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Old 14th April 2013, 05:59 PM   #1
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Default pa amp repair.

hello.i am repairing a pa amp unknown brand.a couple of transistors are shorted at one channel.
do i need to replace all the transistors or only the shorted ones?
they are 6 pairs 2sa1295 and 2sc 3264 so they cost is a bit high.
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Old 14th April 2013, 07:24 PM   #2
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Not enough information.

Is this unknown brand amp worth repairing?

How much power?

Generally speaking, if you don't replace all of the bad parts, the new parts will become bad parts.

So, the answer for you is basically to find someone who knows how to repair amps, and has the test gear, pay them and get it fixed.

Alternately, you can use this amp to learn how to troubleshoot and repair amps... assuming you have basic test gear: Variac, DVM/multimeter, oscilloscope, solder, desoldering tool, soldering iron.

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Old 14th April 2013, 08:18 PM   #3
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Quote:
Originally Posted by bear View Post
Not enough information.

Is this unknown brand amp worth repairing?

How much power?

Generally speaking, if you don't replace all of the bad parts, the new parts will become bad parts.

So, the answer for you is basically to find someone who knows how to repair amps, and has the test gear, pay them and get it fixed.

Alternately, you can use this amp to learn how to troubleshoot and repair amps... assuming you have basic test gear: Variac, DVM/multimeter, oscilloscope, solder, desoldering tool, soldering iron.

_-_-
i know how to repair it and i have everything except variac.
my question is because i noticed the emitter resistors are 0.33r and i thought maybe it would help mismatch between bjt`s.it was an opportuninty
to learn something more.

i would never imaging an answer ay diy audio like yours.everybody can find a technician without the help of diyaudio.

it`s a 2x800/8 2x1400/4ohm and it worth repairing even if i replace all of them.

Last edited by back; 14th April 2013 at 08:20 PM.
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Old 14th April 2013, 08:27 PM   #4
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It is not necessary in most cases to change everything.
Do you have a schematic? It would come in very handy!
If there is just a pair of output transistors blown and no more per channel, change the output, drivers and check the bias circuit. Some times the bias can go high and kill the output pair on their own. When you are satisfied that things look OK, remove the 0R33 resistors in the bad channel and fit 10R 1/2 Watt. If there is still a problem, it will take those out and not your expensive Sanken MT-200 semiconductors.

Hope that helps.

Last edited by JonSnell Electronic; 14th April 2013 at 08:40 PM. Reason: Spell check
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Old 14th April 2013, 08:57 PM   #5
back is offline back  Greece
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Quote:
Originally Posted by Harleyjon View Post
It is not necessary in most cases to change everything.
Do you have a schematic? It would come in very handy!
If there is just a pair of output transistors blown and no more per channel, change the output, drivers and check the bias circuit. Some times the bias can go high and kill the output pair on their own. When you are satisfied that things look OK, remove the 0R33 resistors in the bad channel and fit 10R 1/2 Watt. If there is still a problem, it will take those out and not your expensive Sanken MT-200 semiconductors.

Hope that helps.
this is a good trick i never heard before.unfortunately i don`t have a schematic but i think it was caused from damaged speaker.i will check the speakers tomorrow to verify.

p.s.i guess i need to change resistors to all 6 pairs?

Last edited by back; 14th April 2013 at 08:59 PM.
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Old 15th April 2013, 05:29 AM   #6
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Emitter resistors don't fail like semiconductors. They are resistance wire that burns out like a heating element, or not at all. If they measure within spec. of say 0R33+/- 5%, they will be OK. BTW, inexpensive DMMs are not accurate at very low resistance, so compare readings with other parts.
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Old 15th April 2013, 05:36 AM   #7
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I have seen amplifiers blow LTP transistors when the power device(s) fail. Better to confirm they are good before powering.

Gajanan Phadte
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Old 15th April 2013, 08:07 AM   #8
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no ...this is wrong

it is absolutely necessary to replace all the outputs and rebias the amplifier unless you have the exact same type /brand /serial/batch of transistors

Often amplifier manufacturers do not get trans of the market , place specific orders to transistor factories with close specs to enhance performance and minimize matching issues .

In the specific case 1295-3264 pairs manufactured by Sanken popular around many amps fast and quality product and was an alternative for already cannibalized 1302-3281 and for bit bigger applications .


Chances of finding original transistors is either zero or its going to be way too costly IE 26euro for a piece and stil you will need to replace them all ..\

there is an option that you will find original spares at IMA audio in Athens but still the prize will be too high
( place a picture of your amp it might originate from IMa audio anyway )

Kind regards

Sakis
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Last edited by east electronics; 15th April 2013 at 08:11 AM.
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Old 15th April 2013, 06:18 PM   #9
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hi sakis.do you believe even the sanken from farnell are fake?
they are 5.7 pounds each.

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Old 15th April 2013, 06:52 PM   #10
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it will be enough to be different batch with different ratings

The amplifier looks to me either like an Acoustica or the first ELDER or B3 models may i also see the front plate ?

Both Elder and Acoustica originate from IMA audio in Greece , so you may have a chance there

Kind regards
Sakis
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Last edited by east electronics; 15th April 2013 at 06:54 PM.
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