping its the relay going bad. Readings at the relay while its on is 24.4VDC so its pretty spot on. Any other suggestions before I spend many hours in the schematics?
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Old 17th February 2015, 03:25 AM   #2
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It would be helpful to know what is happening after 8 mins or however long it takes for things to warm up and go pear-shaped. How about measuring any DC on the output, before the relay, so you can see what caused the protection relay to drop out, as it should when there is a DC problem in the amplifier.
Sansui AU-6900 Manual - Integrated Stereo Amplifier - HiFi Engine

This could easily be a semi that has developed a short but with 4 pairs of parallel output devices, it may not be straightforward checking which, if any of these, may be a cause. Otherwise, its a very simple design with basic VI limiters and and DC sensing for protection.
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Old 17th February 2015, 04:03 AM   #3
mpitogo is offline mpitogo  United States
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Quote:
Originally Posted by Ian Finch View Post
It would be helpful to know what is happening after 8 mins or however long it takes for things to warm up and go pear-shaped. How about measuring any DC on the output, before the relay, so you can see what caused the protection relay to drop out, as it should when there is a DC problem in the amplifier.
DC on the output, good call will try that. Thanks!
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Old 17th February 2015, 11:22 AM   #4
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These old Sansui amps frequently suffer from bad small-signal transistors. I believe this one even has the infamous 2SA798 dual transistor (five legs). These will need to be replaced.

KSA992 and KSC1845 will do the trick for most.
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Old 18th February 2015, 04:18 AM   #5
mpitogo is offline mpitogo  United States
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The original resistors at R612 and R613 on the Amp board are 2W carbon resistors running at 3.3K ohm. One of themwent out of value testing high reading 4k ohm and still tests high when removed. I purchased a new pair of 3.3k ohm 2W resistors which I though were suitable, I tried it and it gets very hot. The originals are monster sized compared to these:

SPR2CT521R332J KOA Speer | Mouser

Any suggestions for a suitable replacement?
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Old 18th February 2015, 04:49 AM   #6
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Unfortunately, in the craze for smaller sized parts, resistors have been optimistically re-rated. Use a part that is rated "5W" these days - it should run cooler.
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Old 18th February 2015, 06:38 PM   #7
mpitogo is offline mpitogo  United States
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Originally Posted by wrenchone View Post
Unfortunately, in the craze for smaller sized parts, resistors have been optimistically re-rated. Use a part that is rated "5W" these days - it should run cooler.
I think it maybe ok, introducing the new resistor in the power path may have exposed a possible weak part, it was going into protection right away and likely burned it out. I'm now getting no amplifier output, bad, but it is also going out of protection and the part is running fine.

will now have to follow the power amp troubleshooting guide. out of value resistor @4k may have just been masking the he real problem a possible bad tiny transistor.
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Old 18th February 2015, 09:33 PM   #8
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A bad 2SA798 differential pair transistor (if used, which is likely) could cause excessive DC offset, which would trip the relay.
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Old 18th February 2015, 10:39 PM   #9
mpitogo is offline mpitogo  United States
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welcome, yes that is what I am seeing. I put the meter on the output of L and R and seeing around 2v DC. Will check out replacements for 2SA798.
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Old 19th February 2015, 08:12 AM   #10
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The replacement is KSA992, two pieces, matched, with both emitters soldered together.
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