Xref: utzoo sci.electronics:6942 rec.audio:13897
Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ames!ncar!asuvax!hrc!xroads!wiz
From: wiz@xroads.UUCP (Mike Carter)
Newsgroups: sci.electronics,rec.audio
Subject: Re: Help with intermintant stereo problem
Summary: Some things to look for
Keywords: Pioneer SX1980
Message-ID: <763@xroads.UUCP>
Date: 14 Jul 89 09:03:44 GMT
References: <1364@esunix.UUCP>
Followup-To: sci.electronics
Organization: Crossroads, Phoenix, Az
Lines: 63

In article <1364@esunix.UUCP>, hcooper@esunix.UUCP (Harrison Cooper) writes:
> I have a friend who owns a Pioneer SX1980 reciever.  He has had it
> in the shop once before (a couple of months back) and had one
> channels final amp replaced.  However, now when he uses it (after
> it has been turned on for some time) and shuts it off, it will not
> come back up and run.  It goes thru some sort of diagnostics before
> it will operate (user can see lights come on the front, etc).
> 
                   <<<   EXPLANATIONS DUMPED TO BIT BUCKET   >>>
   
> ventalation (not rack mounted), and it was working earlier in the day.
> He also said sometimes it will start to power up (the lights will come
> on, but slowly fade out).  The service tech said he "might have a bad
> zener".
> 
> 
> Harrison Cooper  N7KSt 


While I question the period of time AFTER the first visit that the problem
showed up (Take it back, Jack!)
I will offer some suggestions and things to check here.
(1) Check DC bias level comming into the amp
(2) Check the two large resistors (normally Flame-proof types..same value)
    found near the output amplifier. Look for signs of abnormal heating
    around by the base of the leads and an open in the resistor itself
    can cause the amp to blow that channel.
(3) Watch out for shorted diiodes!!!
(4) Check the big mamma capacitors at the power supply WHILE it's running.
    you will want to keep a sharp eye out for voltage droop. If it does
    remove all the caps in that section and replace.
(5) WHen the original tech replaced the Amp, he might have left some
    solder entrails or scorched a wire. Check VERY closely.
(6) Look at the bottom of the board...especially where the amp was soldered
    in...any missing solder (i.e. He missed a lead?)
(7) Check the DC resistance from the output plugs to those resistors I
    mentioned. Are they within 1% of each other? NO? Take it back!
 
DEpending on models and methods used in final stages, things can vary
quite a bit. You should have a very small amount of DC volatge on the
output to the speakers while it is running. (2-4V is usually a rectified
reading from the audio waveform..if you get something like
40VDC..well, kiss your speakers goodbye)
 
A lot of service techs can forget to plug the power cables in correctly
or ruin one while removing it. When I worked as a stereo repair tech
I found a jillion complications that resulted from prior service with
bad soldering, scorched wires, wires tightly wound around heat sinks,
solder beads roaming around or even scraps of solder wick, wrong
value components stuck in or "close enough" replacements that arent
wattage rated as high as the originals.
If the lights go dim slowly it really sounds like a thermal run-away
in the supply or *something* connected to it...lots of current drain
pulling the supply down. The placebo "It's probably a zener diode"
is more than often a "I don't know, bring it in so we can charge you"
response.
 
	-Mikey
-- 
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= Mike Carter  N7GYX, Phoenix AZ| Q: Why did the Chicken cross the road  ?  =
= hplabs!hp-sdd!crash!xroads!wiz| A: To ESCape the Main Menu .              =
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