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Old 2nd Dec 2009, 3:40 pm   #1
sbarber
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Default Stereo Amp - Distorted right output

Hi There,

Having found a few similar posts on google this seems like a good place to ask this question, so if anyone can help me out i would greatly appreciate it. - Apologies for the long post, but probably best to give all the information.

A friend has given me a Sony TA V3 Stereo Amplifier from the early 80's - not exactly vintage, but most of the parts are now unavailable as far as i can tell, and the schematic is nowhere to be found on google.

I should probably start by saying my technical knowledge and experience is minimal, so heres what i've been able to work out:

The left output is absolutely fine and works as it should.

The right output is distorted, significantly lower in volume but does change with the volume control.

Bypassing the power amp - speaker direct to the input preamp - is the same on both sides, thus the input is not a problem.

Swapping the transistors on the poweramp (one linked to the left, one to the right) yielded no change.

The problem is the same in the headphones -so its not a connection on the output.

Having switched any capacitors that appeared to give abnormal readings, which has now amounted to most of the capacitors in the poweramp section on the right hand side, no change has occurred.

Most interestingly: when turning it off, as it powers down but before the relay kicks in, the 'broken' channel seems to increase in volume and the signal clears up a little.

Also, disconnecting the 'black wire' from the poweramp board to the input board (one assumes, the ground) caused it to go silent, for roughly 15 seconds, then sound gradually came back. left channel still output clearly, but not nearly as loud. reconnecting it had the same effect - silence and then gradual return of sound, left channel went back to normal.

Any suggestions, however vague, would be very greatly appreciated.

Many thanks,

Sam
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Old 2nd Dec 2009, 4:23 pm   #2
Nickthedentist
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Default Re: Stereo Amp - Distorted right output

Hi Sam and welcome.

I'm afraid amps are very hard to work on compared with most other hifi. As you say, parts are scarce, and it's all too easy to wreck every semiconductor in sight with a single slip of the test prod etc.

The nice thing about stereo amps, though, is that you can compare left and right sides.

Some ideas...
  • Using another small amp/speaker as a monitor, progress through the power amp circuit from volume control to the speaker output, comparing left with right, to see where the signal starts to deteriorate. Make sure you put a cap (say 100nF) in series with the signal lead to prevent DC voltages from wrecking your monitor/test amp
  • Measure voltages on each terminal of each transistor (with no input signal). Repeat for each channel. Be very careful not to short anything with your test probes. Always have the black lead of your meter connected to chassis.
  • Bypass the speaker relay temporarily, as these can play funny tricks.
Good luck,

Nick.
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Old 2nd Dec 2009, 5:57 pm   #3
Boom
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Default Re: Stereo Amp - Distorted right output

Hi Sam, This place seems to have the manual for $6.95. You could well find that only the correct manufacturers parts will do the job though as it will amost certainly be a Jap' made amplifier and whether Sony will be able to supply parts after all these years is questionable.

Might be worth spending some time checking all of the resistors around the output stage. As Nick says you have one good channel to compare with but if it's a dc coupled amplifier your problem could lie anywhere from the input to output stages as all stages will interact.

Dave

http://www.ephemera-etcetera.com/ind...roducts_id=309

[EDIT] Oh dear.... Just checked and the TA-V3 seems to be using an STK463. Is this correct?

Last edited by Boom; 2nd Dec 2009 at 6:03 pm.
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Old 2nd Dec 2009, 7:05 pm   #4
Patrick Dixon
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Default Re: Stereo Amp - Distorted right output

Quote:
Originally Posted by sbarber View Post
Most interestingly: when turning it off, as it powers down but before the relay kicks in, the 'broken' channel seems to increase in volume and the signal clears up a little.
Not much to go on, but sounds like a bias might be off somewhere causing clipping. Maybe a broken resistor after an AC coupled signal?
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Old 2nd Dec 2009, 8:13 pm   #5
theoldtrout
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Default Re: Stereo Amp - Distorted right output

Assuming you swapped ALL the transistors then there isn't a lot left as these type of power amps tend to be d.c. coupled so - as already suggested - check resistors and double check the caps you haven't changed. If the PA has a small preset pot to set the bias, check the wiper hasn't lifted of the track, I've come accross this quite a few times.
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Old 2nd Dec 2009, 8:50 pm   #6
Boom
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Default Re: Stereo Amp - Distorted right output

Sorry to harp on but the unit uses an STK463 integrated circuit for its output not transistors.

