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Nikko ALpha III

philster

philster

Active Member
I just rebuilt a Nikko Alpha III that was aquired from an estate sale and was housed in a rack. The amp did not power on when I got it on the work bench. Both power supply fuses were blown and since its a very straight forward power supply I just rebuilt it. New caps, diodes and resistors. Caps were way out of tolerance and one diode was bad. Once rebuilt it powered up just fine but did not come out of protection. Next I tested the mosfet outputs and all four were bad. Due to the age of this amp and condition of it I just refurbished the protection/reg board and recapped the main amp boards. I aquired some used mosfets from a seller on DIY Audio. They tested good before installing. Powered up the unit thru a DBT of course and it came out of protection just fine.

Next I set the DC offset. I am not a fan of the old trim pots, very sensative and hard to zero in on a voltage. Left channel offset adjusted just fine +/- 10mv. with the shorting plug removed but with the shorting plug in it was way off a little and would not adjust down but close. The right channel not so good. The lowest I could get the offset was -345mv.

The bias voltage (idle current) adjusted fine for both channels. The left/right channel adjusted right to 44mv. I did notice as i adjusted the bias current to the specified voltage the dbt bulb got brighter. Is this because the offset voltage is so high on the right channel??

I'm going to order some replacement trim pots as the old ones are way to sensative. I really dont trust them to hold the correct voltage. Anyone have any thoughts why the right channel offset is so far off with new components installed?

Phil
 
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philster said:
Is this because the offset voltage is so high on the right channel??
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No. It is caused by the bias adjustment -- expected. If you adjusted the bias with the dim bulb inline.

Then tweek the bias back down to zero before going to straight mains. Then readjust with the straight mains.

Otherwise, your bias will be way to high running on straight mains.
 
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avionic said:
Offset -- Amplifier front end --- differential pair or long tailed pair. May no longer be a matched pair.
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Thanks Avionic. I have dealt with that differential pair before on another Alpha III. That differential pair is the only thing I haven't touched on the main amp boards. 2SA872 and 2SC1775. I believe the most common replacements are the KSC1845 and KSA992. That differential pair is part of the offset circuit. I think I can buy matched pairs from digikey

phil
 
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avionic said:
Looks like your going to have to match them yourself
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When matching HFE, do they have to match exactly or can the be within point or 2. For example I have 2 KSC1845 with HFE of 440 and 441? Also do all 4 transistors have to match each other, pnp and npn all have to have same hfe
 
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