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Growers 16th April 2004 11:49 AM

Pioneer A400 faint distortion on right channel
Hi everyone,

I have recently purchased a 2nd hand A400 on ebay and unfortunately there is a slight but audible distortion on the right hand channel when signal is present. The distortion sounds like a partially blown speaker (I spent an hour or so checking my speakers over before realising it was coming from the amp!) a sort of signal dependant mechanical buzzing. It is most apparent with a low level mid-range signal such as an a bass guitar note and is present using all inputs and also on the right headphone output.

Does anyone have any suggestions as to what may be causing the distortion?

Also, I did a search on this forum and found a thread regarding A400 schematics. Would anyone be kind enough to email me the schematics?


SVI2004A 16th April 2004 12:17 PM

possible o/c transistor - tried cheching output transistor(s)

subwo1 16th April 2004 12:21 PM

Two things I would check for are bad transistors and dried up electrolytics.

K-amps 16th April 2004 08:49 PM

Also no bias can be audible as crackling. Check bias too.

lucpes 16th April 2004 11:41 PM

Had some similar problems and the culprit was a blown diode :) but in your case it could be almost anything. Take a digital VOM and compare against the 'good channel', whilst shut down first and then when fired up, taking G R E A T care not to slip connectors, but first read:

check these first:

Growers 18th April 2004 11:55 AM

Thanks for your replies. Since my initial post I've had a closer inspection of the PCB and noticed some evidence of electrolyte leakage which could have come from one of two caps which I will try replacing.

Without schematics (and with little familiarity of amp design) I find it quite difficult to do fault diagnostics on a good channel vs faulty channel basis.

Thanks again.

lucpes 18th April 2004 03:20 PM

Replacing all the low value electrolytics you find on both L/R PCBs isn't a bad idea at all + it's cheap.

mudan 30th April 2004 11:14 AM

Hi all,

I've replaced the old PSU smoothing Elna black/gold caps with Panasonic TSUP as I couldn't source any better of the same spec.

I'm soon about to replace the coupling caps with Audyn MKP QS , hoping this will make a worthwhile improvement. I'm also replacing some of the larger Elna/Nichicons for the L/R channels with Black Gates.

Any recommendations for mods to the A400, including my choice of the Audyn MKP QS would be appreciated.



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