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Need help on identifying amp defect
soulsearchingsun
post Mar 1 2012, 14:26
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I'm having issues with my mid-90s Sony solid state amp.
In particular the right channel is of lower volume (~-12dB in my measurements) and the sound is distorted in a way that there is noise added when I feed a signal to the amp. On silent parts, there is no additional noise. I ruled out the signal source and the speakers as the cause, it has to be the amp. I looked inside to see if I can find burned components, but I haven't had luck yet. I can't see the whole circuit board without disassembly (which I haven't done yet) though.

I uploaded a file here, with the following setup:
sound card output -> amp line level input; amp speaker out -> sound card line level input & speakers

Could anyone give me hints where I should look for the defect? Is this typical for some specific defect?
Thanks a lot!

edit:link now pointing to upload thread

This post has been edited by soulsearchingsun: Mar 1 2012, 14:32
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pdq
post Mar 1 2012, 16:08
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We could probably look for some characteristic distortion resulting from a common failure mode (without even looking/listening I am betting blow drive transistor), but is it really worth it? The likelihood of it being something simple/inexpensive to fix is small, and the cost of an excellent replacement is quite modest, so I think you should just bite the bullet, so to speak.
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soulsearchingsun
post Mar 1 2012, 17:45
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QUOTE (pdq @ Mar 1 2012, 16:08) *
I am betting blow drive transistor
I'm not too proficient with the technical terms, although I have some basic knowledge about how amplifiers work. Are you talking about the output transistors or something before that stage?

QUOTE (pdq @ Mar 1 2012, 16:08) *
but is it really worth it? The likelihood of it being something simple/inexpensive to fix is small, and the cost of an excellent replacement is quite modest, so I think you should just bite the bullet, so to speak.
I'm on a very low budget and refuse to throw things away that could be repaired with modest effort. A replacement amp with remote will cost about 40-50€ (~50-65USD) w/ shipping, I won't have a guarantee that my replacement unit doesn't die for the same/different reasons shortly after buying. I know there's a sweet spot between the effort of fixing something and buying a replacement, but I can't see how I possibly could hit this with buying some components. Thanks nevertheless for your advice, I'll at least keep my eyes open.
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pdq
post Mar 1 2012, 18:10
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The output stage of a solid state amplifier generally consists of a pair of transistors, operated in two different modes (one common emitter and one common collector, if that matters). They operate in what is referred to as "push-pull", i.e one drives the speaker output positive and the other negative.

If one of the transistors dies then you only get "pushing" or pulling". Without the current draw from its complement, the surviving transistor is starved for current and its gain is greatly reduced. More importantly, either the positive or negative lobes of the waves are practically cut off, making for a severely distorted waveform with lots of nasty overtones.

Back in the '60s I had a receiver that blew output transistors several times. I built it from a kit, so it was not difficult to replace them. But, since the problem was a bad design that made it prone to this, I eventually gave up and replaced it.
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soulsearchingsun
post Mar 1 2012, 18:51
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QUOTE (pdq @ Mar 1 2012, 18:10) *
If one of the transistors dies then you only get "pushing" or pulling". Without the current draw from its complement, the surviving transistor is starved for current and its gain is greatly reduced. More importantly, either the positive or negative lobes of the waves are practically cut off, making for a severely distorted waveform with lots of nasty overtones.
IMHO this is not the case with my amp. The recorded material shows a full sine wave. Plus it's not that much distorted. It's mainly added noise. I also thought I could hear a pitch difference between L & R, but this of course can't be true. Something must have tricked me. I suggest to listen to my file and comparing left to right by using the balance control.

Uploaded FFT images to this thread.
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Nessuno
post Mar 1 2012, 20:00
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Try to repeatedly turn back and forth the balance and tone knobs, if there are, while playing or the volume one to full scale if is a coaxial one (this latter with no signal in, of course!) and see if anything changes: it could simply be oxidation on rotative contacts.

