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One channel of AMP has distortion. Where do I start?

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gtr33m

New Member
I realise it could be a multitude of things, but I'd love some help on where I should start.

It's a 5 channel AC3/DTS sony amp that I really want to get working again as the rest of it is great.

I've tested it by swapping the speakers and it's definitively the amp. I've also tested it by swaping between Speaker A and B and the problem is the same on either setup.

I'd also appreciate some advice on caveats of working with amps in terms of safety. Do I need to worry if everything is unplugged, or is it possible that there a capacitors that will give me a jolt.

Thanks,

Mark
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Safety should be fine, no particularly high voltages inside with it unplugged - this isn't to say you couldn't get a small shock, but it's small enough not to be dangerous.

Repair wise, the first thing you need is a circuit diagram.
 

gtr33m

New Member
OK, how do I go about getting a circuit diagram? The model is a Sony STR-DB925. I've done some searching online but no luck. I did find out that the model is very similar to the STR-DE915 but is a PAL/240V version.
 

gtr33m

New Member
I sort of need to be led by the hand here, sorry.

At my disposal I have a reasonably decent volt meter, a basic understanding of the difference between a resistor and a capacitor, but that's about it. I can follow a circuit diagram to a basic degree, but I'm at a loss to know how to test wether the problem is with the pre-amp or the power-amp.

Fixing anything electronic previously has been a bit of visual observation. See a blown part, replace it. Tracking down a problem from testing, I could really use a few pointers.

Regards,

Mark
 

Nigel Goodwin

Super Moderator
Most Helpful Member
You could really use an oscilloscope to check for distortion.

But for a start disconnect the speakers, and measure the DC voltage across each speaker output - they should be zero, or very close to it.
 

unclejed613

Well-Known Member
Most Helpful Member
ok, which channel?
i'm guessing it's a front channel since you said it's in both A and B speaker outputs.
check R526 and R576. they are the ceramic resistors with 3 wires. they should read ).22 ohms from the center wire to each end. if you have one open, you probably have a bad output transistor as well. these output devices are darlingtons and should be replaced as a pair, which is how sony sells them. try to get them from sony if possible. second sourcing them is a very IFFY proposition, since half the time you will get counterfeits, the other half of the time they will have differing beta range codes.
 

unclejed613

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also, this might help something else from getting broken in the future. i see this all the time and it just takes a few minutes of preventative action to keep a serious problem from occurring.

on some of sony's receivers, the section of the main board where the display board ribbon cable connects can get broken. if that section of the main board is an L-shaped section of board, then the following can keep it from breaking and creating a world of headaches repairing it. remove the screw that holds the L-shaped section to the receiver frame, and bend the metal tab that the screw attached to down and away from the board. this will keep that section of the board (where a lot of control signals pass through) from breaking if the unit is dropped or moderately bumped. this is a recurring problem with sony receivers, since they've kept the same chassis stamping and basic board layout for the past 10 years or more. there are about 20 traces in that small area that are tuner and display control lines, and the cmos buffer chip for some of these signals is right there too. if that's not an L-shaped area of the board, don't worry about it.
 

gtr33m

New Member
Thanks guys, I'll give that a check later today.

I do have something called a labjack, which supposedly can act as a basic computer based oscilloscope but I have never used it as one. I will check the resistors first and see if that isolates the problem.

It is the front left channel and I should point out that the problem started happening after moving house. To my knowledge it was not dropped, and there is no evidence of external damage, but maybe just being jostled around was enough to cause problems.

Thanks again for all the help. I'll post more info once I've tested it.
 

unclejed613

Well-Known Member
Most Helpful Member
you could have broken solder connections on one of those resistors........

be VERY careful using a soundcard oscope.... the max input to a soundcard is +/-5V, and the rail voltages and output voltages from your receiver are in excess of +/-30V. there is no isolation in a soundcard's input, so that 30V or more could easily find it's way onto your PC's motherboard....leaving behind a smoking ruin.
 
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gtr33m

New Member
Thanks for the info.

