Nakamichi PA-7AII Amplifier distortion

[tenelson]

I recently was given a complete Nakamichi system, including preamp, tuner, cassette player, CD player, and a PA-7AII power amp. After I connected everything up, I noticed that the left channel occasionally distorts at all volumes. In addition, after a while, it sometimes starts to hum, fairly quietly. The hum volume didn't change when I changed the preamp volume.

I tried another set of speakers, and the distortion was still there. Then I used my Marantz 2230 as a preamp with the PA-7AII, and the distortion was still present. The distortion was completely gone when I switched back to my 2230 as amp+preamp.

So it seems obvious to me that the PA-7AII has a problem. I took the top of the case off, and all of the caps seemed intact. (I've seen the tops of electrolytic caps bulge in circuits that have overheated). What should I do next?

 

[mhedges]

Is there a bias adjustment?

 

[damacman]

Take the amp to a reputable service center and have it repaired.

 

[for_p1]

Take the amp to a reputable service center and have it repaired.

But prepare a few hundred $.

 

[tenelson]

I do like to fix things myself if possible. I'm handy with a soldering iron, multimeter, ordering from Digikey, and so on. I just found the service manual. Right off the bat, I see how to verify DC offset and idle current. I will check those and see where I'm at.

 

[stoutblock]

^^^ Good start ^^^^

I always check DC offset and idle bias on an unfamiliar amp before installing in my system. I also have a PA-7AII but mine works flawlessly. There were some reliability problems with the earlier PA-7A amps but I've not heard of excessive issues with the II version. Hopefully it is something simple.

Good luck!

 

[tenelson]

According to the service manual, the idle current should be 40mV both each channel. My left channel measured 2mV, and the right channel measured 3mV. I was able to adjust the trimpot on the right channel to 40.9mV, but the left channel will not adjust, it just stays at the 2mV reading. Does this imply a blown transistor?

 

[stoutblock]

Could be a lot of things including a bad trim pot. Take it to a pro...