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Old 24th January 2009, 01:35 AM   #21
gveugenio is offline gveugenio  Philippines
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Not really an electronics expert here. Have a pair or A3N, and a pair of A3P (rebadged Mj2955, and 2N3055). I'm gonna replace them with MJ15003 and MJ15004. Does the MJ15003 or the MJ15004 go to the positive??
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Old 24th January 2009, 01:35 AM   #22
gveugenio is offline gveugenio  Philippines
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After looking at the data sheet I assume the MJ15004 is the PNP and the MJ15003 is the NPN. So MJ15004 can replace the Musical Fidelity A3P output stage transistor, and the MJ15003 can replace the Musical Fidelity A3N. Did I get that right??? Sorry guys not an expert here....
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Old 24th January 2009, 01:46 AM   #23
jacco vermeulen is offline jacco vermeulen  Netherlands
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Quote:
Originally posted by gveugenio
MJ15003 goes and which side the MJ15004 goes
Provided the PCB is the same as the one for the MA50 : the circuit on one side is mirrored compared to the other.
Think rails : both P's on the same side, both N's on the same side of the board.
You can see the orientation at the Hennessy home page pics.

Right.
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Old 24th January 2009, 05:35 PM   #24
jaycee is offline jaycee  United Kingdom
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Correct, MJ15003 substitutes for A3N and MJ15004 substitutes for A3P.
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Old 25th January 2009, 12:02 AM   #25
gveugenio is offline gveugenio  Philippines
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Thanks. I plugged one of my Musical Fidelity MA65 mono blocks by accident while I was upgrading the electrolytic capacitors.

Instantly I saw smoke coming from the output stages so I unplugged right away. What else could have been damaged?? I also intend to change the driver transistors (bd139 and bd140). Any help is appreciated.
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Old 25th January 2009, 12:46 AM   #26
Zen Mod is online now Zen Mod  Serbia
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musical fidelity MA50
Quote:
Originally posted by gveugenio
............I plugged one of my Musical Fidelity MA65 mono blocks by accident while I was upgrading the electrolytic capacitors.
..

wait a sec - are you sure you are still alive ?

what "accident" was that you have connected amplifier with mains , while you're working on PSU ??
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Old 25th January 2009, 09:39 AM   #27
gveugenio is offline gveugenio  Philippines
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Hehe I'm still alive. I removed the cover and heatsink of my amp so I could upgrade the electrolytic capacitors (my MA65 is around 15years old).

I de-soldered all the old capacitors and inserted the new ones (not yet soldered). I then took a coffee break so I unplugged my soldering iron.

When I resumed my work I plugged the amplifier by accident thinking it was my soldering iron. I noticed burning smell coming from the output stages of my Musical Fidelity MA65 Mono Block. Upon noticing the output transistors being fried I unplugged the amp right away. The amp was just plugged around 30-40 seconds...

My friend a electronics engineer said class A amps will burn the output transistors instantly when run without the heat sink or without a load. He told me to just replace the output stages(MJ15003/4). What else could have been damaged?? Should I replace the driver transistors (BD139 and BD140) too??
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Old 25th January 2009, 11:21 AM   #28
Zen Mod is online now Zen Mod  Serbia
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Quote:
Originally posted by gveugenio
Hehe I'm still alive. .....

hehe

I'm glad ........
besides that - that sort of mix-up sounds somewhat familiar

do you know how to check bipolar transistor with diode test on your DVM ?
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Old 26th January 2009, 01:34 AM   #29
gveugenio is offline gveugenio  Philippines
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I read about the diode test with a Digital Voltmeter. Purchased Onsemi MJ15003/4 for my amps already, will try installing them when I get free time.
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Old 30th January 2009, 09:22 AM   #30
jez is offline jez  United Kingdom
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A1N, A1P = 2N3055 2N2955

A3N, A3P = MJ15003, MJ15004

MA50 is both channels of A1 in parallel, not bridged, but with higher voltage mains transformer and a few minor resistor value changes.

Mains transformers often (very often!!) go on these due to the heat.
I used to work for MF and often would leave several of these I had repaired on over-night soak test. It was a regular (2-3 times a week) thing to come into work and find an amp that had been fitted with a new transformer the day before dead, with a blown mains fuse and sorry looking toroid

These and the A1 run so hot that we used to warm up pasties and samosas for our lunch breaks on the amplifiers ! (yes we used grease proof paper...)

Tip: don't even think of doing anything to one of these amps before replacing all the electrolytics, preferably with 105 C ones.
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