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Old 4th January 2007, 08:02 AM   #1
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Default Using higher rated ouput transistors??

Hi All,
I have a question about output transistor wattage ratings.
I have repaired an Onkyo A-8067 integrated amplifier(80 watts per ch.) that had 3 bad output transistors. I accidentally used the parts list from a lower rated A-8057 model (65 watts per channel).
So now I have transistors 2sa1301/2sc280(rated at 120 watts) instead of 2sa1186/2sc2837(rated at 100 watts). These transitors are rated 20 watts more despite being for the lower power amp model.

I'm not sure whether to order the correct ones or use these. I was going to replace all 8 and do have all of them. Even though the amp works I'm wondering about what affect on fidelity and reliability these incorrect transistors migh cause.


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Old 4th January 2007, 12:56 PM   #2
Gigapod is offline Gigapod  France
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The power rating is not the only figure you should look at when substituting a transistor with another one.
You need to consider the entire datasheet and the circuit schematics. If you post the relevant information here somebody may be able to analyze it and give you some valid feedback, otherwise it's just empty talk.
Nice picture, btw.
Also note that most manufacturers will do a little transistor matching, but right now you seem to have an unmatched pair there...
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Old 4th January 2007, 01:24 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
what type of use blew the amp originally?

Are the original outputs up to the job you expect?

Do you want the same again or less or more?

80W into 8r implies a supply of about +-45Vdc.

You need to compare the devices Vce=45V and find what current (Ic) each type can take.
It looks like two pair per channel, if so you are looking for 2.5A or preferably 3A @ 45Vce & Tc=25degC.
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Old 4th January 2007, 11:23 PM   #4
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I think the fuse blew because the amp did not have adequate circualtion because of a glass doored cabinet along with it's
age-1985.

Here is the transistor datasheets for the2sa1301 and it's complementary 2sc3280
These are the new higher wattage transistors that I was going to use for replacment

These are the transistor datasheets for the original transistors
2sa1186 and it's
complementary 2sc2837

Here is the schematic (page 3).
Transistors 2sc2837 are Q609, Q610, Q613, Q614
transistors 2sa1186 are Q611, Q612, Q615, Q616

Any help would be greatly appreciated.
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Old 5th January 2007, 12:17 AM   #5
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i've been doing repair work for many years, and if i have an opportunity to upgrade a repair, i do it. if an amp has 250W transistors (such as MJ15022/23), and i can get MJ15024/25's to replace them, i will. it increases the reliability margin. if that amp blew the fuse, and it didn't take out the output devices, that says something about the output devices. i've seen amps built with mixed up BOM's before, so what you're describing isn't really uncommon. if you have access to the BOM for the higher power amp, go for it. a little overkill can help reliability astronomically.

btw..... that amp is 20 yrs old, and was in a limited airspace.... recap it..... redo all of the heatsink grease too......
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Old 5th January 2007, 12:22 AM   #6
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The link does not seem to work.

Here is a good link for the datasheets and schematic.
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Old 5th January 2007, 12:30 AM   #7
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go for it....... you get a 20% "safety margin", which probably equates to an amp that will last another 30 years..... but like i said, recap it, and replace ALL of the heatsink grease (not just on the transistors you're replacing).... while you're at it, replace all the zeners.... with 1 watt in place of 1/2 watt, etc...
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Old 5th January 2007, 12:40 AM   #8
Gigapod is offline Gigapod  France
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Quote:
Originally posted by tgferg67
[B]I think the fuse blew because the amp did not have adequate circualtion because of a glass doored cabinet along with it's
age-1985.
...
Lack of air flow is a killer. I agree with your explanation.

I didn't manage to download either the datasheets or the schematics from the links you provided, however I googled for the datasheets. The original transistors are faster and imho you should just try to stick to them or find better equivalences.

In terms of SOA the replacements you already soldered in are OK (in fact they are better than the original), in terms of gain I guess they are roughly OK too, so they should work, but I would just change all the output transistors, or at least all the ones in the channel that blew up.
If you have a few spares try to match them in term of gain.
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Old 5th January 2007, 01:00 AM   #9
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I am very green when it comes to electronics.
I have had some people with a stronger electronic background than me say go ahead with the repair with the bigger transistors,
however...

I find it very peculiar that Onkyo used these larger transistors
in the lower wattage amp model below mine(15 watts per ch).
When I look at the service manual of both amps and compare it looks like Onkyo tried to carry-over as many parts/components between the models.
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Old 5th January 2007, 03:13 PM   #10
ilimzn is offline ilimzn  Croatia
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It's not only about the power dissipation rating of the transistor, so within reasonm, you should stop wondering about that. Especially if higher ratings are used to replace lower ratings.
In some cases it may be as simple as the original transistors being obsolete, or even direct replacements having a different, higher rating capable case.

Finally, two things to mention with regards to your picture:

1) NEVER EVER replace just one transistor in a pair, even if the other appears to test fine. Unless you are strung out for the last cent, replace all power transistors in the channel.

2) 1301/3280 and 1302/3281 are perhaps the most faked transistors ever, and their original sole maker has long obsoleted them. Current parts are 2SA1943 and 2SC5200, unfortunately, even these are being counterfeited now. Be very careful where you get them!
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