Repairing a Nikko Alpha II power amp

Long story short, I was given a Nikko Alpha II power amplifier as payment for helping my friend move a literal truckload of stereo equipment, but that's a story for another post. Most of that equipment was busted, so I expected this to be... but to my surprise, I plugged it in and flipped the switch, and it powered on just fine. Plugged in some speakers and it sounded pretty good, except some clipping and distortion mostly in the right channel.

When I opened the thing up, I saw that the "PROTECT. PCB" between the transformers has been worked on and the electrolytic caps are all new. The amp boards, on the other hand, still have the original caps and haven't been worked on at all.

I'm planning on getting some new electrolytic capacitors for the thing (about 10 in total), but is there anything else I should check or look for? This is my first amplifier repair project, so any tips or advice are welcome.

Cheers

 

There's a service manual over at hifiengine for this thing! You'll have to register to download it, but its all there! What a great site. They had the service manual for my old HK 430 receiver too.

http://www.hifiengine.com/manual_library/nikko/alpha-ii.shtml

If you're handy with a schematic, it could be of some service. At very least it lists every single component in the thing.

If you're getting clipping only on the right channel, I'd confirm that the power supply voltage is the same on both channels. If it isnt, power supply needs some TLC. If it is, its probably a bias problem somewhere. The fact that one side works and the other doesn't will make your life muuuuuuuch easier. Simply compare values between the sides. I'd start with the output transistors (looks like Q2 and Q3 on each channel) and check their bias voltages first.

 

I think I've traced my right chanel problems to a busted leaky capacitor. It cost me seven bucks and change plus shipping for a set of new capacitors for both amp boards.

Thanks for the tip on the manual! I'll check the voltage biases too. It's pretty hard to resolve the details on the schematics but I think I can make due.

 

Here's the busted cap in case anyone is curious

 

Turns out there were several busted capacitors, that was just the most obvious one. I replaced all the electrolytic capacitors on both amplifier boards (10 caps in total, $7). Probably took me an hour or so. Unfortunately, this didn't resolve the issue in the right channel, so I took apart the heatsinks to look at the transistors and saw this:



The PCB which is holding onto this transistor is simply coming apart. The transistor itself is loose and can be wiggled back and forth, because the solder has come off of the PCB. The left channel transistor isn't in much better shape, with the top layer of the PCB beginning to peel off.

I'm thinking once I get back home, I can get some perforated board and re-mount the transistors onto that. Hopefully that fixes it... I'm excited to get this thing working. Once it's singing, I'm going to use it while I recap my Hafler DH500 power amp.

 

Here's a photo of the amplifier's guts with new capacitors. I used Nichicon "high-end" stuff. Total cost of parts was like $7 from Digikey.