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sansui: G-9000;

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Forum » Radios and other type of sets (Physics) etc. » MODELS DISPLAYED » sansui: G-9000;
           
Roee Sulimarski
 
 
IL  Articles: 6
Schem.: 4
Pict.: 0
22.Feb.09 15:02

Count of Thanks: 15
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Hi,
my G-9000 right channel is making me a hard time.
it some times works well , but often sounds very poor (unlike the left channel).
it sounds very weak... it is happening at all inputs respectively (aux,tape,phono and even radio).
what do you suggest? where to start?

I have the schematics, I tried to upload it to the G-9000 web-page several times, but didn't managed!!!
if someone manages, I will be glad to send it to him...


 

Thomas Albrecht
Thomas Albrecht
Officer
USA  Articles: 384
Schem.: 6679
Pict.: 682
24.Feb.09 18:50

Count of Thanks: 6
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Hi Roee,

While I work with you offline to help you get your schematics posted, here are some suggestions on how to start troubleshooting.

One thing that is very nice about your situation is that you have two stereo audio channels, one working and one not.  So you can compare the two to find where the problem is in the nonworking one.

Since you mention that the problem is intermittent, the first thing to do is check for any loose connections, poor solder joints, or dirty switch or relay contacts.  Dirty switch contacts are what I would suspect first.  Make sure that every switch involved in the audio section (input selector, loudness/tone switches, speaker selector, etc.) is clean (use contact cleaner if necessary).  Also it is common to have dirty contacts on the speaker protection relays (remove cover and clean contacts, or check by physically pressing on contact or jumping with jumper wire when you hear distortion).  Also check cable connection plugs/sockets between boards -- wiggle them to see if any of them affect the distortion.

If none of the above helps, then it's time to go into the circuitry.  Concentrate only on the specific boards in the audio amplifier system, and compare the working and non working channels.  You can do this in several ways:

1.  Use a voltmeter to probe various points in the circuitry for voltage.  Notice that the schematic in the service manual you have (but not on the standalone schematic) has voltages listed at very many points.  Starting at either the input or the output of the audio amplifier, compare the voltages at various circuit points in the working and nonworking channels and also compare against the value given on the schematic.  Work your way all the way through the amplifier circuitry.  If you find a deviation (especially between left and right channel), check all components in the neighborhood, and you may find the problem.

2.  Another alternative, if you have a signal tracer, is to probe and listen to the audio at various points along the way in the nonworking channel.  You have distortion at the output, but none at the input.  By listening to the signal at every stage along the way, you will find where in the circuit it goes bad.  Test components in that area.

3.  Yet another alternative, if you have a scope, is to input a sine wave into the audio input of the bad channel.  When there is distortion you will see a distorted waveform at the output to the speaker.  Probe various points working backwards, and when you see the signal get perfect, that is the stage where the failure is.  Best to also compare the good versus bad channel, since in a complex transistorized amplifier like this, it is sometimes tricky to trace the actual audio path (as opposed to many transistors with other functions, like setting biasing, etc.).

If the problem is not in the relays or switches, the first components to suspect when going through the circuitry are the electrolytic capacitors -- all the little ones interspersed throughout the audio amplifier circuitry on the bad channel, as well as the larger ones at the speaker output.  Most likely the electrolytics in the power supply are not the problem, since this is common to both channels.  You can test electrolytic capacitors with an ESR (effective series resistance) meter if you have one, or you can try testing for shorts, and bridging with new capacitors if they are not shorted (to check for open failures).

It is also not too unusual to find bad transistors (sometimes the failure of the transistor is caused by the failure of another component, such as a nearby electrolytic capacitor).  You can often find exact replacements with an internet search.  There are also some very good forums (such as the DIY Audio Forum) where folks can help find obscure transistors.  Do you have a simple transistor tester?

Good luck,

Tom

Konrad Birkner † 12.08.2014
Konrad Birkner † 12.08.2014
 
D  Articles: 2334
Schem.: 700
Pict.: 3655
25.Feb.09 09:52

Count of Thanks: 8
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I made similar experience with Pioneer Receivers, where the speaker relay was the culprit.

Should Your set also feature a time delayed speaker connection after power on, then check the relay contacts.

Anyway: Good luck!

Torbjörn Forsman
 
 
S  Articles: 119
Schem.: 26
Pict.: 58
25.Feb.09 21:43

Count of Thanks: 5
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In my experience, a very common intermittent fault on japanese power transistors from the 70's is that either the base or emitter connection goes open circuit inside the transistor. Most likely, it is due to bonding wires coming loose. In some cases, a such transistor might seem o.k. when tested with a transistor tester or a multimeter set for diode testing, but then the bad connection fails when subjected to higher currents in the amplifier.

Also, i would like to mention a problem that I had many years ago with a Pioneer amplifier from the early 70's , it also had intermittent problems in one of the output stages. Sometimes it worked as it should, sometimes it gave distorted sound and sometimes a fuse blew. The reason was that some swarf from the threaded fastening hole in the heat sink had got stuck between the heat sink and the mica pad, and finally it had worn through the mica pad causing intermittent short circuit between the transistor's collector and the heat sink.

Roee Sulimarski
 
 
IL  Articles: 6
Schem.: 4
Pict.: 0
27.Feb.09 08:35

Count of Thanks: 3
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Hi,

Thank you all !!!
Double Thanks - Tom ,with the help and patient regarding the schematics scanning and resizing procedure.
I hope this weekend ,to start analizing the problem step by step like you've suggested,
 

Best Regards,
Roee.

 

  
rmXorg