These are also extremely expensive.

Dave
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File Type: pdf datasheet.pdf (66.6 KB, 397 views)
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Old 2nd Dec 2009, 9:04 pm   #7
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Default Re: Stereo Amp - Distorted right output

They were about a tenner when I last bought one, admittedly 7 or 8 years ago.

Maybe one could be had secondhand? They were used in loads of low-end, big-name units, both separates and music centres.

Nick.
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Old 3rd Dec 2009, 12:18 pm   #8
sbarber
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Default Re: Stereo Amp - Distorted right output

Thank you for the suggestions gentleman - not in a position to check at the moment but having previously read it was likely to be a transistor or a capacitor I will admit to having largely ignored the resistors, now it has been mentioned i seem to recall there was one that physically looked a bit 'dodgy' in the protection circuit that i will look in to - although you mention after an AC coupled signal, apologies but im not completely clear on what to look for in terms of locating this - relating to the power section rather than the amplifier?

I believe it is an STK463 thats doing the work - although to be honest if that's gone its more likely to be a case of upgrading the whole thing than trying to replace that - I cannot recall whether i mentioned but it is not my amplifier but i was requested to take a look by the owner, the father in law of a colleague, who's had it since new and wants it fixed not on a "save my beautiful amplifier!" basis, but more from an aesthetic point of view as he has the whole set - tuner, record player, etc, and doesnt want something different that will 'look out of place'.

Anyway, once again thank you for the advice, i shall have a look this evening and let you know if there is any progress.

Sam
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Old 3rd Dec 2009, 11:36 pm   #9
Nickthedentist
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Default Re: Stereo Amp - Distorted right output

Quote:
Originally Posted by Nickthedentist View Post
They were about a tenner when I last bought one, admittedly 7 or 8 years ago.
And still a reasonable £10.45 now: http://www.dalbani.co.uk/catalogue/p...s.php?id=23528

Quick and easy to change, and might get the unit on its feet again.

Nick.
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Old 4th Dec 2009, 1:05 pm   #10
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Default Re: Stereo Amp - Distorted right output

Quote:
Originally Posted by sbarber View Post

Bypassing the power amp - speaker direct to the input preamp - is the same on both sides, thus the input is not a problem.

The problem is the same in the headphones -so its not a connection on the output.
At what point in the circuit do the headphone outputs come into play?
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Regards,
Ben.
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Old 4th Dec 2009, 6:08 pm   #11
sbarber
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Default Re: Stereo Amp - Distorted right output

Post poweramp - amp output, to headphone out, to speaker output.

Nick - thanks for locating the chip. Unfortunately it appears the person i was doing it for was too impatient to wait for me to work out what was wrong with it and wanted it back, so i put it back together and gave it back to him. He promptly took it to a local 'repair shop' who took a £50 deposit, opened it up, and then said 'someone else has fiddled with this, i've got no idea what they've done or haven't done - so i can't do anything sorry'.

If he's not upset enough to throw it in the bin i sense it'll be coming back this way fairly soon in which case i'll get back to testing and let you know what happens.

In the meantime, thoroughly appreciate everyone's input!

Sam
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Old 4th Dec 2009, 10:01 pm   #12
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Default Re: Stereo Amp - Distorted right output

In the circumstances, the Moderators have decided to close this thread. Sam - if the amp comes bouncing back your way, just let us know and we'll reopen the thread for you.
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