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Pioneer A400.  Headphone output fine speakers distorted
Pioneer A400.  Headphone output fine speakers distorted
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Old 1st December 2017, 02:03 PM   #1
bigshuggy is offline bigshuggy
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Default Pioneer A400. Headphone output fine speakers distorted

Just a quicky for now as this amp might be going back in the return post today!

Got a really nice condition (cosmetically at least) A400 off Ebay. Seller said it had a scratchy volume pot so that was first thing I cleaned. Tested it on headphones first and thought - 'It's a keeper!'

Connected up the speakers - BAD distortion.

Checked the idle current and it reading 400-500 mV when the manual says it should be 20 mV!! Can't get it down even close to 20mv.

Should I just punt it back to the seller and look elsewhere?

cheers.

Last edited by bigshuggy; 1st December 2017 at 02:16 PM.
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Old 1st December 2017, 02:20 PM   #2
Mooly is offline Mooly  United Kingdom
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Pioneer A400.  Headphone output fine speakers distorted
The bias current is measured across the 0.33 ohm emitter resistors on the output stage.

If you have 400+ mv there, and there are no speakers connected then you have problems. If the current were that high then things should be getting very hot very quick.

If they are not then I would suspect the resistors and possible output transistors have failed.

Ultimately it should be an easy fix but it will require normal faultfinding to see whats what.
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Old 1st December 2017, 02:26 PM   #3
bigshuggy is offline bigshuggy
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The reading I get across the big white 0.33 resistor is upwards of 400mv no speakers attached.

Paid 70 for this one but another one has come up locally for 80 - just my luck!

So - you think I should chase this fault to save the hassle of sending it back?

I could justify it as a refurb if I got some money back from the seller.


or just get the other one?

Last edited by bigshuggy; 1st December 2017 at 02:40 PM.
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Old 1st December 2017, 04:35 PM   #4
Mooly is offline Mooly  United Kingdom
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Pioneer A400.  Headphone output fine speakers distorted
I can't make the decision for you its up to you whether you want to take it on as a restoration and learning exercise.

400mv across 0.33 ohms means that 1.2 amps is flowing. Now if it really is passing 1.2 amps then something should be getting very hot... that something being one or both output transistors in the channel concerned... which leads to the next question... are both channels the same ? That would be very odd and suggest a common cause.

Could it have suffered impact damage and have a crack on the PCB ?
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Old 1st December 2017, 04:44 PM   #5
RJM1 is offline RJM1  United States
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I would just suspect the 0.33 ohm emitter resistors on the output stage. If the output transistors failed open you would have no voltage across the resistors an if shorted you would have 35V or more. I've seen them blown on both channels before. Some times when a speaker wire gets shorted people change te speaker connections at the receiver to see whats wrong and end up blowing up the other channel. This usually happens at low volumes or the output transistors would short and you would have big problems with nothing coming out of the headphone jack.

Last edited by RJM1; 1st December 2017 at 04:58 PM.
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Old 1st December 2017, 06:17 PM   #6
bigshuggy is offline bigshuggy
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I'll wait on the seller responding before I go poking about too much. I was really hoping for a straight forward purchase - for a change...
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Old 1st December 2017, 08:07 PM   #7
bigshuggy is offline bigshuggy
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Quote:
Originally Posted by Mooly View Post
I

...something should be getting very hot...

Could it have suffered impact damage and have a crack on the PCB ?
PCB and joints looks fine - the inside of the amp is actually quite clean.

I think I've found the issue - or one at least. Not posting here just yet in case it's going back - don't see why I should make the sellers life easy considering they've sold me a faulty amp!

Last edited by bigshuggy; 1st December 2017 at 08:33 PM.
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Old 2nd December 2017, 07:21 AM   #8
Mooly is offline Mooly  United Kingdom
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Pioneer A400.  Headphone output fine speakers distorted
Good luck be interesting to know what you have found.
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Old 5th December 2017, 02:19 PM   #9
bigshuggy is offline bigshuggy
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Still no resolution with this...

Something else I noticed through the headphones though. When using CD as input - I could still hear the source very faintly when selecting other inputs.

I know - I should really walk away from this now...
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Old 5th December 2017, 02:28 PM   #10
Mooly is offline Mooly  United Kingdom
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Pioneer A400.  Headphone output fine speakers distorted
Signal breakthrough from other inputs might actually be a 'characteristic' rather than a real fault.

It is a common issue on many cost effective designs and caused by the use of simple single pole switching as a summing point to then feed the signal to a relatively high impedance amplifier. The signal couples capacitively through adjacent PCB tracks and cabling.

It can be 100% eliminated at a design stage but often isn't given much consideration.
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