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Old 8th January 2013, 04:16 PM   #1
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Default I need troubleshooting advice PLEASE

Hello Guys
I am currently trying to repair a SONY STR-DE597 6.1 receiver, which does the following trouble:
All of it's channels work, but with strong differences in output gain...
when i generate the test tone white noise, from what i hear:
surround BACK = 100%
CENTER = 100%
Front Left = 35%
front right = 20%
Surround Left = 45%
Surround Right = 100 %
the SUB output works OK

is it right to assume that if any output transistors were blown, nothing would be heard from those channels? (MN2488 and MP1620)

Each channel has 4 output transistors and i have all identified them... each channel has the same set. Would it be logical to remove a set from say, the surround right channel (which works 100%) and exchange the set with one of the lower output channels (like front right), to see if the problem follows the transistor set? (or maybe i should only move the two MN2488 and MP1620??)

I don't have signal tracing equipment available, so i am just looking for a way to save this one before ordering transistors or putting the amp to the trash...

i have done a factory reset on it, with no change.

can anyone suggest where else to look for such a problem??

THANKS, any suggestions are welcome.

Pierre D
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Old 8th January 2013, 05:16 PM   #2
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Location: Jeffersonville, Indiana USA
I wouldn't waste money on transistors until I had done a DC voltage check on the outputs. If DC voltage occurs on the output, you have a blown output transistor.
You can't work safely on electronics with a power supply over 25 VDC without a DVM or VOM to measure the supplies at under 25v before you touch any metal. >25 v across your heart can inhibit your ability to let go, and maybe stop your heart.
You need a meter to test the ouputs for DC bias. A good all in one signal tracing and power supply checking meter, is the Simpson 266 100kohm/volt analog meter. You can't buy one. utility is obsolete. You can buy DVM's, but they don't work on AC music, or commercials either. You can't do anything without some sort of meter. To check power supplies resistors and a first check on transistors, I use a Sears 82140 non-autoranging DVM which is fine for DC but not for AC signals except the wall voltage.
If your output transistors aren't blown, you'll need the VOM or a sound probe to trace AC signals. The VOM will follow the beats of rock music, a DVM will not since they average over seconds. Or you need a $400 oscilloscope and a couple of $50 probes. I don't recommend "PC scope"s for debugging, the ground is not isolated and offers the opportunity to blow up your computer, too. You can make a sound probe of a computer powered speaker by 1. connecting a probe to a .047 uf cap 600v and then the back of the caps to the tip of the input plug 2. connecting an alligator clip lead to the back ring of the input plug 3. connecting line to line 2.7 v zener diodes between tip and back ring to prevent DC voltage changes from blowing up the input of the amplifier. You'll need a little board for the 3 parts, I use polycarbonate left over from a window replacement. I drill holes in it. You can also buy sound probe kit from various suppliers.
Transistors, out of circuit you check with the ohms scale of the meter, ohms 2000 scale, and look for 450-650 ohms (.45-65 v) forwards and 1999 or dash or OL backwards. You should be able to do that twice with the base of the transistor being the common parts. However, power transistors can be okay at 2v the meter puts out on ohms scale, and still blow up at rail voltage. You need a leakage test at higher voltage. I use the 200 ma scale of the meter in series with a 17 v power supply in series with a 47k resistor, to test collector leakage. Short the b to the e of the transistor first, that junction isn't rated at 17v. The Vceo off the datasheet of the transistor as a voltage source is a better test, but dangerous to you, the operator, above 25 VDC. I found that used MJ15025 MJ15024 that leak a bit of current at 17 Vce, are the ones that tend to explode at 170 V.
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Last edited by indianajo; 8th January 2013 at 05:34 PM.
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