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Old 31st August 2010, 04:05 PM   #1
kinser is offline kinser  Israel
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Default Transistor Replacments

Hey guys, got a quick Q' for y'all..
I got a Mac Audio mono block car amp (Fearless 1000). its rated at like 900 watts peak at 2 ohms, and around 600 continues, power supply rails run at 75V +/-. it powering a TC Sounds TC2000 at 2 ohm load.
In the output section it has 5x 2SA1941 and 5x 2SC5198 Toshiba output transistors. also per each side it has an additional 2SB817 and a 2SD1047.

I was wondering if i can replace the 2SA1941 and 2SC5198 with some MJL4281's and MJL4302's
and replace the 2SB817 and 2SD1047 with an MJW21195 and an MJW21196.
the on semi transistors seem much better choice for an amp that will be running 2ohms (even though the amp is 2 ohms stable). plus ive got about 25 of each ON Semi transistor i stated above.

What do you think? can i do the replacement? pin outs are the same on all.

Here are some data sheets:
MJL's: http://www.icbase.com/pdf/ONS/ONS28620607.pdf
MJW's: http://www.datasheetcatalog.org/data...MJW21195-D.PDF
2SC5198: http://www.toshiba.com/taec/componen...c//66/7886.pdf
2SB817/D1047: http://www.audiolabga.com/pdf/2SB817.pdf

Thanks,
David
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Old 1st September 2010, 02:33 AM   #2
kinser is offline kinser  Israel
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no one have an idea?
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Old 1st September 2010, 03:03 AM   #3
amc184 is offline amc184  New Zealand
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Why do you want to replace them? The devices you listed are very good, there's no need to replace them unless they're broken.

Anyway, yes, the MJL4281 / 4302 pair should be a acceptable replacement for the 2SA1941 / C5198 pair. However, the MJW21195 / 21196 pair aren't a good replacement for the 2SB817 / D1047 pair, they're much slower and lower gain. The only thing close to the 2SB817 / D1047 pair that I can think of is the 2SA2031 / C5669 pair.

So in short, swap the output transistors if you must, but leave the drivers alone. If you do play around with it, remember to check the bias.
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Old 1st September 2010, 06:26 AM   #4
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hi kinser,

I've been putting this together, it may be of help.

Output Transistors

regards
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Old 1st September 2010, 11:57 AM   #5
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it would be nice to know what is the concept behind that ... in an amplifier like that what actually limits the power is VI limmiter so beyond the bias the VI limiter needs to be recalcultaed according to the soa of the new transistors ( probably also in combination of heatsink capabilty )

But finally replacing a set of tranistors will not effect much on the performance since a few other things beyond the ones stated above may need to be recalculated ... For example faster transisitors than the original might make the amp oscilate while faster drivers might make the amp more hifish ...

once i converted one amplifirer from JBL tour series i think GTO575 that actually was designed to operate at min 4ohms load and 36+36 rails ...

with a few modes on the power supply to increase rails to 56+56 volts recalculate vi limmiters and a few other details i managed to produce almost twice the power but this
amplifier was ment to work on stictly 8 ohms load ...Nice mod ...happilly working for 4 years now
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Last edited by east electronics; 1st September 2010 at 12:01 PM.
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Old 1st September 2010, 01:12 PM   #6
kinser is offline kinser  Israel
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sakis,
there is no concept really..
to be honest when the amp was running it would run pretty hot, i think that it was not designed properly. plus by looking at the mosfets in the PSU section, i doubt that the output transistors are genuine Toshiba's.. so it would be a matter of time till they go kaput. im trying to fix it i just cant fin the darn problem.
i know this aint the place for these kind of questions but..
when i power it up, it draws heavy amounts of current, the voltage drops from 12v to about 7v, i get no voltage on the rails (should be about 75 volts DC +/-). the mosfets heat up.
looking at the board all seems fine, the components all seem fine, mosfets are nice and shiny, so are the rectifiers, all the caps, resistors and diodes are good. no signs of heat or burning anywhere.
so i thought while i have it open and im working on it i would swap out the output transistors to something that has a bit more play, to be on the safe side.

David
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Old 1st September 2010, 01:30 PM   #7
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if your voltage drop from 12 to 7 it means that either your power supply is not enough ( i dont know if you are working from bench power supply or car battery inside a car and a working car )
then either your power leads are to thin for this kind of job

and finally and most importand ther e could be a chance that any of the mosfets in the power supply is leacky causing excesive current draw

also if you have leacky electrolytics in the power supply ( not the big ones just the small arround the tlc 494 which i presume is the pwm controler ) there is also a chance the trafo is oscilating in the wrong frequency resulting excesive curent draw ....

you need to measure a few things
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Old 1st September 2010, 01:38 PM   #8
kinser is offline kinser  Israel
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I'm using a bench power supply, 13.8v@10A. got a headlight bulb in series to limit the current so that i don't end up blowing the bench PS. I've replaced the tl494, will replace the small transistors around the 494 and the mosfet drivers (all of them are 2sa1015's). after that ill pull out the mosfets and test each one. hope to find the problem soon. sure do miss the bass in the system.

David
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Old 1st September 2010, 03:15 PM   #9
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10 amps for this amp is not enough to start ... you need a by far better power supply to see what is going on ... imagine if the power supply states 10 and actually is 7 you are in trouble

i use 35 amps power supply for tetsings like that @ 13.8 volts and some times even this is not enough
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Old 2nd September 2010, 11:49 AM   #10
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i powered this amp with the above PSU to test it when i bought it. turned on and played music. in the car it burnt two of the three on board 30A fuses, and the main fuse (60A, too small but comes to show that i never really pushed the amp hard). that day it blew was a really hot day, it was about 40 degrees outside, and hotter in the car, had the amp going hard, and it just cut out. at first it would power up but id get a sound like a test tone starting high going down low in the Fq range from the sub. when i tested it at home i did hear a crack sound from somewhere on the amp but can see anything burnt.
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