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Old 9th August 2011, 01:14 PM   #1
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Default Technics SU-V7 not functioning

Dear Friends,

I am trying to fix an old Technics SU-V7 amp which is not functioning. The protection relay does not trigger on. On checking, I noted that there is a heat sensing SCR mounted on the heatsink. When checking with a multitester, I realised that, the is continuity between anode and cathode even without triggering the gate. I guess that this SCR has shorted. The part number on the SCR is TT201. I was told that this component is no more available. Can someone who came across this problem suggest a substitute or replacement for this part.

Thank you in anticipation

Carlos
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Old 9th August 2011, 01:51 PM   #2
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obviously there is something seriously wrong here i dont recall ever seing a protection circuit based on scr ...

there is a couple of thermistors on the amplifier that one works for the idle of the amplifier and the other senses high temperature on the amp and shunts the drive as a precaution ...

non of these ever fail ( to my understanding ) and if relay desnt click number one reason is dc ofset present ....

i recon that your approach/procedure is wrong and fault tracing should start from there .

Finally it is not uncommon that all works fine and simply some electrolytic that is very normal to be dry or leacky in this age of amplifier doesnt allow protection circuit to operate properly .

Keep in mind that as i said its likelly that most electrolytics inside your amp is ot be replaced anyway ...your amplifier is too old ...still a very nice machine ...

if you need aditional help post here i also have a schematic for your amplifier and you are welocme to it .

kind regards sakis
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SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
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Old 10th August 2011, 12:23 PM   #3
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Default Technics SU-V7 not functioning

Quote:
Originally Posted by sakis View Post
obviously there is something seriously wrong here i dont recall ever seing a protection circuit based on scr ...

there is a couple of thermistors on the amplifier that one works for the idle of the amplifier and the other senses high temperature on the amp and shunts the drive as a precaution ...

non of these ever fail ( to my understanding ) and if relay desnt click number one reason is dc ofset present ....

i recon that your approach/procedure is wrong and fault tracing should start from there .

Finally it is not uncommon that all works fine and simply some electrolytic that is very normal to be dry or leacky in this age of amplifier doesnt allow protection circuit to operate properly .

Keep in mind that as i said its likelly that most electrolytics inside your amp is ot be replaced anyway ...your amplifier is too old ...still a very nice machine ...

if you need aditional help post here i also have a schematic for your amplifier and you are welocme to it .

kind regards sakis
************************************************** *******
Thanks alot Sakis for your advice. I resolder the power amp board and removed the scr from the board this morning and powered on the amp. After about 30 seconds, the relay clicked on but immediately turned off again. I powered off and on back again. The same thing happened. The relay clicked on and the off again. As your advice, I will check all the electrolytics and the thermistors and get back to you again.

Thanks again

Carlos
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Old 10th August 2011, 04:31 PM   #4
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see if you measure DC on the output, if you don't have a meter, I suggest getting someone else to repair the damage., which my be small in parts cost, and generaly cheaper in labour if the owner doesn't "help".
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Old 11th August 2011, 05:06 AM   #5
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Dear Digits,

Now when I on the amp, after about 20 secs, the relay comes on and then cuts off, then clicks on and cuts offs and it keeps repeating this. There is hum sound from the speakers when the relay comes on. I measured the output and it shows a DC voltage of about 0.5v when the relay is off and it goes up to more than 2v when the relay comes on and then the relay cuts off. Where can the problem be.

Thanks

Carlos
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Old 11th August 2011, 06:20 AM   #6
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your approach/procedure remains wrong and sorry to say beyond common logic ....

when an amplifier is faulty number obne thing you DON'T do is to conect speakers on it .Don't expect the amplifier to shelf repair while you are observing the relay clicking on and off pattern ....

soldering , electrolytics ,open resistors ,leacky drivers,dirty trimmers , and a few other things might be the reason for a thing like that ...

Repairs is about procedure and understanding safety and operation of circuit ... If you dont have any of these tools do not atempt a repair . seek for expert help .
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