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Old 16th April 2013, 01:48 PM   #11
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Thanks for all the replys!

I've hooked up a test speaker DIRECTLY to the output on the circuit board bypassing the speaker switch and the problem is still there. Has nothing to do with the speaker switch. Also if it were a bad transistor, wouldn't it be distorted even through the headphones? The sound is very clean through the phones until you connect the speaker wire to the output, then it becomes distorted, with ANY speaker, through the headphones as well!
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Old 16th April 2013, 01:49 PM   #12
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Originally Posted by Ian Finch View Post
Well, generally there is a headphone socket and when you plug the headphones in, on most older models, the speakers are switched off. That's one set of switches.
Next, there is usually a selector for connecting A, B or both pairs of speaker connectors. That's another set of switch contacts and that's from times before relays were also included!

Why not post the model number so we can check for ourselves what really is present between output stage and speaker connectors.

Actually, since you have probably been giving those switches a workout in trying to find something that makes a difference, I doubt that a speaker selector will be the main problem. Headphone sockets can remain bad depending on their design. Check this out before looking further, as Mooly suggests.

I'm jumping the gun to my impression from your fault description, which to me sounds like the usual output stage transistor failure. Headphones often don't sound so bad because drivers can often supply enough current through the shorted output transistor but eventually, the driver fails too if you turn it up in the hope of more volume but the sound, along with the repair bill, goes from bad to worse.
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Old 16th April 2013, 01:51 PM   #13
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Originally Posted by Ian Finch View Post
Well, generally there is a headphone socket and when you plug the headphones in, on most older models, the speakers are switched off. That's one set of switches.
Next, there is usually a selector for connecting A, B or both pairs of speaker connectors. That's another set of switch contacts and that's from times before relays were also included!

Why not post the model number so we can check for ourselves what really is present between output stage and speaker connectors.

Actually, since you have probably been giving those switches a workout in trying to find something that makes a difference, I doubt that a speaker selector will be the main problem. Headphone sockets can remain bad depending on their design. Check this out before looking further, as Mooly suggests.



I'm jumping the gun to my impression from your fault description, which to me sounds like the usual output stage transistor failure. Headphones often don't sound so bad because drivers can often supply enough current through the shorted output transistor but eventually, the driver fails too if you turn it up in the hope of more volume but the sound, along with the repair bill, goes from bad to worse.
There is a shematic on the first post.
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Old 16th April 2013, 02:17 PM   #14
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Originally Posted by Mooly View Post
Connect your speaker directly between chassis and the appropriate output side of cap C13 or 14 and see if the distortion is present. That will bypass any internal wiring and switches.
Thanks...did that and problem still exist.
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Old 16th April 2013, 07:03 PM   #15
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Originally Posted by Rudes View Post
Thanks...did that and problem still exist.
In that case a basic check of the quiescent current in the outputs should be performed just for starters to see what is going on. Bigun mentioned the manual as quoting only 2mv across the 0.5 ohm resistors (which is 4milliamps). so check it and compare with the good channel.
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Old 16th April 2013, 07:34 PM   #16
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There is a shematic on the first post.
Your attached schematic only covers the power amplifier itself. I'm sure you can see that there is more in the box than is on that schematic and this is what we were asking about.
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Old 16th April 2013, 07:40 PM   #17
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Originally Posted by toprepairman View Post
1000uF caps for speaker coupling is a bit on the low side. Bass performance will be a bit compromised. Up it to at least 2200uF or up to a max of 4700uF. Don't go any further as your turnon thump will get worse, and you risk blowing the 2A fuses, or worse. Use low ESR and 105* types.
Henry
I've read in Douglas Self's power amplifier handbook that the output coupling capacitors will need to be as big as 100mF to minimize distortion, but is not possible due to the high price (and physical size) of such massive capacitors. Are you saying that using them would not actually work?
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Old 16th April 2013, 07:46 PM   #18
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Originally Posted by Welcome View Post
I've read in Douglas Self's power amplifier handbook that the output coupling capacitors will need to be as big as 100mF to minimize distortion, but is not possible due to the high price (and physical size) of such massive capacitors. Are you saying that using them would not actually work?
They would work fine when fully charged in circuit. 100mF (100millifarads) is 100,000 uF and would draw considerable current for a considerable time via the 8 ohm speaker to fully charge. The speaker would object to that and so to the output stage and PSU
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Old 16th April 2013, 07:47 PM   #19
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Obviously headphones of usual 32R load plus 220R protective resistors existing in the plug equals to a very easy load of 250 R aprox.

meaning that any amplifier could play that with no problems even if one of the output transistors is totally open, loose contacts, or bad soldering and so on Yet again it could be only the drivers playing and still you will get some sound in the HPs ..

It could be as well any feedback related issues like a dry capacitor or so

verifying some bias will let you know if output transistors even exist in the circuit.

Finally even though you noticed that many forum members run to bail you out do not expect any fast food solutions like change resistor R125 and you are done ....it will take a bit more than that

Kind regards
sakis
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Old 16th April 2013, 07:52 PM   #20
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Hei Mooly just noticed your signature ....i begin to like you more and more every day
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