Maker Pro
Maker Pro

Sony STR DE-425 centre channel malfunction

M

Main Account

Jan 1, 1970
0
I got an old 5.1 Sony STR DE-425 (doesn't have a 5.1 decoder inside) which I
mainly use as a stereo receive. Some couple of weeks ago I connected it to
the 5.1 output of a DVD player and found that the center channel acts like
it only reproduces "half" of the sine wave... if you know what I mean. Took
the cove off and noticed that it uses an STK 350-230. Don't have the
schematic of this amplifier nor the pin layout for the IC to test it
properly but while looking at the back of the circuit board I touched the
IC's pins close to the right hand side of the circuit (holding it in your
hand, looking toward front side with the pins pointing down...) and the
problem vanished. Took my finger off and the sound went bad... As long as I
keep my finger on the circuit board the thing works properly but I can't
keep my finger there all time...
I was thinking to buy a new IC (if I can find one...) but this "magic
finger" issue brought a new question: what if the IC is ok, something else
(like a capacitor for example) causes the problem? Replacing the IC may not
be a solution. Any thoughts?

....SM
 
J

Jerry G.

Jan 1, 1970
0
I would be looking for a bad trace, or bad solder connections on the
board. It is a lesser of a chance that the output IC is bad. Pressing
on the board to make the unit work is an indication more of bad solders
more so, than bad components!

It is possible however, to have a part that is mechanicaly defective
inside, and when pressing on it, makes it work temporarily. This type
of event is very rare, and is less than 1% of the time.

As for the output IC, I have not bought these for a while, but I would
think it may still be available from Sony. It is advisable with Sony
products to use origional parts only, if you want to have long term
reliablity from them. When doing service work, I make it a habbit to
use origional parts to avoid having returns on my work.


Jerry G.
======
 
A

Andy Cuffe

Jan 1, 1970
0
I got an old 5.1 Sony STR DE-425 (doesn't have a 5.1 decoder inside) which I
mainly use as a stereo receive. Some couple of weeks ago I connected it to
the 5.1 output of a DVD player and found that the center channel acts like
it only reproduces "half" of the sine wave... if you know what I mean. Took
the cove off and noticed that it uses an STK 350-230. Don't have the
schematic of this amplifier nor the pin layout for the IC to test it
properly but while looking at the back of the circuit board I touched the
IC's pins close to the right hand side of the circuit (holding it in your
hand, looking toward front side with the pins pointing down...) and the
problem vanished. Took my finger off and the sound went bad... As long as I
keep my finger on the circuit board the thing works properly but I can't
keep my finger there all time...
I was thinking to buy a new IC (if I can find one...) but this "magic
finger" issue brought a new question: what if the IC is ok, something else
(like a capacitor for example) causes the problem? Replacing the IC may not
be a solution. Any thoughts?

...SM

Does it also work if you press on it with something insulated like a
screwdriver handle? If so, it's probably a cracked solder connection.
Check carefully around hot spots on the board for bad soldering. If
it's the conductivity of your skin that's fixing it, I would look for
an open resistor in the area you're touching.

Andy Cuffe

[email protected] <-- Use this address until 12/31/2005

[email protected] <-- Use this address after 12/31/2005
 
M

Mark D. Zacharias

Jan 1, 1970
0
As for the output IC, I have not bought these for a >while, but I would
think it may still be available from Sony

The STK350-230 is a driver IC, not an output. They are available from Sony
and from generic suppliers like MCM.

Mark Z.
 
M

Main Account

Jan 1, 1970
0
I was actually looking for bad soldering when touching the board. I noticed
that sometimes you cannot see it (before I start touching it I use a bright
light and a magnifying glass to visually check all connection in that area)
the solder crack is so thin that only by moving the terminals you'll get a
result. So when I noticed the change I thought that I found it but very soon
I got disappointed: I don't need to apply any pressure (ok... let's say
"significant" because if there's no pressure at all there's no contact) to
make it work. I tend to believe that is more the skin resistance
"short-circuiting" something than bad soldering. More precisely: if I
connect the voltmeter (digital) on the 200V scale between the negative pin
of C553 and (any end of) JW118 it measures 46V, the speaker's cone goes a
few millimeters up (DC voltage on the speaker goes from 0.05 to 2.5V)) but
the amplifier works properly. This doesn't look good to me, the amplifier
should work well with zero DC voltage across the speaker. I replaced the
capacitor (100uF/10V) and there's no change. This capacitor connects pin# 2
of the IC to the "+". I measured the voltages around Q553 and Q445 and
everything looks balanced (when I don't have the voltmeter connected as
mentioned above, when I connect it the voltage goes unbalanced on the two
transistors).
I couldn't find any open resistor in that area.. as much as I could verify,
without the schematic I'm like a blind man walking down the street with a
white stick in my hand. After a couple more tests I noticed that if I use a
different voltmeter (a more expensive one) the amplifier works just a little
better, not as well as with the cheap voltmeter.... First conclusion: Sony
likes cheap staff. I'm inclined to think that the IC has some internal
problem and must be replaced. What do you think?

....sm
 
M

Mark D. Zacharias

Jan 1, 1970
0
Main said:
I was actually looking for bad soldering when touching the board.

At least try resoldering the driver IC's. This is a pretty common problem.

I went to look at the PDF service manual, but it doesn't include the
schematic. Bummer.

Mark Z.
 
M

Main Account

Jan 1, 1970
0
Already did. And not only the driver but other things I thought may be wrong
around it. Nothing changed. Then I decided... don't tell anyone! - I
connected a 1Mohm resistor between that capacitor and the link, the thing
works fine now. For how long it'll work... I wouldn't do this for a client
but this is my own thing I'm talking about so what the heck. Not prepared to
buy a new one yet. Also noticed that the driver for front channels is
operating red hot, installed a little heatsink on it. Thanks guys for all
your help!

....sm
 
Top