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Old 15th August 2011, 01:02 PM   #1
samlo is offline samlo  India
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Join Date: Aug 2010
Default pioneer sx 3800

Hello, All experts I just repaired Pio. sx3800.(PSU, POWER AMP, TONE. I removed tuner section because i want to use it as power amp for aux input only) ) it was dead and some components were missing. some components are not available at local market, so i used equivalents. mainly, i made 3 changes (1) i used C5242, A1943 as output tr. (2) i used C5171, A1930 as driver (3) In schematics there is D109 ( STV 2H ) in bias chain, which is outdated and not available, so i used ordinary 1N4007 diode. except these changes rest circuit is original. i checked and set dc offset about
+- 5mv, then set bias preset at middle position , then checked voltages at bases of drivers, it shows 1.10 V( + and -), then i started amp with cd player conected to input and i put 22 ohm res.( series connection) between main power rail for safety. Amp came alive. then i removed 22 ohms res. and powered directly . Amp is working ok. But there are two minor problems. (1) Q115 ( vas ccs) and Q109 (pre driver) becomes hot ( not very very hot) (2) sound is very good (not excellent) upto 40---50 % vol. level but afterwards it becomes distorted. i dont know how to solve this problem because i dont have any CRO or Analyzer. I need some tips and tricks from experts to rectify these problems. kindly help and reply as soon as possible. thanks in advance, sorry for bad english. PIO sx-3800sm_01.jpg

PIO sx-3800sm_02.jpg

Last edited by samlo; 15th August 2011 at 01:14 PM.
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Old 15th August 2011, 01:24 PM   #2
wahab is offline wahab  Algeria
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Location: algeria/france
Quote:
Originally Posted by samlo View Post
(1) Q115 ( vas ccs) and Q109 (pre driver) becomes hot ( not very very hot) (2) sound is very good (not excellent) upto 40---50 % vol. level but afterwards it becomes distorted.
Q115/109 are respectively CCS and VAS and dissipate about 200mW
each, so no surprise that they are a little hot , moreover if they have
no heatsink.

As for volume , it could be that the source output level is such
that the amp reach full power well before the volume pot is set
at maximum value.

This is very common when using old 70s/80s amps that have
input sensitivity of about 0.2/0.4V while any CD player
will easily have 1V output level at least.
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Old 15th August 2011, 03:09 PM   #3
samlo is offline samlo  India
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Join Date: Aug 2010
Hi Wahab, thanks for reply. i also think tr.heating is normal. but i dont think that distortion is due to over loading of the input, because at about half vol., sound is not so loud. you said that o/p of cd player is 1V. is it rms or peak? secondly, i also have pioneer A221 integrated amp. its rail volt. is +-35V, o/p about 35 watts, it has cd input , its sound quality is exccellent. i studied its schematic but there is no any attentuation in cd input to decrease signal level. kindly reply.
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