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Old 21st November 2014, 07:38 PM   #1
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Default Onkyo A7 question

I am refurbishing an Onkyo A7 and in the circuit for each of the four output transistors there are 3.3uf 63v electrolytic capacitors bypassed by 0.001(?) ceramics that connect between VCC (+ and -) and the collectors of the output transistors.

I am struggling to understand the purpose of these

I have attached the circuit

Thanks
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File Type: pdf hfe_onkyo_a-7_circuit.pdf (393.9 KB, 60 views)
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Old 21st November 2014, 07:47 PM   #2
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AFAICS they are between the rails and ground!

Perfectly normal.
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Old 21st November 2014, 07:55 PM   #3
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thanks Cliff, but what is their purpose?
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Old 21st November 2014, 08:05 PM   #4
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Quote:
Originally Posted by pwdiya12 View Post
thanks Cliff, but what is their purpose?
Decoupling. Google it.

Supply fast current pulses into the output transistors (and then the speaker!) which may not be available from the main PSU reservoir cap which may be a long way away through (inductive) wire.

Local decoupling is "A GOOD THING", and never does any harm. The higher the working frequencies the more important it becomes.
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Old 21st November 2014, 08:12 PM   #5
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great thanks.

so if I increased the 3.3uf to 4.7uf this would have a marginal effect (if any)?
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Old 21st November 2014, 08:16 PM   #6
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You would be VERY hard pressed to measure any difference, let alone hear it.
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Old 21st November 2014, 08:20 PM   #7
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thanks Cliff, much appreciated!

I do have 4.7uf 100v Nichicon electrolytics, but no 3.3uf 63v, so will substitute these.

As an aside would film caps make better replacements in this instance, or do these need to be polarized?

Best

peter
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Old 21st November 2014, 08:34 PM   #8
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Why are you changing them??

Are they faulty, or are you infected with the audiophool's "must swap caps" bug?

They are either alu or tantalum polarized electrolytics.

I am not the person to empathise about cap rolling.

Good luck. Do read stuff. Lots of it.
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Old 21st November 2014, 08:53 PM   #9
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you may go up to 100uf at 63 volt if you like and this will happily work with 1uf film bypass capacitor

you might find that after that you get a sense of better impact , better transients and cleaner high ... don't expect miracles if any improvement should be marginal but audible

Kind regards
Sakis
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Old 21st November 2014, 08:53 PM   #10
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ok thanks again cliff
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