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hyuna
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Last seen: 3 years 5 months ago
Joined: Dec 15 2016 - 6:11pm
Testing an Audio Power Amp with Paralleled Transistors

A question concerning my current repair project of an audio amp that uses big (600 W) but otherwise rather classic complementary output stages:

I assume when testing an audio power amp with 6 pairs of paralleled npn/pnp transistors, it is o.k. to assemble just 1 pair while testing as long as you have no significant output load connected. The advantage would be that, while testing, I would not blow as many of the precius MJ15022/23 transistors(see datasheet at http://www.kynix.com/uploadfiles/pdf2286/MJ15022.pdf) I have available as spare parts.

In what ways (if any) will the bias current circuit be affected?

Any problems I may not have considered but may be important for this repair technique?

Any other really good tricks for fixing big amps?

Once I am closer to finishing the repair: Does 10...15 mA of bias current per transitor branch sound like a good number to you?

commsysman
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Joined: Apr 4 2006 - 11:33am
Transistors
hyuna wrote:

A question concerning my current repair project of an audio amp that uses big (600 W) but otherwise rather classic complementary output stages:

I assume when testing an audio power amp with 6 pairs of paralleled npn/pnp transistors, it is o.k. to assemble just 1 pair while testing as long as you have no significant output load connected. The advantage would be that, while testing, I would not blow as many of the precius MJ15022/23 transistors(see datasheet at http://www.kynix.com/uploadfiles/pdf2286/MJ15022.pdf) I have available as spare parts.

In what ways (if any) will the bias current circuit be affected?

Any problems I may not have considered but may be important for this repair technique?

Any other really good tricks for fixing big amps?

Once I am closer to finishing the repair: Does 10...15 mA of bias current per transitor branch sound like a good number to you?

That might be a reasonable current for the collector of each transistor with no signal. The main thing is that each transistor should have around the same base current and collector current. If not, one or more might need replacing.

With a 600 watt amplifier (Class AB), I would expect the total idle current for the + and - side of the power supply to be something like 200 mA or so. If there are fuses going to each side of each amp, you can pull the fuses one at a time and check it with an ammeter inserted in place of the fuse.

commsysman
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Joined: Apr 4 2006 - 11:33am
Precious Transistors??

Those tranistors are readily available and very cheap.

Mouser and Newark electronics sell them for around $5 each (or less if you buy 100 or more).

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