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Old 11th November 2008, 12:23 PM   #1
Dan2 is offline Dan2  South Africa
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Default Repair or Replace??

i have an old mosfet amplifier that (you can see) is a DIY job. it says 600W output but im not sure about that. it has overload protection in the form of relays on the output, and one channel kicks out even at low output levels. and the amp sounds horrible.

so i was wandering what is the best thing i can do with this amp?? should i try and fix it or build a new amp from scratch?? i will obviously use the transformer, and maybe the output transisters???

i know i haven't given many details about the amp, but if there some measurements i should take please let me know.

i want to use this amp in a PA/ small disco system, and add a limiter to it so i (or anyone else using it) cant blow the amp or speakers. this will be my first amp project, so i would like to have things as simple as possible
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Old 11th November 2008, 03:47 PM   #2
jaycee is offline jaycee  United Kingdom
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Find out first what you have.. what size transformer is in there, how big are the heatsinks, what power supply capacitors.

Some pictures of it, including the insides might help us.
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Old 13th November 2008, 02:34 PM   #3
Dan2 is offline Dan2  South Africa
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so the transformer is pretty big (and heavy). its 38-0-38 which pushes out about +- 50v dc. i have no clue how much current i can draw from it but this thing looks pretty industrial!!


transisters are MJ15024

got some pics here, sorry about the quality but i had to compress them to attach:
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File Type: jpg amp1.jpg (50.1 KB, 504 views)
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Old 13th November 2008, 02:35 PM   #4
Dan2 is offline Dan2  South Africa
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another pic
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File Type: jpg amp2.jpg (29.3 KB, 456 views)
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Old 13th November 2008, 02:36 PM   #5
Dan2 is offline Dan2  South Africa
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and another
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File Type: jpg amp3.jpg (45.6 KB, 405 views)
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Old 13th November 2008, 02:52 PM   #6
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Default Hi!

Firs of all 15024 isn't Mos fet.

Pcbs look like PA 300, origin.

600W?, maybe 2x300W.

Repair ofcourse!!

Regards zeoN_Rider
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Old 13th November 2008, 02:59 PM   #7
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Quote:
i have an old mosfet amplifier that (you can see) is a DIY job.
It seem you have a BJT amp here , not a mosfet , as MJ 15024
is npn 250w. With the hardware you have here, you have many
choices.

First , with 38-0-38VAC , that gives about 52-0-52 VDC
allowing for a 120w amp. looking closely at it shows its age,
so the caps should be replaced.

The present amp has design considerations (VAS on main HS,etc
)but there are many amps here that would work with
this "shell". the "power amp under development' (NMOS 200)
or in a couple days , my amp "frugal amp"
are modern amp designs which would seemlessly just
"bolt on" to what you have already.(they use the same L channel
for outputs)You could repair it but that is up to you.

Dont throw it out because I just had to spend 250$+ to get what
you have. (Case , trafo, Heatsink)
OS
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Old 13th November 2008, 04:46 PM   #8
KISS is offline KISS  United States
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Well, if you really want to scrap it, you could build the Leach Amp. Do a Google Search. Sound quality is excellent.
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Old 13th November 2008, 05:31 PM   #9
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I'd repair.
it adds a worthwhile skillset.
__________________
Just because you can't hear it doesn't mean no one can.
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Old 13th November 2008, 06:05 PM   #10
jaycee is offline jaycee  United Kingdom
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I can confirm it's a PA300 amplifier board. Look at the diagrams here - the layout is identical.

http://www.reber.si/ojacevalniki/PA300/PA300.htm

Is the closeup pic of the PCB the faulty channel? I see heavy scorching around several resistors.

As you now have the schematic and PCB layout, you could repair it... but i would be wondering why it failed and why those resistors overheated in the first place... it could be a design flaw.

Maybe someone here knows the PA300 circuit better?

edit: from the look of it the fried resistors are R13, R15, R18 and R19. That would suggest the VI limiting circuit has been activating frequently, ie the amp has been severely abused. I would test the output and driver transistors, replace the scorched resistors, replace T1, T3, T4 and T5. You might also want to check the diodes around that part of the circuit.
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