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Old 29th July 2011, 09:56 PM   #1
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Default Just got a Nakamichi PA-7A Mark II

It seems to be in great condition but I have a couple of questions:

1. Should I replace the caps replaced/upgraded? Rough idea of investment?
2. Who can do this for me in the San Francisco bay area (I'd rather not deal with shipping if possible)

This is my first exposure to Pass amps and am excited to learn more.
thanks in advance for the advice!
carlos
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Old 29th September 2011, 07:18 AM   #2
mirolab is offline mirolab  United States
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Default Nakamichi PA-7 mkII bypass caps

Hello Carlos,

I bought my Nakamichi PA-7A mkII exactly 20 yrs ago, and have always loved the sound (and looks) of it. It was always a bit dark & romantic sounding, with big warm bass, and smooth highs, and wide imaging. Lately... when compared to my newer amps (Bryston) it was sounding sluggish with compressed dynamics.

I got to looking at the schematics and noticed that the four HUGE 33,000uF electrolytics were not bypassed at all... and they are likely degraded after 20 years. I have not yet found suitable replacements, but I thought i'd at least add some bypass caps.

First I put 4.0uF Dayton Metallized Polys across each of the 4 caps, and WOW what a big difference. Suddenly it had dynamics again.... but mostly in the highs.... the amp was actually bright sounding... but the mids and lows were still sluggish. Clearly the 4.0uF's were supplying current for HF, but not enough for lows....

SO... I bought a set of Elna Silmic 100uF/100v elctrolytics. BTW..... I am putting these bypass caps in the bottom of the amp, under the big main caps. (see pic) There is only about 1 inch of space under there, so that’s the max diameter of cap you can stick in there. The good thing is that it’s very well protected under there. Anyway, the 100uF’s really did the trick, and opened up the dynamics & resolution of the midrange and into the midbass. The Polys are still there too. I only installed the Elna’s yesterday, so I need to do more listening and let them break-in. It still sounds a bit bright, so I might disconnect the polys and try just the Elna’s. I also bought a set of Elna 47uF Cerafines, which I understand might sound different than the Silmics.

I am fairly new to messing around with amp mods like this, and I am blown away with what a drastic change 4 little bypass caps made on this amp. There are bypass caps on the input side of the amp circuitry, but the output devices are being supplied directly from the large 33,000 uF caps (Nichicon Great Supply, 90v). I’d like to replace these, but they are tall cylinders (about 1.5”d x 5” tall) and Nichicon does not make them like this anymore. Anybody have any recommendations??

The next big step will be to replace the caps on the driver boards, but that will require major disassembly. Also the bias & offset trim pots need replacing…. They are very noisy when I adjust them.

As for servicing your amp Carlos, there is a company in central California called vintageamprepair.net that specializes in Threshold service… and thus the Nakamichi Stasis amps too. I was tempted to send mine in, but thought I’d give it a go myself first…. Especially considering the 2-way shipping of a 60 pound amp! I love this thing.... i'll never sell it.

..... Miro.
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Old 20th October 2011, 06:57 PM   #3
mirolab is offline mirolab  United States
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Default Nakamichi PA-7A mkII bypass caps

I am now on my 3rd configuration of bypassing the main PS caps on my PA-7A. Here's a summary of configurations in case anyone is interested...

1.) Stock 20yr old 33,000uF electrolytics:
Sound was very coherent, but dark tonal balance and restricted dynamics.

2.) 33,000uF + 4.0uF Dayton Polypropylenes:
Whoahhh... Brought back the highs and transients, but mids + lows sounded sluggish. Amp now too bright and a disjointed sound.

3.) 33,000uF + 4uF polys + 100uF Elna Silmic's
Adding the 100uF's increased the clarity and dynamics into the mids + lows now. Much better overall sound, but the highs still sound harsh & disconnected.

4.) 33,000uf + 100uF Silmic's
Got rid of the 4uF Daytons and that tamed the highs. Just adding the 100uF Silmics is a better balance. This simple mod costs like $10 in parts for 4 caps, and makes a HUGE difference in reviving the life of this 20 yr old amp!

Ideally, i would like to replace the 33,000uF PS caps, but i don't know where to get suitable replacements. My next big step is to disassemble the amp and replace the driver board caps. Not sure yet how to deal with the 18 transistors attached to the main heat sink! It could get messy.

Any comments welcome!................. Mirolab.
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Old 24th October 2011, 08:58 PM   #4
cviller is offline cviller  Denmark
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You can easily buy replacement cans at digikey, but they are expensive.
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