Broken Sony TA-N330ES (2SC2921 Transistor) - diyAudio
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Old 29th May 2010, 10:20 PM   #1
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Default Broken Sony TA-N330ES (2SC2921 Transistor)

Well, I can't say that I know much of anything about High end stereo amplifiers or electronics, but I am trying to get into it a little and started off with a used amp for my home set up. I purchaced this amp for $125 at a yard sale, knowing that it was mid to early 90's but that it would be much better than any new equipment that I could afford. Unfortunately it does not work, the protect light is on. I had looked over the boards and caps before I purchaced it and all looked very good, but I missed a burnt out transistor. (2SC2921) I don't see anything else that is damaged so I am planning on ordering this transistor and replacing it. Is it common for these transistors to fail and not damage anything else? Also I need to make sure that this is the same transistor as what I have. Mine is labeled
Sanken 2SC2921 with 19f below that line. The one I am looking to purchace is labeled 2SC2921 with an 82p below that line : Sanken Sanken Transistor Mt200 160v 15A 150w BCE | SANKEN | Sanken

I don't know what the 19f or 82p mean...

Hopefully I can get this amplifier back up and running.

Thanks, Neil
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Old 30th May 2010, 12:49 AM   #2
ilimzn is offline ilimzn  Croatia
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Rhis is one of a pair (or multiple pairs) of output transistor(s), a NPN part. It's complementary sounterpart is the 2SA1215 if memory serves me right.
When one in a complementary pair fails, it is almost certain the other is also damaged even though it might test good using the usual simple transistor test.
Things to check:
1) There is usually a small value (fraction of an ohm) large size (2-5W) resistor connected between the emitter and the output of the amp (prior to the protection relay). Check that it is not open on all output transistors in that channel.
2) There is usually a small value low wattage (few ohms) resistor in the base circuit. Check that it is not open on all output transistors in that channel.
3) Check driver transistors (usually one pair of smaller types in TO220 case, possibly mounted on small separate heatsinks)

The usual procedure to do this right is to prevejtively replace the emitter resistors, all output transistors in the channel, and the driver transistors, also base stopper resistors if present. It sounds like an unreasonable investment if only one transistor seems to have failed, but it saves you multiple repairs and associated possibility of PCB and mechanical damage, as the whole output circuit operates in concert, so if one transistor has gone dead (which is always due to some kind of abuse), the other parts have been abused as well. It's only that the one that failed managed to fail completely before the others did. If you leave them in there, expect sudden 'inexplicable' failures and the very real possibility of further parts failing due to operation outside of the normal conditions during repeated failures.

One more thing - buy parts from reputable suppliers. These transistors are very often faked and the fakes are either complete rubbish and will go up in smoke taking other parts with them immediately or shortly after turn-on, or are not up to spec, so even if you get an operating amplifier, it's performance will not be as designed and intended.
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Old 30th May 2010, 01:52 AM   #3
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Transistors rarely go for no reason.

Could have been a short on the output.
Possibly a bias problem causing the output transistors to short out the power supplies.

What I do with blown output transistors is remove the lot and wire the VAS output back into the LTP. Then I power up the amp (without a speaker) and see what bias voltage and DC offset I am getting.

Only when I am satisfied the driver stage is OK do I consider putting output transistors back in, even then I only put one pair in to start with.

This strategy has saved a lot of heartache with repeat blown output transistors,
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Old 30th May 2010, 03:07 AM   #4
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Nigel, I have heard of this technique before but it is a very unspecific instruction, to the inexperienced, to say "wire the VAS back to the Long Tailed Pair".
I for one (and I expect qtoche) would appreciate a little flesh on the bones of where on the LTP this point might be and with what, if any additional components or precautions.
In ignorance but with respect,
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