Nikko Alpha II amp troubleshooting

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Hi Tom,
The post from WorkingAtHome is correct. We generally measure bias current by measuring the voltage drop across a low value resistor. Most amps have this built in as the emitter resistor (or source resistor for FETs).
There is something damaged, transistors and diodes tend to go short when they fail, resistors tend to open or change their value if overheated. If the transistor measures okay out of circuit, then something else has shorted. Look at the schematic for something else across those leads. If you aren't sure you checked something, check it again and make notes. Make sure you check everything, including lead orientation and proper transistor number.

-Chris
 
output 2sd555

For some reason the newly installed 2sd555 (right channel) was creating a short. It checks okay. Zero voltage when removed. It seems when mounted with the screws to the heat sink, it shorts out. I tested it without the screws, and not allowing the T to touch the sink. The test bulb glows, and dims out. The power meter illuminates, but still remains in "protect" mode, as the left channel is now problematic.

The original 2sd555 had a thin plastic like gasket. Should the new T have the same?

Tom
 
That was a mica insulator. It's purpose is to electrically isolate the back of the transistor (connected to one of the pins) from the grounded heatsink. You can likely reuse the old one if you still have it. It should be coated (both sides) with thermal grease to improve heat trandfer from the transistor to the heatsink. Without it, is will likely die an early death due to heat.

Thermal paste is available from most electronics places. Radio Shack even had a little tube.

Incidently, this applied to all heatsink mounted transistors.
 
change in status in RT Channel

Did a comparision with another new transistor and found that it measured .6 voltage drop. Also compared measure of the 2sd555 on the left channel, reading .6 voltage drop. The one I just replaced measured Zero. Uh oh. I think its bad.
So I Replaced it and now it seems we're back to constant bright illumination of the 60w test bulb.

Tom
 
Similar issue with Nikko Alpha II Amp

Hi Chris,

I was onlin looking for some info on my Nikko Alpha II and saw this thread. Unfortunately I see it ended without know whether "livetoski" resolved his issue. I have a similar problem with my amp except the fuse is blowing (same channel, left). So far I replaced all the power transistors and the four transistors with the heat sinks on the Main Amp circuit board. Still no luck! I've also replaced the four large caps, they were leaking! I thought maybe they were shorted and caused my problem at first but no luck and this is why I went after the transistors. Any obvious causes come to mind that would blow the fuse?

Guy (Georgetown, Ont)
 
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Hi Ken,
440's & 450's or the 220's and 230's as well?

The II was a simpler design and did not use a DC servo loop. It was a very good design but pushed the outputs a little hard as there was only one pair on 55V rails. It would have been a very good amplifier if it had two pairs per channel. The original outputs were 2SD555 and 2SB600. Only rated for 10 A continuous.

-Chris
 
Nikko Alpha II

Thanks for the reply KBK. If all else fails, it may be my very last approach! Chris has offered (on his bench) to assist me if I cannot get it going on my own. I plan to take him up on his offer as we are practically neighbours. I may have induced the problem myself so right now I'm looking at replacing the power transistors again. I've checked all the semis on the main amp board and they seem to check ok. In the mean time, I need to buy better power transistors.

Guy
 
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Hi Brian,
I own an Alpha 220 and a 230. You wouldn't have any info on these would you?

So, your protection circuit had a "bake off"? I guess those two screw flat pack outputs went as well. They had an odd pseudo class "A" bias circuit. Mine are still working right now but I was considering some mods to improve the sound quality. I think the Alpha II may have been the better sounding amp.

-Chris
 
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Hi Brian,
Any information you could share would be wonderful! I'm completely in the dark as my manuals went when I sold my shop.

I would suspect the drivers were bad. I have repaired more than a few of these but my memory is only so good. I do recall the drivers, predrivers and bias circuit often went on these. I've had the DC servo op amps blown as well. Yank the op amp until you have it running, then install a new one.

I would have a look at On Semi MJW devices to see if they would fit on those. It's been over 10 years since I last looked at one.

-Chris
 
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