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Old 20th February 2013, 10:23 AM   #1
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Default Technics SU8044

Dear Friends,

I am having a problem with the above mentioned amp. When I increase the volume from 0 to a quarter level the sound is OK. If I increase the level above quarter onwards , the sound becomes fuzzy and distorted. I have tried replacing the 2 filter capacitors(8200uf 50v) with 2 (15000uf 50v) caps but it is the same. Can someone give me a clue what can be causing this problem.

Thanks

Carlos Rajoo
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Old 20th February 2013, 10:33 AM   #2
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It really needs a scope to see what is happening.

Are both channels affected ?

If both are affected, then does altering the balance control alter the level at which the problem occurs. In other words with one channel only running does it go louder before the distortion occurs which could indicate a PSU issue.

Does it do it on headphones ?

Have you tried different speakers ?

Beyond that, and its a scope job which would show in seconds what was happening.
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Old 21st February 2013, 10:31 AM   #3
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Originally Posted by Mooly View Post
It really needs a scope to see what is happening.

Are both channels affected ?

If both are affected, then does altering the balance control alter the level at which the problem occurs. In other words with one channel only running does it go louder before the distortion occurs which could indicate a PSU issue.

Does it do it on headphones ?

Have you tried different speakers ?

Beyond that, and its a scope job which would show in seconds what was
happening.

Yes both channels are affected and the balance control does not has any effect. I haven't tried with the headphones yet. What I realise is that it becomes even worst if I engage the "loudness" switch. I have replaced all the small capacitors around the power supply and power output board. When I off and on the amp, for the first few seconds it sound OK and then it distorts again. I have tried different speakers but still the same. The AC secondary voltages are correct. Can the transformer or the rectifier diodes cause this problem.
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Old 21st February 2013, 10:45 AM   #4
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I trust you will put the 8200uF caps back in to avoid over current issues with the power supply. 8200uF was chosen as the optimum value exceeding that will only have a detrimental effect.
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Old 21st February 2013, 10:55 AM   #5
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From what you describe it does seem as though it could be PSU related. Both channels affected is a big clue. It won't (as in 99.99999%) be the transformer. More likely a low voltage rail that is common to both channels is doing something odd. This is where a scope is essential for fault finding. All you can realistically do without is check all the DC conditions and monitor any rails and see if they alter when the problem appears.
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Old 5th March 2013, 10:38 AM   #6
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Quote:
Originally Posted by Mooly View Post
From what you describe it does seem as though it could be PSU related. Both channels affected is a big clue. It won't (as in 99.99999%) be the transformer. More likely a low voltage rail that is common to both channels is doing something odd. This is where a scope is essential for fault finding. All you can realistically do without is check all the DC conditions and monitor any rails and see if they alter when the problem appears.
My scope display tube is dead. I took the DC voltage across both the caps today. As I increase the volume, the voltage drops slightly, from 41volts to about 39.9 volts. I connected the headphone output to a different amp and it also distorts as I increase the volume above quarter level. When I turn off the amp, set the volume at one third level and ON the amp, for the first 2 to 3 secs the amps sounds OK, then it starts to distort again. I have replaced all the small caps and transistors in the power supply/mute circuit. It shows no difference.

Thanks

Carlos
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Old 5th March 2013, 11:58 AM   #7
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If the headphone feed distorts (and I assume you had no speakers connected at the same time) then it sounds like some bias or operating point is wrong but very difficult to say for sure.

Its normal for the voltage to fall on the reservoir caps as the volume is increased.

Without a scope your only realistic hope is that you get lucky and can find a definite voltage that changes dramatically in those first couple of seconds.
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Old 5th March 2013, 01:08 PM   #8
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You could check that the preamp output is clean before it connects to the power amplifier and thus eliminate the many possibilities there. However, I see that there is a single positive regulated 30V supply to the output stage modules and this is dropped to 14V for the input stage and 1.2 V which must be a bias voltage for the output stage. The balancing negative 1.3V supply comes from the discrete part of the P.A.

If you want to point the finger of suspicion at a Japanese electronic design, it is the simple zener/transistor regulator. Check the output there at the emitter of Q503 as no proper bias to the power amp = horrid sound, right? To be certain, check for ~1.2 V at
pin 0 of IC401,402.

No signal or speakers connected when testing please and take care not to slip with probes and smoke it all.
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Last edited by Ian Finch; 5th March 2013 at 01:31 PM. Reason: Add link
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