pa amp repair.

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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Member
2005-11-18 1:01 pm
crete
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.
 
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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.
 
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back

Member
2005-11-18 1:01 pm
crete
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?
 
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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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Chinese import that looks like a Crown?

maybe a jpeg of the front and the inside... and no, a lot of folks do not know where to find a tech, and usually the poster who asks a question like yours is lost and probably will make a big mess of the insides of a solid state amplifier... otherwise they'd not ask the sort of question that was posted.

But fine if you can handle it... sorry.

_-_-
 

back

Member
2005-11-18 1:01 pm
crete
Chinese import that looks like a Crown?

maybe a jpeg of the front and the inside... and no, a lot of folks do not know where to find a tech, and usually the poster who asks a question like yours is lost and probably will make a big mess of the insides of a solid state amplifier... otherwise they'd not ask the sort of question that was posted.

But fine if you can handle it... sorry.

_-_-

your post was very helpful.

thanks.
 
Short answer (and agreeing with earlier ones): you do not need to match NPN to PNP but yes all which are in parallel.
So if, say, you have 2 NPN dead, replace all 3 with same brand, bought at the same time, which *should* imply same batch, and no need to replace the PNP, but if you have at least one bad on each side, yes, you'll need to replace all 6. Sorry.
The good ones you pull are not lost ($$$), can be used in a smaller amp; say, a 100/150W per channel unit with just 1 device per side.
And remember, if amp makers provide for adjustable bias is because all transistors are not created equal. :(
 

back

Member
2005-11-18 1:01 pm
crete
Short answer (and agreeing with earlier ones): you do not need to match NPN to PNP but yes all which are in parallel.
So if, say, you have 2 NPN dead, replace all 3 with same brand, bought at the same time, which *should* imply same batch, and no need to replace the PNP, but if you have at least one bad on each side, yes, you'll need to replace all 6. Sorry.
The good ones you pull are not lost ($$$), can be used in a smaller amp; say, a 100/150W per channel unit with just 1 device per side.
And remember, if amp makers provide for adjustable bias is because all transistors are not created equal. :(

thanks a lot.this is what i knew i must do but i asked just in case.unfortunately there is one at each side and the total is 6 pairs!!!.anyway i already order them all.

thank you all for the help.
 
150W with one pair is kinda optimistic ...even with MOT200 pairs but he will get the point anyway !!

It was an example.:)
I had to "spread" the example because if I said "100W" we would have 1000 *angry* :eek: posts claiming "XXYYZZ amplifier has 150W with a pair :mad: " ... and viceversa ;)
Oh well, delights of DIY audio Forum :p

And yes, the basic point is that "pulls" are not "lost", specially comsidering they are relatively expensive.

In fact, my old "junkbox" has saved my skin many times, specially on Saturday afternoons and such.

I'm into Musical Instrument amplification, and amps are needed "tonight", "Monday is too late" ... which is true for people who play mainly Friday night and weekends.

@ back: did you check the drivers also?
Outputs don't like to die alone.
Sometimes bias and protection transistors also die.
Good luck.
 
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.

It depends on if the other transistors have gone open circuit between 2 pins or not.

I always check transistors on a DMM for Hfe.
If they are amplifying ok then chances are they are fine.
The only other ways is to build a test circuit for them with a limited current supply in case of faults.

In my experience most of the faulty transistors I have found were short CE or DS. But I have found the odd one with the BE or GS shorted.
 
Agree.
I also regularly find bipolars shorted BC (which *murders* the driver :( ) and emitter open.
I guess the brutal overload blows the chip into a massive short and the internal thin wire which joins the actual emitter on the chip to the external emitter leg blows open, like a fuse.
The resulting explosion also cracks the epoxy case open or even blows a crater in it.

In my field (MI amplification) this happens often because desperate Musicians try to "repair" an amp blown in the middle of a show by replacing the dead fuse with, say, a 20A one they pulled out of a car, rolled up cigarette paper or even a small nail or something.
Obviously *something* else has to act as a fuse !!!!