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Old 21st February 2005, 06:58 PM   #1
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Default Nikko Alpha II amp troubleshooting

Hoping someone out there can put me in the right direction. I have a Nikko Alpha II power amp with audio output thru only the left channel. It gives equal voltage at the 5.0 amp fuse holders with the fuses removed. If you replace both fuses and power up, it remains in "protect" mode. Now, if I remove the right channel fuse, power up, audio out of the left channel is established. In this state, with power still on, attempting to replace the right channel fuse causes it to immediately blow. Any ideas short of bring it to a pro ? Thanks
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Old 21st February 2005, 07:23 PM   #2
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Not definite, but I would bet you have dead output transistors in the right channel. You can test this pretty easily by (with power off) checking the resistance between the pins of the transistors mounted on the heatsink. If they are bad, they will show 0 ohms.

These can be hard to find (I couldn't) but I now think I *might* know someone who has a few (2sc2565/2sa1950)
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Old 21st February 2005, 07:23 PM   #3
anatech is offline anatech  Canada
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Hi livetoski,
How much experience / test equipment do you have?
While in protect mode, what are the amplifier's DC voltages before the relay?

One thing to consider, a good tech is worth his / her weight in gold. A bad tech is everyones worst nightmare.

-Chris
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Old 21st February 2005, 08:13 PM   #4
anatech is offline anatech  Canada
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Hi Tom,
The original o/ps were 2SD555 and B600. You can use the new On Semi devices or MJ15024 and MJ15025.

Protection circuit will detect either high DC offset (above a few volts) or excessive current through the emitter resistors. You need to find out which is tripping the protection circuit.

-Chris
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Old 21st February 2005, 08:39 PM   #5
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Default getting started...

What components do I specifically begin testing and what am I looking for ? I'm a little weak on some of the terminolgy here... what is o/ps 2SD555 and B600 referring to?

Tom
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Old 22nd February 2005, 12:25 AM   #6
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Sorry, I was confusing it with the Alpha 230 when I mentioned the 2sc2565's.

"o/ps 2SD555 and B600" means output devices were 2SD555 and 2SB600.
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Old 22nd February 2005, 12:54 AM   #7
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Hi Tom,
I don't think the output transistors are shorted. Please measure the DC offset from the emitter resistor center and let us know what it is. Is there any way you coud post the schematic for the Alpha II. Mine is gone.
Thanks Tom!

-Chris
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Old 22nd February 2005, 06:13 AM   #8
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Default proceeding with testing

Couple more questions before I proceed here... Having some trouble locating/identifying the emitter resistor. I can scan the schematic and attach tomorrow if that will help. Next, what "state" should I perform the voltage check, that is, with the 5.0 amp fuse in place, which puts the amp in protect mode, or without the fuse, which releases the protect mode, enabling the left channel ? Sorry for all the questions, as this is my first amplifier repair attempt.

Tom
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Old 22nd February 2005, 06:13 AM   #9
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Default Answered one of my own questions

After submitting the last post, I realized that I obviously need to have the fuse in place to send current to the right channel main amp. Now to locate the emmitter resistor.

Tom
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Old 22nd February 2005, 06:04 PM   #10
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Default Alpha II schematics

Here the schematics of the Alpha II main cb and Protector cb. I've included the voltages measured at the noted points.

Tom
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File Type: jpg main&protector_cb1.jpg (85.3 KB, 1037 views)
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