Autostudio e-80.2 amplifier in protection mode - diyAudio
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Old 6th July 2010, 06:51 PM   #1
Mote is offline Mote  Estonia
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Default Autostudio e-80.2 amplifier in protection mode

Hello

Autostudio is not so well known finnish brand, but I have heard that they are copies of Kenwood.
Picture of the insides attached:

If this really is copy of some Kenwood model then it would be great to know which one, because I think schematic for Kenwood is much more easier to find than for Autostudio.

The problem:
Wiring and all that was OK. So I started to look around on the inside. Everything looked good, but on the output transistor resistance between BE, BC and CE were all 0 ohms. That transistor is blown, and there are npn and pnp transistors working together, so I need to change them both.
(Aluminium thing that presses them to heatsink was there, I removed it to check which transistors are used.)

Found a local store which has both of those transistors in stock and also the thermal tape thing.

The amplifier belongs to a friend of mine who got it for free, if he is willing to invest some money in the amp I would like to change those.

I understood that if I replace the 30 A fuse with a smaller one (10 A?) then I can test the amplifier without blowing transistors again if there are other problems as well.

If one channel consists of 4 output transistors and you remove 2 then it is possible to turn on the amplifier without damaging it. What about if there are only two and I remove those and try to power up the amplifier to see if the protection light goes on?

How high is the probability that only output transistors are shorted and evrything else is intact?

Thanks in advance for answers and other tips for noob are also welcome.
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Last edited by Mote; 6th July 2010 at 07:01 PM. Reason: Tried to repair links for pictures, cant delete the old links :(
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Old 6th July 2010, 10:06 PM   #2
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It's not likely a copy of a Kenwood. It's more likely to be similar to the Power Acoustik amps. Email me if you want the schematic diagram.
babin_perry@yahoo.com

It's possible that there is no further damage. If you can't find any other defective components, you'll have to replace the ones that you found to be defective.

It's possible to remove the blown outputs (leaving the good ones) to continue troubleshooting but this one has only 2 per channel so if you remove them from the blown channel, only one channel will produce audio.

The 10 amp fuse can only protect the semiconductors if they are all clamped tightly to the heatsink. The chance of damaging the clamped semiconductors with the 10 amp fuse inline is minimal but it will not entirely remove the risk of component failure.
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Old 7th July 2010, 08:31 AM   #3
m-tech is offline m-tech  Serbia
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Quote:
Originally Posted by Mote View Post
I understood that if I replace the 30 A fuse with a smaller one (10 A?) then I can test the amplifier without blowing transistors again if there are other problems as well.
For testing, you can put a 12V/50W light bulb in series with power supply. It will protect your amp from blowing.
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Old 7th July 2010, 10:07 AM   #4
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Even with a 2 ohm resistor or an automotive headlamp as a limiter, the transistors MUST be clamped if they are to be protected.
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Old 7th July 2010, 10:15 AM   #5
Mote is offline Mote  Estonia
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OK, the owner decided to give it a try.
Thanks for quick answers.
What is the safest way to proceed?
Other transistors in the supply seemed to be OK, which details are also likely to be defected?
Should I remove the blown transistors, try if the amp still goes to protection mode?
Or just replace them and see what happens with a smaller fuse in place.
If I test the amp and clamp the transistors do i need to install the thermal tape at once or can I do it afterwards?
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Old 7th July 2010, 12:24 PM   #6
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The transistors MUST be insulated from the heatsink. You can reuse the original insulators, especially for testing.

Removing the outputs from the defective channel shouldn't cause any problems. With those removed, the amp should power up and produce audio from the remaining channel.

The driver transistors and the base resistors may be damaged. You can check the resistors in the board after removing the transistors. The drivers should be removed for definitive readings.

It's also possible that one of the large emitter resistors has been damaged. Confirm that they're within tolerance. Since it's unlikely that your meter goes to 0 ohms when you touch the probes, touch them before measuring the resistance across the emitter resistors. Then subtract what the meter reads with the probes shorted from the reading you get across the emitter resistors.
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Old 8th July 2010, 05:48 AM   #7
Mote is offline Mote  Estonia
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Resistance of base resistors on the broken channel was identical to the working channel 220 ohms for 2sa1694 base and 2700 ohms for the other transistor. Colored stripes tell me the same values as well.

Large emitter resistors I could not check because my digital multimeter shows me 0 ohms for both emitter resistors on both channels. They have very small values? Checked the stripes and came to conclusion that they have the value of 0,1 ohms, is that correct?

By driver transistors you mean transistors in power supply? If yes then those I soldered out yesterday and checked with multimeter and they are OK.
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Old 8th July 2010, 06:08 AM   #8
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The emitter resistors are likely 0.1 ohm. If they read 0 ohms, they're probably OK.

The driver transistors are directly connected to the base resistors for the output transistors. They appear to be the ones that have the flat back instead of the rounded back.
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Old 8th July 2010, 04:53 PM   #9
Mote is offline Mote  Estonia
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Checked the driver transistors and they are good.
Put the new 2sa1694 and 2sc4467 in and the amp back together and instead of "Protection" LED i saw the green "Power" LED lighting up.

Perry Babin you are the best. You and your site were a big help.
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