Just my two cents… smile.gif


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... I live by long distance.
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DVDdoug
post Mar 1 2012, 20:05
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The waveforms don't look like a blown output transistor. It could be another transistor or IC, or maybe a capacitor that's throwing-off a bias voltage somewhere.

Of course it's always good to wiggle the wires & connectors around and push & pull the components around, because poor connections are the cause of many problems.

If you have an oscilloscope and a schematic, you should be able to trace-down the problem by comparing left & right channels at various stages/points.
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Arnold B. Kruege...
post Mar 2 2012, 01:33
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QUOTE (soulsearchingsun @ Mar 1 2012, 08:26) *
I'm having issues with my mid-90s Sony solid state amp.
In particular the right channel is of lower volume (~-12dB in my measurements) and the sound is distorted in a way that there is noise added when I feed a signal to the amp. On silent parts, there is no additional noise. I ruled out the signal source and the speakers as the cause, it has to be the amp. I looked inside to see if I can find burned components, but I haven't had luck yet. I can't see the whole circuit board without disassembly (which I haven't done yet) though.

I uploaded a file here, with the following setup:
sound card output -> amp line level input; amp speaker out -> sound card line level input & speakers

Could anyone give me hints where I should look for the defect? Is this typical for some specific defect?
Thanks a lot!

edit:link now pointing to upload thread


I downloaded your files and don't get the same FFT spectral analysis showing a rise in the noise floor when tones are present.

In my spectral analyses, the noise floor stays about the same whether or not the tone is being added.

I can obtain spectral plots like yours by including the areas where the tone turns on or turns off.
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soulsearchingsun
post Mar 2 2012, 03:32
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QUOTE (Arnold B. Krueger @ Mar 2 2012, 01:33) *
I downloaded your files and don't get the same FFT spectral analysis showing a rise in the noise floor when tones are present.
In my spectral analyses, the noise floor stays about the same whether or not the tone is being added.
I can obtain spectral plots like yours by including the areas where the tone turns on or turns off.

Yes, you're right of course. I may have been too careless chosing the windows. They were overlapping into the silent parts. I uploaded new plots that only include areas where the sine wave is present (respectively only silence).
Still, if I compare the plots for the right channel with a 440 Hz sine wave and silence, I can see a difference in noise floor. Not much, though (~3dB? Just FFT effects? IDK).
When I said that there was no noise added when I output silence, it was just a subjective impression from listening through my speakers. I can clearly hear a 'scratching' noise on top of the sound playing on my right speaker which I can't on the left one. With silence, the difference is far less obvious.
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stephan_g
post Mar 4 2012, 20:53
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The first thing I'd associate with scratchy low-level output on a mid-'90s Sony unit is bad contact in the protection relay. (In which case you should try the other pair of outputs.) Next, a resistor near the output gone high, a bad transistor, or bad solder joints. Obtain a service manual if possible, and search for other people's problems with this model.
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soulsearchingsun
post Mar 5 2012, 20:16
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Thank you for your opinions. Seems I (sort of) solved the problem today. I wiggled the Input/Source selection PCB with it's electric source selection component and the noise was gone. The channels are 0,9 dB apart now. So it seems to be either mechanical (something inside source selection mechanism) or solder joint related. As soon as I have a free weekend, I'll pull said PCB out and resolder suspicious joints (if I can find any) and/or clean contacts.
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DonP
post Mar 5 2012, 22:33
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QUOTE (soulsearchingsun @ Mar 5 2012, 15:16) *
Thank you for your opinions. Seems I (sort of) solved the problem today. I wiggled the Input/Source selection PCB with it's electric source selection component and the noise was gone. The channels are 0,9 dB apart now. So it seems to be either mechanical (something inside source selection mechanism) or solder joint related. As soon as I have a free weekend, I'll pull said PCB out and resolder suspicious joints (if I can find any) and/or clean contacts.


One common source for solder failures is connectors or switches that have their soldered connections double as the mechanical mounting. Every time you connect/disconnect a cable you stress the joints, and it is all the worse if you use "gourmet" cables that require more force to get on and off.
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