Author Topic: Need an obscure Nakamichi amplifier schematic  (Read 630 times)

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Offline cvanc

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Need an obscure Nakamichi amplifier schematic
« on: April 26, 2019, 02:39:17 pm »
Hi all-

Anyone able to point me to an obscure schematic for a Nak amp?  It seems I have an oddball version here.

I'm working on one from the good old days and can't find docs that match this particular unit.  The model is PA-7Aii but the boards do not match the service manuals I have found at the usual places (hifiengine, etc.).

The manuals I have found say there is only one internal pot and it sets bias.  No offset adjustment exists.  But that's not what I see in my unit.  Mine has 2 trimpots (presumably for offset and bias) but it seems both pots strongly affect both parameters so it's a bit of a challenge to sort out how to adjust it.  Any insight is most welcome.  Thanks!
 

Offline floobydust

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Re: Need an obscure Nakamichi amplifier schematic
« Reply #1 on: April 27, 2019, 06:55:44 pm »
PA-7, PA-7E have two trimpots, one for bias current and another for DC offset.
PA-7_II, PA-7A_II, PA-7E_II has one trimpot for bias current.
But I found a "PA-7E_II 6" pic with two. Maybe post pics of the region with the mystery trimpots. Nakamichi may have rolled back to two adjustments on later board revs. PCB Part No. BA07259A or BA07446A?

Sometimes there are issues with mismatch in the JFET input diff amp. I'm not sure how good the dual 2SK240 was in production, they may have done an update to the PCB and design that is not in the available schematics. Other times I have seen an extra balance trimpot for the JFET front end. That adjustment is not for the faint of heart. A noisy trimpot can cause drama when at the JFET sources.
 
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Offline ebastler

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Re: Need an obscure Nakamichi amplifier schematic
« Reply #2 on: April 27, 2019, 08:10:00 pm »
This site claims to have the service manual for the PA-7A II:
http://www.hifi-manuals.com/Nakamichi/PA-7-A-Mk2/downloads

It's supposed to be free, but you need to create an account, which I have not done. So I can't vouch for it.
 
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Offline cvanc

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Re: Need an obscure Nakamichi amplifier schematic
« Reply #3 on: May 01, 2019, 01:09:44 pm »
Floobydust-
Thank you; the photo you posted appears to be the same board version I have.  Can you share where you found it?

Ebastler-
Thank you too; that version of the manual is the closest I have seen yet (but still not an exact match).

Appreciate the input!
 

Offline floobydust

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Re: Need an obscure Nakamichi amplifier schematic
« Reply #4 on: May 01, 2019, 08:01:14 pm »
The image is from HiFi Shock Museum, I got lost in the amplifier porn looking at images of this Nakamichi. There are many PCB pics out there.
It's the only amplifier I've seen that does not use emitter-follower output stage. It's common-collector with no overall loop feedback, just local feedback and under 0.05%THD is impressive. But 62lbs  :o
 


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