Nikko Alpha III that I can't set the idle current on

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#42
OK, back in the saddle. I apologize for not getting on this but the family issues had to be dealt with and I just had to let the hobby go for awhile and concentrate all my efforts else where.
So I have the jumpers back on and all the mosfets installed. It is plugged in thru the DBT and it cam up fine and the relay engaged. Light bulb is barely showing any light or current running thru it.
 
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#44
I'll have a look in a little while. I can't just remember where it was all at but it does all seem promising. If the -56 and + 56 volt rails are now good then it sounds good to go.

Check there is near zero volts at the speaker outputs (less than -/+50 millivolts).

I will look later. You need to reset the bias but we do that in stages so leave that for now.
 
#45
^ if the rails and offset is good you can test the amp with a speaker (still with the bulb in place) but keep the volume low. The speaker current will cause the bulb to brighten and flash with the music. Do not turn the volume up high.

If that is OK you can repeat the all the above on full mains (no bulb) and this time turn it up louder. Even with no bias the amp should sound essentially normal.

If we are still OK switch off and disconnect the speaker and put the bulb back.

We now see if the bias adjusts (remove any shorting links we added) and also make sure the bias preset is turned to the end that give minimum bias current. Now turn the bias up very slowly on the repaired channel and the bulb should start to brighten. When it begins to brighten stop. Do not turn it up beyond a dullish glow. All we are doing is proving it adjusts OK. We now turn the bias back to zero again.

Now go to full mains (no bulb) and carefully set the bias for real.

If you are not sure on anything then ask first 🙂
 
#46
OK before hooking anything up I measured the speaker connections. I have 4mv at the outputs. I also found 2 very loos wires that were going to the speaker outputs. If i wiggled the signal wire the DBT bulb would go bright and it would kick into protect mode. I cleaned up those connections rewired and soldered therm back in and its fine now.. I think I am ready to test. I will let you know the results
 
#47
Testedf with rt speaker, tested good. Hooked up left speaker and it clicked, so I shut it down. Turn on and bulb gets very bright until I unplug the input cable and then it goes dim and the relay engages. So back to troubleshooting just from connecting left speaker
 
#48
There is with out a doubt an issue with the left channel. If I hook up my preamp and connect a speaker to the right channel I get good sounding music. If I connect the speaker to the left channel no music and it will go into protection and the DBT starts pulsating.
 
#49
Looking at this regulator board there are a lot of loose wires that solder directly into the board. It might be something it might not but I am going to take the time to clean up all these connections and resolder them. One is only held in by one strand of wire so this needs to be rectified before I go any further
 
#50
This sounds like a possible tricky fault to locate.

If you have 4mv at the outputs and if (without you disturbing anything) connecting a speaker causes the amp to trip out and if that is repeatable behaviour (meaning you are 100% sure you are not disturbing anything in the amp) then it may be that the amp goes unstable with a load. It is a possibility.

If the DC offset jumps around as you poke/prod/waggle things (with no speaker attached) then you most likely have a poor connection somewhere.
 
#53
Success!!!!! Both channels are working fine. The cables to the reg/prot board have been cleaned up. I did find a small ceramic cap on the left channel board right next to the connections for the input signal had a broke lead and a small piece of solder wedged between the leads, sigh!!!😒😢 I am mad at myself for not being more attentive to cleaning things up. That little solder shorting out that cap who knows how long that damn thing has been there.
Well it's jamming with a Nikko Beta 30 preamp thats been recapped with new opamps LM4562. Sounds fantastic. I still have to recap the right channel board on the Alpha III when this is all said and done.
Whats next, Full mains??? No DBT scares me, lol but it needs to be done I suppose
 
#54
If you are confident then yes. I always say turn the bias back down first when moving from a bulb to mains in case the higher rail voltage causes the already 'bulb set' bias to increase dramatically. It should not do in a well designed amp (with constant current sources to define critical operating points in the circuit) but its a bit of an unknown so I always say to play it safe.

Well done, a good result 👍
 
#56
I bet you can't hear any difference at zero bias 😉

All I'm saying is turn the bias to zero before adjusting to a final value on full mains.

If you are happy all is OK on full mains then set the bias correctly. 100ma is a recognised figure for a single pair of Lateral FET's and that means adjusting for 22 millivolts across either of the 0.22 ohm resistors. It doesn't matter which resistor you use but ideally follow the procedure in the manual... which just looking says 44mv between test points 9 and 10 and which is across one of the 0.22 ohms.

So Nikko recommend 200ma bias current. Lower = cooler. You've listened to it at zero. Try it and see if you hear any difference 🙂
 
#60
Sigh, blew a fuse and replaced the fuse and left channel is not happening. Also when I bump the back of the amp it kicks into protection mode or if I try to button up the back of the amp. Very strange but tells me I still have some loose connections some where
 
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