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Diagnosing Nakamichi PA-7 Amplifier - Recap?

D

Decibel_116

AK Subscriber
I picked up a couple of Nak PA-7 STASIS amplifiers a while back along with a CA-5 and a CA-5II preamp. I put them in my main system one-at-a-time to see what each component would do to the sound. Well, the good news is that the first PA-7 does make sound. Although, I was initially impressed with the midrange purity when I inserted the PA-7 into the system, the sound lost some of its presence and width of sound stage. I downloaded the service manual. There are two adjustments that I found in the service manual. One is for DC offset, the other is the bias adjustment. I have not tried making those adjustments yet. I started thinking that I should consider replacing all of the electrolytic capacitors first. It has four 33,000 µF 100Volt capacitors for the mains filtering. Anyone ever had to replace these before? If so, are there better ones? Are certain models better for this application? There does not seem to be any caps this large that are designed / recommended specifically for an audio power amplifier application. Most seem to be designed for large switching power supplies.

BTW, the preamps are experiencing similar issues with dull sound quality. These are supposed to be Nelson Pass designs. I expected better than what I am hearing. Any other specific recommendations for these particular models? The CA-5II has large Silmic Cerafine coupling capacitors. Should these need replacing?

Apologies for the long post. Perhaps I am expecting too much from these old units.
 
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G'day;
If they're a matched set perhaps put together and play on and off conservatively. Nothing wrong with checking bias and dc offset if you're comfortable doing it. Caps can reform. You can also clean the pots and any switches?

I'm working on my Fathers 53 year old Sherwood that I know has low hours on it and its sounding better and better with less than 3 real time hours playing time so far.
 
The pots and switches seem to be fine. They are all Alps Blue types. Perhaps the larger caps are fine and they will settle in. I may try to set the DC offset this weekend and see if things improve. The CA-5II is an enigma. I have never had a preamp that sounded hard before...
 
If its a Nelson Pass design, you should go to the diyaudio site, Pass labs forms. Mr. Pass is on there about every day, maybe he can give you some pointers on your equipment and how it should preform.
 
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