"GregS" wrote in message
...
In article , Readily Visible
wrote:
Yesterday with the unit on its side and the top and bottom removed, it
played for 10 hours without the problem left channel dropping out.
This was using 16 ohm speakers.
Today I hooked the unit up to 8 ohm speakers, still on its side and the
top and bottom removed, and after less than two hours of play, the left
channel dropped out. It could have been sooner, I set it to stereo and
set the balance to the midpoint. I checked it after about two hours of
play and the left channel was gone.
Here is where it gets pinpointy. First I tried jumping the speaker relay
solder joints and got nothing, so that rules out the speaker relay as
the problem.
Then I got a nice Japanese style pine chopstick and started poking on
the amp board. First, since the bottom was oriented to me, I started
poking all the cold solder joints that I had earlier hot soldered. No
change. The left channel remained dead. Then I rotated the unit, still
standing on it's end and began poking all the trannies, resistors,
coils, caps and whatnot. First time through, nothing. Then, when I poked
the leads of one of those flat trannies screwed to a heat sink on the
left side of the amp board, the left channel kicked back in.
For those who are really interested, here is a link to the service
manual with parts lists and schematics:
http://www.hifiengine.com/manuals/pioneer/sx-838.shtml (relatively
painless registration required)
The tranny in question is 2SD358 on the left channel side. It is one of
those that drives one of the output trannies. The solder joints are hard
to access, but look to be well soldered, so I am going to assume, at
this point, that poking the leads corrected, for the moment, an internal
fault in the tranny. If the local electronics supply shop has the NTE
equivalent, I may swap it out at some point, but for now I will just let
the unit play to see what may develop.
Chopstick will work nicely especially if its plastic. You could have also
tapped
the output relay and avoided attaching leads.
greg
Is it just me, or is this getting (made) ridiculously complicated ? It's an
old amplifier with a well defined repeating intermittent problem, that can
be provoked pretty much at will, most of the time, by poking with a
chopstick. It has now reached the point where different thread respondents,
are offering the same advice. Just solder up all the joints on all the
devices attached to the heatsink, and if that doesn't work, swap the suspect
transistor with the corresponding one from the other channel. if the problem
moves to the right channel, then you know it's the transistor, so just stick
a new one in. If neither of those things cure it, then you need to apply
some more detailed fault finding. This sort of problem is a fundamental
'bread and butter' one that you would put the workshop junior on. I don't
mean to be unkind, but if you are having this much trouble tracking down a
straightforward problem like this, I'm not sure that you should actually be
inside it in the first place ?
Arfa