The labjack U12 is USB based data logger which has some oscilloscope software built for it. It's well isolated from the PC
 

unclejed613

Well-Known Member
Most Helpful Member
just make sure you don't exceed the input voltage ratings.... a real scope is usually somewhat forgiving of overloaded inputs. but smaller devices aren't.

you may want to inspect the bottom of the pc board carefully to see if there are any cracks in it. my usual way of looking for cracked pc boards is to backlight it and look for light or dark lines that go through the substrate. some cracks will be too small see on the surface, but cause discontinuities to light passing through the material. when possible i use a light table which is the best way to do it, but having a lamp behind the board works almost as good
 

gtr33m

New Member
Ok, I've opened it up and done a good inspection of the parts

I've checked the L shaped boards, which look like the input connection boards. There are 3 levels of boards and all seem fine.

I've also checked r526, r576 and also r776, r726, r626 which I assume are for the other channels. They all measured 1.1 ohms when unpowered on my voltmeter, none measured open. I'll check when powered and see if there's any difference, but it seems unlike that there is a problem with these resistors.

I've also measured the dc voltage across the front channels and there is nothing.

I can confirm the problem is still present and it is the right channel that is causing the problem. It does work, but it distorts even at relatively low volume. As near as I can tell, it's across the spectrum, not a hight or low problem. I hear it most distinctly when listening to voice, but that's probably because it's easier to distinguish the problem.

Do anything else you can suggest I check? I've got the oscilloscope working, what should I be checking next?
 

gtr33m

New Member
A little more interesting when measuring the resistors when powered. I turned on all 5 channels on the amp and each dropped to .4 omhs except for R576. Don't know what it means, but at least it's different. From my reading of the diagram, this is for the right channel which is the problem one.

Does this mean that R576 is faulty, or that something leading to it is faulty?
 
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Nigel Goodwin

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A little more interesting when measuring the resistors when powered. I turned on all 5 channels on the amp and each dropped to .4 omhs except for R576.
You should NEVER attempt to measure resistance whilst powered, you're likely to kill both your meter, the amp, and get no meaningful readings anyway.
 

gtr33m

New Member
I'm clearly showing my lack of knowledge.

Thanks for the advice, I'll not do it in the future. I don't think I caused any damage though this time.
 

unclejed613

Well-Known Member
Most Helpful Member
just curious, what is R576? if it's a ceramic resistor, and it gave you a different reading than the others like it (even though you shouldn't measure resistance with power applied), then something is different here. i would measure the voltages across those resistors with it powered up. R576 is the emitter resistor for the output transistors in the Rch. this tells me that it's R576 and the output transistors, and their associated components you should be checking. one of the output transistors could be bad, or one of the 180 ohm resistors feeding the bases of the outputs. Q554 and Q555 aren't likely to cause distortion, but if there is a problem with the output devices, it's likely one or both of these transistors is bad too. with the POWER OFF. use the ohmmeter in diode check mode to test the 2 power transistors. the picture shows the pinout of both devices. on the MN2488 from B-E and B-C you should read open with the negative (black) ohmmeter lead on B, and you should read low resistance with the red lead on B. from C-E you should read open with the red lead on C, and you MIGHT read low resistance with the black lead on C. with the MP1620, you should read low resistance B-C and B-E with the black lead on B and open with the red lead on B. you should read open C-E with the black lead on C, and again you MIGHT read low resistance with the red lead on C. if all of these readings are ok, proceed to checking the resistors with the ohmmeter back in resistance mode. the ceramic resistor R576 should read less than an ohm end-to-end (meter leads have some resistance that makes reading low resistances somewhat inaccurate). R574 and R575 should read close to 180 ohms. when testing D551, D552, D553 and D554 should all read open with the red meter lead on the end with the stripe (make sure you return to diode check mode) and low resistance with the black lead on the end with the stripe. since we're looking first for the cause of distortion, don't worry about Q554 and Q555 yet.

do these checks first, since if you don't find anything wrong here, we're headed back to IC501, and static testing of IC501 isn't so simple, but we do have identical IC's near it to make comparisons with
 

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gtr33m

New Member
unclejed613, first off, thanks for all the help.

I'm at work right now so can't check anything, but I can tell you that R576 is I believe a ceramic resistor. It's white, about 3cm W x 2cm T. The posts extend through the top resistor, presumably for testing as they don't connect to anything on top. I'll get the model number off them tonight.

On a positive note, I happen to have at my disposal a working unit which is exactly the same model. I borrowed it from a friend when mine stopped working properly. I'll try everything you suggested first, but at least there is some what of comparing.
 
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unclejed613

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you also have the other channels to compare against.
 
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