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Old 11th February 2014, 09:22 PM   #1
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Default Technics SU-V470 out of balance

Hello everyone,

I'm new to this forum and new to the audio electronics. I hope you guys can help me out.

I've recently picked up a used Technics SU-V470. Generally it is in good shape and sounds OK, however I noticed there is a slight difference in the R and L channel output. The right output sounds clearer and a bit louder than the left channel. If I turn the balance knob to 10 or 11 O'clock they sound balanced again. This applies for all inputs (CD/tuner/aux ect) and also for the headphone jack output.
Does somebody recognize this behaviour? Could there be something wrong in the pre-amp or power amp (faulty transistors i.e.)?

I have some background in electronics, however audio is completely new to me. Where can I start measuring? I already have the circuit schematics.
I hope you guys can help me out!
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Old 11th February 2014, 10:32 PM   #2
mjf is offline mjf  Austria
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could be the volume pot (or annother contact)...........

i would clean all contacts (rca jacks,fuses,pots,switches...........) with contact cleaner(spray). if it does not help,it could be a part failure.
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Old 12th February 2014, 06:22 AM   #3
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Originally Posted by mjf View Post
could be the volume pot (or annother contact)...........

i would clean all contacts (rca jacks,fuses,pots,switches...........) with contact cleaner(spray). if it does not help,it could be a part failure.
I think there is no problem with pots or switches, since I don't hear any cracking when I turn them. Also, I'm not a fan of contact spray, since it is only a temporary fix..
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Old 12th February 2014, 07:38 AM   #4
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Tape monitor switch might be an issue since all signal pass from there . We often bypass them since hardly ever anymore one uses cassette players .

On this amplifier exist also a couple of thermistors located on the volume control pcb which the use of them is totally unknown to me . Even though i have repaired something over than 5-6000 amplifiers in my life i have never seen anything like that ever again

I will have to presume that thermistors sense temp inside the amplifier and decrease volume accordingly. you could remove them or swap them to see how it will work

In general these amplifiers suffer only from soldering issues down under and all over the board and cleaning issues ....Unit is quite young to have capacitor issues but still one feedback capacitor that has a failure might also present similar type of fault behavior .

Kind regards
Sakis
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Old 12th February 2014, 07:39 AM   #5
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Does sound like an issue with the volume control to me rather than a circuitry issue. Sometimes they can go like that and not crackle.

Given that the problem position seems to be where it would have been likely to have been nudged up and down a bit then it becomes more likely too.

If it was something with the circuitry it wold happen with similar level not similar volume position.
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Old 12th February 2014, 07:42 AM   #6
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Just seen sakis' post. He has some good suggestions. I've never encountered thermistors in a volume circuit before so it would be a good starting point.
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Old 12th February 2014, 10:05 AM   #7
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Originally Posted by east electronics View Post
On this amplifier exist also a couple of thermistors located on the volume control pcb which the use of them is totally unknown to me . Even though i have repaired something over than 5-6000 amplifiers in my life i have never seen anything like that ever again

I will have to presume that thermistors sense temp inside the amplifier and decrease volume accordingly. you could remove them or swap them to see how it will work
That makes sense! I will take them out and replace them with equivalent regular resistors.

I assume these are the thermistors on the volume pcb?

Click the image to open in full size.
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Old 12th February 2014, 10:04 PM   #8
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Ok I measured the resistance from each thermistor and they seem to be giving both 50kohm at room temperature. So no problem there, however I'm going to replace them anyway.

Next I reheated all solderjoints on the volume pcb. After I had done this, I noticed the volume pot didn't turn as smooth as before. I can hear the pot actually grinding when I turn it. Obviously some kind of lubrication has vaporized during reheating of the solderjoints, or dirt came loose inside the pot..

So I guess now I'll have to spray it in anyway. I will use Tuner 600, however this will not lubricate the pot I think? Do I need it to spray with some other solution?
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Old 19th February 2014, 09:16 PM   #9
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Success!

I replaced the thermistors with regular resistors and sprayed in the volume pot with Tuner 600. The amp is fully in balance again and sounds perfectly!

Thanks for helping me out!
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Old 20th February 2014, 07:42 AM   #10
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Great result, good old sakis! A wealth of experience.
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