M250 Parts Replacement Querry....

hi everyone....
'm just asking if anybody have tried replacing the m250 output transistor. the original output transistor MJ15024/MJ15025. 'm planning to replace the MJ15024/MJ15025 with MJL4281A(NPN)/MJL4302A(PNP). anybody comments and analysis for these particular replacement is it working? or not? thanks just waiting.... see you soooooon.


if you are referring the attachment,
you are not wrong
the amp will operate happily with those devices you want to replace
with, assuming that there are no PCB layout errors
and also you are close or equal to those assigned biasing voltages.....

:smash: enjoy building you amps.......




  • m250voltref1.jpg
    50 KB · Views: 823

thanks hienrich and joan2 for your reply... thanks also hienrich for the schematic with voltage ref... anyway i already have the data sheet of mjl4281/mjl4302.... by the way have you tried mje15034/mje15035 as drivers for mjl4281/mjl4302? these two transistor mje15034/mje15035 rated as 350volts 50watts....
anyway soon ill be starting to build this m250 using these new set of transistors.... whatever problems incountered or outcome ill just post it here....

actually you can still do the same with that scheme I've attached
( by the way let us also thank somebody who originally posted the schemes with biasing voltages, I forgot were that thread that is...)

here, looking to the schemes with the given output devices, you will
have no problems if you directly replace those NPN's with your MJL4281's and those PNP's with your MJL4302's, still
the same voltages appears on those pins designated by the scheme
after it is properly biased.....

here some hint :smash: assuming PCB lay-outs are done correctly
and everything from input differentials to the voltage amp stage
and the driver stage are in place correctly ( never put the output stage yet:att'n: ).
when powering up the boards measure the voltage drop across R22
(75 ohm res) ,mean while turn the trimmer res until you can
measure a voltage drop of about 0.6volts ( positive probe @ point A and the negative probe @ point B)

after which, turn off , then connect now the out put devices....
assuming that all connections are correct ....

turn the power on and quickly, measure the voltage drop across R22 (75ohm res), turn the 1k trimpot until a voltage drop
of about 1 to 1.1 volt across R22 (pos. probe @ A, neg probe @B)

play some music for 15 min. , by this time the heat sinks might
be warmer,
finally measure it again, readjust if it goes high and get it back to 1.1 volts...

or for the most final test there should be a 0.55 Volt drop across each of the Base to Emitter of the output devices.... ( 0.6 volts is a bit high and operate the outputs hotter)

these are just simple biasing when we don't have oscilloscopes...




diyAudio Moderator Emeritus
2003-05-29 2:57 am
Maybunga, Pasig City
or for the most final test there should be a 0.55 Volt drop across each of the Base to Emitter of the output devices.... ( 0.6 volts is a bit high and operate the outputs hotter)

not a good practice!
better measure the quiscient collector current of the output transistors, a range of 150 to 250 ma. assuming 4 pairs output devices will give good result in most cases, cross-over distortion is minimised this way!

base emitter diode measurement is a tricky business!
m250 follow-up

hi hienrich kumusta? i don't have any parts yet of the m250 but i already have tried looking and ask for the parts availability here in manila.... i only have gone to one store yet in Alexan here particularly in fairview actually its just one of the braches of alexan.... i only have one problem coz they are out of stock of TIP121 which was used in the dc protection circuit.... mybe this week i have to go the RAON in Quiapo Manila... there have DEECO and ALEXAN main branch.... but i haven't tried yer asking if they have these new transistor MJL4281A & MJL4302A. I also wanted to check the availability of MJE15034 & MJE15035 which will be used as the driver transistors... by the way do have any idea about what transistor will be used as a replacement of TIP121? have you tried these M250 power amp? how does it sound? and have you tried powering higher voltage than the original +-70Vdc?
again thank you very much....



diyAudio Moderator Emeritus
2003-05-29 2:57 am
Maybunga, Pasig City
you can get free samples from www.onsemi.com by signing up, registration is free, but nowadays you will have to pay for freight charges! you can get everything you need from there!

buying stuff from raon is risky, lots of fakes going around. stores that carry genuine parts are alexan, oriental, newport, hirose, with deeco you have to be able to spot the originals from the fakes. at times they will ast you if you want originals because the pricing is much different for fakes and originals!

well, these are only some ways of doing things most specially when
we don't have scopes.....

I have always done this without encountering problems....

Tony: you've also a good point here...

but measuring voltages across devices are more convinient..

and since the ON state of bipolar transistors are approximately from

0.5 volt to 0.65 volt, 0.7 volt is for ideal BJT's....

if you have a working project right now try testing the Base to Emmiter voltages of the output devices (standby state). Assuming they are biased properly....


for me, I would rather run for (original) Sankens, since they are much
rugged and reliable: 2sc3264(npn), 2sa1295(pnp)





diyAudio Moderator Emeritus
2003-05-29 2:57 am
Maybunga, Pasig City
quiscient power dissipation

the whole point of knowing your idle currents is to know your output transistor quiscient power dissipation! measuring Vbe alone will not let you find that out!

and it is not hard to do either! what i do is to solder a 10ohm 1watt resistor accross a busted fuse, this i plug into the fuse clips so that when i power up the amp i will know the current by measuring the voltage accross the resistor as i turn the bias pots! i do this on both the +/- rails.

a second benefit from this is in case there is a mistake in the wiring somewhere, the resistors will burn first rather that the output transistors getting fried!

so you see, there are reasons why we do things!