Sansui AU-20000 DC on Speaker output no audio - diyAudio
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Old 21st January 2013, 07:03 PM   #1
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Default Sansui AU-20000 DC on Speaker output no audio

HI Folks, I have a Sansui AU-20000 that has a 1.4V DC voltage on both speaker outputs. There is not audio output from either channel. The preamplifier section runs perfectly fine (I can connect to an external amplifier and it plays well). I have tested all the Power transistors and they all look good. I cannot find anything wrong with the audio boards too. Do you have any tips / suggestions on what can be done to resolve this issue? I'd like to get this unit working again. Regards Wala.
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Old 21st January 2013, 07:28 PM   #2
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With the DC, protection relays are kicking in. Being as old as it is.... If the outputs are good... Could be alot of things. Bad capacitors in the bias circuit??? Did you try zeroing the bias pots to zero the DC on the speaker outputs?
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Old 21st January 2013, 08:14 PM   #3
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Check for dry joints and resistors going high in value..
Next check the in put transistor pair followed by the vas circuit.
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Old 21st January 2013, 08:35 PM   #4
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Take as a fact that there is no working capacitors in a unit with this age all of them need to be replaced no questions asked .

Normally there is no chance that the protection will kick in and lock the output relay while still there is 1.5 volt DC present in the output terminals .

the specific unit might be cannibalized in the past and some clever guy bypassed the relay ...

The specific unit features 4 different power supply's inside one for the protection circuit one for the amplifier first stage one for the preamp and one for the amplifier

Start from there ... check all voltage according o the schematic pop the hatch to see for cannibalization replace all electrolytics and then check soldering psu

expect 85% of the problems of your amplifier to be shelf cured after that .

@ amptech
You surprise me for a person with such a nick name :""amptech"" ( putting resistors before electrolytics for an amplifier of that age )

Kind regards
sakis
PS :
Email me if you need a schematic
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Old 21st January 2013, 09:03 PM   #5
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Sakis, over many years of servicing countless amplifier's and building many...
Iv'e Encountered capacitors going leaky with age I've noticed resistors within vintage equipment going open and or high thus long tail transistor pair going leaky has well...amplifier's can be ikesome with many faults. Even the pcb itself can be part of the fault..

With out the schematic to hand one would start by checking voltages etc ..

Yes if you've got a circuit of this amplifier it would be interesting to see as when there's request for help on trouble shooting we can only give common answers...

Regards.
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Old 21st January 2013, 09:06 PM   #6
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Thanks for the useful information guys. I have now replaced all the capacitors from both audio boards. Also replaced the two heat sink mounted transistors with MJE15032 and MJE15033 which have better specs. Also replaced both the trimmers for bias and offset with multi-turn units. BUT, the offset issue remains. I did a thorough check of the boards and it all looks good. If I connect 3V on the speaker output (small button battery), the protection immediately kicks in and turns OFF the speaker output. So, I know that the protection is working. I have checked for dry solders and reflowed any that look suspicious. Jasoneyes01 suggested that I turn the bias pot to set DC offset. Is that really the case? Never done that before. Currently, when I replaced the trimmers, I pre-set them to the original trimmer settings. I was wondering if I need to screw in the boards before taking measurements? They have a plate that screws into the body and was wondering if that is where the ground connection comes from? Regards Wala.
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Old 21st January 2013, 09:19 PM   #7
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Hi,
Here it is the link where you can find the schematic.
Link:Sansui AU-20000 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine
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Old 21st January 2013, 09:20 PM   #8
amptech is offline amptech  Scotland
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Computerwala, did you check the voltages across the emitter resistors at the o/p these will be the high watt types say 5watt and .22 of a ohm or .47 of an ohm.. A reading of say 20mv or more can be seen.

Biasing sets up the q current for the output devices and if you over set this you notice the heat sinks getting hot!!! When replacing o/ps devices within a amplifier that has a trim a reset is a must or you could damage the replacements..
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Old 21st January 2013, 09:27 PM   #9
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Hi,
By way it is looked that this amplifier was built when the transistor were cheap.
I would check the voltage at the base of the output transistors to see where the 1.5 volt it is coming. If the base read 1.5 volts then the voltage it is coming from the drivers.
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Old 21st January 2013, 09:31 PM   #10
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Thanks amptech. I took a measurement of the AC current while the unit is turned ON and saw approximately 0.3A being used from the mains. No heatsink heating. I'll check the emitter resistors voltages tonight. Also plan on reading the service manual Tauro0221 sent (thanks). I have checked the positive and negative voltage rails on the Power Output transistors and those look good. So whatever the issue is, it has to be coming from the audio boards (it seems). I did measure the resistance of each of the 5W power resistors and they all measured at 0.3 ohms (look good).
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