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C-Audio SR-707 Protect Issue

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qwerp View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote qwerp Quote  Post ReplyReply Direct Link To This Post Posted: 03 March 2019 at 12:56pm

thanks, I shall give it a go later on

I forgot the mention there are little variable pots ( Phillips screw driver type) on each channel board and they had oxidisation on them, like white crystals or fluff.  Im not sure if this made its way onto the contacts


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Post Options Post Options   Thanks (0) Thanks(0)   Quote qwerp Quote  Post ReplyReply Direct Link To This Post Posted: 03 March 2019 at 1:14pm
Hi

yes there is amplification on both channels and sounds clean too Smile

I was using an old cheap plastic speaker ( not sure of the ohms) wasn't all that loud.

So it is the protection circuit.   Oh my,  I shall have to lever it out and have a look.   : /  :  )





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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 03 March 2019 at 1:17pm
Originally posted by simonp1100 simonp1100 wrote:

The only other thing you could try is to put a stereo signal into the amplifier and put the front volumes on low and then with an old scrap speaker (cheap car speaker etc), connect between the back ground speaker connection and the metal cans on one of the MOSFET's on channel A and see if sound comes out, if it does do the same for channel B and again if it does this would then give 100% that the problem is the protection circuit at fault. 

This above test can ONLY be done if you have 10mv DC or less on each output.  Thumbs Up


Yes - I was going to suggest this - but with a sig gen & oscilliscope! The best way to see if it is just a faulty protection circuit - or if there is more going on, eg high frequency instability/oscillation on the o/p. A simple multimeter check won´t pick that up.   Ouch


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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 03 March 2019 at 1:23pm
Originally posted by Earplug Earplug wrote:

Originally posted by simonp1100 simonp1100 wrote:

The only other thing you could try is to put a stereo signal into the amplifier and put the front volumes on low and then with an old scrap speaker (cheap car speaker etc), connect between the back ground speaker connection and the metal cans on one of the MOSFET's on channel A and see if sound comes out, if it does do the same for channel B and again if it does this would then give 100% that the problem is the protection circuit at fault. 

This above test can ONLY be done if you have 10mv DC or less on each output.  Thumbs Up


Yes - I was going to suggest this - but with a sig gen & oscilliscope! The best way to see if it is just a faulty protection circuit - or if there is more going on, eg high frequency instability/oscillation on the o/p. A simple multimeter check won´t pick that up.   Ouch




I suggested this as i assumed he does not have this type of equipment and would give a quick way of finding out if the protection circuitry is faulty or not and if sound is coming out whether it is distorting etc.


Edited by simonp1100 - 03 March 2019 at 1:24pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote qwerp Quote  Post ReplyReply Direct Link To This Post Posted: 03 March 2019 at 1:28pm
owch, i did the test with a speaker,  with 44mv DC present on one chn and 83mv DC on the other chn.  (These DC readings were taken before when with nothing connected in or out.) 

I do not have an oscilloscope unfortunately.



Edited by qwerp - 03 March 2019 at 1:32pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 03 March 2019 at 1:32pm
Did it work ??. The DC offset for these amplifiers are normally around + or - 10mv so the readings are a bit high and maybe need adjusting (after a warm up for about 10-15 minutes).


Edited by simonp1100 - 03 March 2019 at 1:34pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote qwerp Quote  Post ReplyReply Direct Link To This Post Posted: 03 March 2019 at 1:33pm
yes copy of post above >


yes there is amplification on both channels and sounds clean too Smile

I was using an old cheap plastic cased speaker ( not sure of the ohms) wasn't all that loud.

So it is the protection circuit.   Oh my,  I shall have to lever it out and have a look.   : /  :  )



Edited by qwerp - 03 March 2019 at 1:33pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 03 March 2019 at 1:35pm
Originally posted by simonp1100 simonp1100 wrote:

Originally posted by Earplug Earplug wrote:

Originally posted by simonp1100 simonp1100 wrote:

The only other thing you could try is to put a stereo signal into the amplifier and put the front volumes on low and then with an old scrap speaker (cheap car speaker etc), connect between the back ground speaker connection and the metal cans on one of the MOSFET's on channel A and see if sound comes out, if it does do the same for channel B and again if it does this would then give 100% that the problem is the protection circuit at fault. 

This above test can ONLY be done if you have 10mv DC or less on each output.  Thumbs Up


Yes - I was going to suggest this - but with a sig gen & oscilliscope! The best way to see if it is just a faulty protection circuit - or if there is more going on, eg high frequency instability/oscillation on the o/p. A simple multimeter check won´t pick that up.   Ouch





I suggested this as i assumed he does not have this type of equipment and would give a quick way of finding out if the protection circuitry is faulty or not and if sound is coming out whether it is distorting etc.



Yes - I noted the 'scrap speaker' comment! I have a few of those around the place as well for 'real life' tests. Sometimes circuits will behave differently under load.  Smile



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Post Options Post Options   Thanks (0) Thanks(0)   Quote qwerp Quote  Post ReplyReply Direct Link To This Post Posted: 03 March 2019 at 1:35pm

if there is hf oscillation confusing the protection circuit

could i trick the protection circuit by disconnecting something ( the feedback?)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 03 March 2019 at 1:37pm
Glad we are getting there.  ClapClapClap

You may need to take the board out and start measuring the components on the protection circuit. The boards come out easily as the are just on plastic PCB spacers (don't forget to push the small plastic release pin in before the board passes past the pillar).


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 03 March 2019 at 1:40pm
Originally posted by qwerp qwerp wrote:

yes copy of post above >


yes there is amplification on both channels and sounds clean too Smile

I was using an old cheap plastic cased speaker ( not sure of the ohms) wasn't all that loud.

So it is the protection circuit.   Oh my,  I shall have to lever it out and have a look.   : /  :  )



Ok - at least you now know.

Another question: are the output devices getting very warm? That would be a sign of high frequency oscillation - fairly common with MOSFETs.

(Or bias too high.)







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Post Options Post Options   Thanks (0) Thanks(0)   Quote qwerp Quote  Post ReplyReply Direct Link To This Post Posted: 03 March 2019 at 1:42pm


Thank you very much

I shall try and  fault find the protection circuit later on :  )   Tongue
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