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Sansui AU-7900 Problem
Sansui AU-7900 Problem
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Old 9th June 2019, 04:22 AM   #1
Nagal is offline Nagal
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Default Sansui AU-7900 Problem

Hi....

i have the sansui au-7900, the sound is normal but, but when I turn off the amplifier. there is sound Bukk in the speaker, there is a solution to this problem, I am afraid it will damage the speaker.

Thanks in advance
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Old 9th June 2019, 07:31 AM   #2
donpetru is offline donpetru  Romania
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In the first phase, you should check the relay that switches on loudspeakers. It is possible that the contacts of this relay have remained welded. So, check the relay RL601.
If the relay is OK, the DC Offset level should be checked on the output of each channel with volume potentiometer: minimum and maximum. And of course, will have to be checked, all capacitors on the audio signal path of the amplifier.
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Old 10th June 2019, 04:47 PM   #3
Nagal is offline Nagal
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Hi donpetru
How to cek dc offset level.
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Old 10th June 2019, 06:08 PM   #4
huggygood is offline huggygood  France
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sansui au-7900 service manual - Google Search
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Old 10th June 2019, 06:48 PM   #5
donpetru is offline donpetru  Romania
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Quote:
Originally Posted by Nagal View Post
Hi donpetru
How to cek dc offset level.
How to Measure DC Offset: 5 Steps (with Pictures) - wikiHow
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Old 11th June 2019, 04:13 AM   #6
BobtheBuilderMLG is offline BobtheBuilderMLG  United States
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Also, Nagal has the amp ever been restored? I just got done repairing the 7700 which is a similar amp that had a similar problem and then it blew the relay coil.
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Last edited by BobtheBuilderMLG; 11th June 2019 at 04:15 AM. Reason: cause
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Old 11th June 2019, 04:52 AM   #7
Ian Finch is offline Ian Finch  Australia
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Relays do fail with bad contacts but it may not be the case here, since you have sound but only have the dying amp. noise problem when you switch off. The noise is likely normal but the speaker relay should block it by opening as soon as power is cut. Probably, the protection relay is not being switched properly and many old amps show capacitor failures in the protection circuit that powers it. You can easily measure with a multimeter, when the relay coil is powered with 24 volts or not, when you turn on/off and deduce whether the contacts are opening. The manual describes and shows the details of the circuit it but be warned that this is an old design and some components will now be unobtanium. If you can't test this function or measure caps and semis properly, you will need practical help.
Sansui AU-7900 - Manual - Integrated Stereo Amplifier - HiFi Engine (register to receive free downloads)
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Last edited by Ian Finch; 11th June 2019 at 05:18 AM.
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Old 11th June 2019, 03:01 PM   #8
Nagal is offline Nagal
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thanks to all, this Saturday I will check the sansui amplifier, because I have time on Saturday only.
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Old 12th June 2019, 05:50 AM   #9
Nagal is offline Nagal
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Click the image to open in full size.

Hi all.
I tried to move the switch to connect (see picture) I only played the Sansui power with the Bluetooth pre-amp, the sound was normal and when the amplifier was turned off, the speaker had no sound Bukk.
according to the master, the damage is at the pre amp or power amplifier.

Thanks in Advance
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Old 12th June 2019, 04:44 PM   #10
Mark Tillotson is offline Mark Tillotson
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There's a worrying amount of corrosion on the outside - I hope its less affected internally.
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