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Old 11th November 2011, 01:13 AM   #1
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Talking Help needed: speaker internal amp blows fuse

Hi,
I have a pair of vintage speakers with built in amps (to power the woofers). But one of the amp keeps blowing fuse (the one in the RED circle highlighted)
Click the image to open in full size.
I understand it is very hard to diagnoise this kind of issue. But I want to know which components that most likely failed has caused the issue.

Thanks a lot for the help,
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Old 11th November 2011, 03:12 AM   #2
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Everything is easy for the person who knows it and ...

The transistors that r fitted on the heatsink r blown. If u test them on a Multimeter, u will find that the Collector & Emitter is shorted. Replacing them may not lead to a total repair as other transistors will generally follow suit (design dependent).

In case u intend to fix it yourself, replace the fuses with 100-150 ohms, 2W resistor and test after u replaced the faulty components. U will prevent further damage if the amp is still having a fault.

In this amp, u cannot disconnect the driver, while testing.

Never change only one transistor, also change the corresponding transiator that complements it.

Gajanan Phadte

Last edited by gmphadte; 11th November 2011 at 03:18 AM. Reason: added content
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Old 11th November 2011, 03:18 AM   #3
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Hi Gajanan,

Thank you so much for the info. I will try it out and let you know how it goes.

These speakers were made in France. You must know them well. There was a thread here about these speakers:

http://forum.vintage-audio-laser.com...rol-t5336.html

Best regards,
Singzh

Last edited by timnet; 11th November 2011 at 03:30 AM.
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Old 11th November 2011, 04:07 AM   #4
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Hi,
Since the negative fuse is the only one that is blowing the transistors that will caused to blow it are TR107,TR105 and possible D107.
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Old 11th November 2011, 04:18 AM   #5
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While you are at it replace C112 and C113...'Tantalums' they age badly and end up shorting....use standard electrolytics.
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Old 11th November 2011, 10:56 PM   #6
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Sorry, my bad.

I just realized that the blown fuse was at the positive side.
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Old 11th November 2011, 11:20 PM   #7
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Hi,
Them you most check TR101, TR106 and the diode D106.
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Old 11th November 2011, 11:22 PM   #8
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Just change the outputs and the drivers, check the W/W resistors and change those tantalums. Should be fine. Test with 47 ohm resistor where the speaker fuse is and make sure no d.c. on the output with volume down. (no signal)
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Old 11th November 2011, 11:26 PM   #9
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Take out D106 and D107 and throw them away.
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Old 11th November 2011, 11:46 PM   #10
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Thanks gentlemen for the suggestions.

I measured all transistors and I could find any obvious failures.

New observation: if I do not connect the woofer, then the fuse is fine. as long as I connect a woofer to the circult (I even tried with another new woofer), the fuse blows right away.

Any idea?
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