Nikko Power Amplifier Alpha II Stuck On Protect

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caps need to be replaced one way or another ...there is no point tetsing them ...simply replace them

Nikko amps after some years fail on the psu caps also so you need to take a look at that also

Some of the designs have also two sets of voltage one for the outpout and one for the front end there you need to take a look also the rails are fused with diferent fuses so you need to check everything before proceeding

points to take care of is the electrolytics arround the protection circuits and also the vbe multipliers attached in heatsink cused trouble in a couple of times

also a schematic will be a good place to start ( which i am sorry to say that i am looking for also ) eventhough all nikko amps that come in to my hands have been repaired allready but having it is a good thing to

some extra help you will get from here mostly regarding the procedure
http://www.diyaudio.com/forums/solid-state/136261-vintage-amplifier-repair-upgrade-manual.html
 
for fun ....

this particular Nikko belongs to a costumer that realy wants to play with the amp ... so this one we will strip down and replace almost everything ...given the oportunity i will try to upgrade a few things ( a good point is to remove all ceramics beyond all the elctro's )

the amp has blown one channel ( fair enough since one of the rails was almost AC since about half a glass of juice was missing from the cap bank )

will remove all capacitors ( all means all ) and present my work in details in a few days

regards sakis
 
Havin one of these can make life easier :cool:
 

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what i found inside

damages :
----both output tranistors where gone in the wind from one channels
---- took with them one of the drivers + one of the predrivers
---- from the four caps ( bank ) i found one with extensve juice leak
---- all other small caps where also gone
---- the lamps of the wonderfull VU meters also gone ...

procedure

//since outpouts where gone and driver also i thought about replacing all outs with something more modern and more rowbust if possible MJ15024-25 was a good choise since it was available in excelent quality ( i do have a wonderfull stock from many good quality tranistors ) replaced including all the mechanical

// pre driver and driver pnp 2SB536 ( 40 mhz !!! ) was impossible to find and if found the quality is going to be questionable ...so decided to replace all drivers with MJE 15032-33 that sounded like a wise choise but also proceeding like that saved me the trouble to find or replace predrivers since pre driverss and drivers was the same tranistor so i was despert for one gone predriver

// all electrolytics ahere replaced both banks and small here and there some of them beefed up and all of them at least double the voltage ... then again banks where rated at 63 volt which i found kinda low takeing also in mind that amp now works at 240 volt ....so i put inside 10000 mfd /100v ,added mundorf 100 nf 63 on the top of every cap

// replaced all ceramics with better quality ( styroflex) input filter is silver mica now and after extensive tests 100 pf ceramics at drivers miller caps where replaced with 68 pf styroflex, bypassed the input cap 220nf ,

// add multiturn trimers for the idle , biassed at 120 ma , offset was 16 mv for the left channel and 20 for the right

// replaced all lammps with 12 volt 2.5 W ( no daylight filter though available )



after that i have never seen eveything so square at 100 khz !!!!!!

done with scope readings didnt have time to listen to which i ma going to with in the next days

regards sakis
 
nikko alpha II : missing protection card

Hello,
I purchased a Nikko Alpha II amplifier which work great, but has a missing protector circuit card.
I planned built one with the help of the service manual provided by Sakis (thanks again, mate ;)).
But after having searched for components, I had to admit that I am not good enough for that : I am able to assemble a kit and solder, but not to find a replacement for the discontinuted relay or diode :(
So, here is my question : is it worth building a new protector circuit, or is it cool to live without it ? What is the risk, as far as I don't switch the speaker on to soon after power up ?
 
Some on another forum told me about Velleman K4700 card. Do you guys think that it will be good for the Alpha II ?
Reading the schematics I do understand that connexions are quite different between Nikko's original card and the Velleman's one (4 cables coming from amp cards connected to Nikko protector card, but only two connectors on the Velleman). Is that an issue ?

Regards,
Luc
 
Hello,
here is the Velleman k4700 diagram :
An externally hosted image should be here but it was not working when we last tested it.


Other information on it :

  • switch-delay: ± 6 seconds
  • DC protection: +1V/-1V
  • max. input voltage: 200Vpp + DC
  • max. switching current: 10A
  • LED indication for: WAIT (switch-on delay) and ERROR (DC on speaker output)
  • supply voltage: 220VAC
  • PCB dimensions: 55 x 125mm / 2.2 x 4.9"
I would say it's ok, but I am not sure about this. Other questions I can not answer is :
- what kind of power supply do we have on Alpha II : symetrical or asymetrical ?
- from Alpha II schematic, I can see 2 wires from Left Amp card and 2 wires from Right Amp card connected to protector board. I can identify the two wires carrying the left and right channel amplified signal, but I don't know what are the two other wires.
They both connect on a same point on Protector Board, with "OCS" or something like that written. The question is : what is this, and where should I connect them on k4700 kit ?

Sorry if my questions are silly, but I can not figure this out by myself :(

cheers,
Luc
 
Wow, I see this thread is old, but I'll throw this out anyway: have a 220 that initially burnt up a resistor on the PCB, replaced and burned again. Think I have identified one of the right channel transistors as the probable cause.

I want to make this my 1st big project and leap in to repair/refurb, but can't find many caps and transistors at the local Radio Shack. Any suggestions on where to find a full kit, and any on possible upgrades? Thanks!
 
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