nakamichi pa7 mrk2

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ive been thinking of buying a pa-7 amp for a while now and up pops a mark2 European model.
reading up about it on the web and i found this and i quote,

" They were supposed to just license his STASIS technology, which combines the benefits of Class A amplification without the drawbacks (runs stupid hot = very low reliability) into a hybrid Class A/Class AB power amplifier with optical bias. Well, they didn’t just license STASIS. Rather, the first generation of the PA-7 is a direct copy of the equivalent Threshold amp. A lawsuit ensues, then Nakamichi releases the PA-7II with an altered design providing 25 more WPC (watts per channel) and a higher price tag. But everyone knew then and knows now that the original PA-7 was, “the one to get”.

is this true? is the first model better? or would a threshold s 200 be better for driving yamaha ns1000's speakers?

 
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" They were supposed to just license his STASIS technology, which combines the benefits of Class A amplification without the drawbacks (runs stupid hot = very low reliability) into a hybrid Class A/Class AB power amplifier with optical bias. Well, they didn’t just license STASIS. Rather, the first generation of the PA-7 is a direct copy of the equivalent Threshold amp. A lawsuit ensues, then Nakamichi releases the PA-7II with an altered design providing 25 more WPC (watts per channel) and a higher price tag. But everyone knew then and knows now that the original PA-7 was, “the one to get”.

This story is speculation that doesn't match the facts.
 
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If you read through some of the other threads on this site about Nakamichi's Stasis amps you find more about history of how they came to be, as well as questions and answers that dispel some of the urban myths that surround that part of history.

One of the interesting things noted by others is that Nakamichi actually used the wrong capacitor in the feedback circuit on the original PA-7 (they transposed voltage and capacitance), compromising its function. Nakamichi made a number of changes on the MK2 (improved layout, parts changes, power supply bypass caps on the amp boards, better internal wiring), and also rectified the feedback capacitor error. Given these facts, it's hard to believe the MKI is "the one to own", but I've never heard them side by side.
 
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electrafixion71 is right, MK2 is the fixeruper or the bug fix version of the mk1, there has been many post here talking about how to make the mk1 better, aka mk2, go for the mk2; an owner here of both mk1 and mk2, both to me sound the same. but keep in mind, your buying 30+ year old electronics and capacitors needs changing. including the BIG PSU one that cost $100 or so each, there are 4.

but its worth every penny, also you need a full recap with calibrations, service manual is out there with little bit of googling.
 
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I do not recall bug fixes for the first version. I do recall Nakamichi's lead
designer modifying the circuit, specifically by decoupling the front end supply.

This probably gave some improvement at audio frequencies and power levels
but did result in issues at high frequency clipping.

Fortunately, this was not a normal operating condition, but the recovery
was a rail-to-rail spike.

I informed the head of Nakamichi USA of this, but we were not communicating
well, and I did not hear back about it.
 
Thank you for clearing that out, i was under the impression, the MK2 was improvement/fixes over the mk1.

this just made me to look at both the mk1 and mk2 designes. I've also notice some trip pots like VR12 thats not even on the mk2 service manuals. maybe they had versions thats not even including in the original service manual for mk2

so the mk2 maybe better then mk1 then ?
 
Dear All,


It is my first post on this forum. I have a Nakamichi PA7 II (european 220V version). It is been running very well until few months ago where right channel started to sound bad, saturated. When warming up, it sometimes desapears, but it is not stable, and the right heat sink remains cold for hours, slightly warm when listening at high levels for a long time whereas left channel stabilizes in 15 minutes to a pretty warm, hot to the touch but acceptable temperature, just as the right channel did for years.
After few searches, I realizes it was likely related to idle current adjustement.
I followed the procedur in the manual, Warmed the amp and then tried to find the screw postion that gives 40 mA idle current. This was pretty straight forward for left channel, which was at about 45 mA, I played with it and set it up at 40. Right channel was much harder to stabilize. The screw had almost no effect on current, until it started going versy high and run hot... Then I managed to stabilize it around 40, but when starting from cold, does not reach 40 any more, and doesn't warm up as Left channel does.
It seems to me that the thermistor might be out of order.
What so you thnik? Are those thermistors easy to find? and hard to replace?
Thanks for reading.
Best regards.
Lionel
 
Dear Mr Pass,
thanks a lot for your answer.
I just measured the resistance from both right and left channel pots (VR11).
They behave exctly the same, from 0 to 1000 Ohm, pretty linearly.
I also checked the resistances (R163 on attached scheme) which are directly next to the pot and they are both 1000 ohm, as the schematic indicates.
When turned on, Left channel starts at about 130mA idle current and stabilizes around 40.
Right channel remains at 2mA. I measured the tension at the resistance next to the pot, it is at 470 mV on both sides.
When I turn the right channel pot from 0 to max, the tension at the resistance vary from 370 to 700 mV, this has some very eratic consequences on idle current (sometimes indicating 8mA), which always end up stabilizing at about 2mA what ever the position of the pot.
What do you think ?
Thanks again for your help
Lionel
 

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Dear Mr Pass,


thanks a lot for your answer.
I just measured VR11 pots on both channels with the amp switched off. They behave exactly the same, ranging from 0 to 1000 Ohm pretty linearly and consistently.
I also measured the resistance R163 which is 1000 Ohm on both sides.


I then turned the amp on and checked idle current.
Left channel (working well) : Starts at 130 mA when cold, and stabilizes at about 45 mA. Tension at R163 is 462mV.
Right channel (saturated) : Remains at 2mV. Tension at R163 is 470mV. I rotated fully on both sides the VR11 pot : Tension at R163 vary from 350 to 710mV. Idle current remains at 2mV (few months ago, VR11 had erratic consequences on idle current, now it has no effect anymore).
(see attached file for nomenclature)
What do you think ?
Thanks again for your help.
Lionel
 

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