I haven't been able to recreate the even resistor issue I saw previously. Maybe it was a mis-measurement, but that doesn't seem terribly likely. Maybe one of my probes was not making good contact?
I wanted to see if the noise situation improved with the RCA inputs soldered in and the heat sinks attached to the main chassis. Sure enough, there's no noise I can discern in the speakers (admittedly tiny old AudioSource things). I re-set bias and things seem to be humming along quite nicely.
I figure I'll use it in this configuration in my garage for a few days/weeks and see if anything weird happens. I'd still like to swap out the left channel bias pot to a new one, but I can wait a bit.
I wanted to see if the noise situation improved with the RCA inputs soldered in and the heat sinks attached to the main chassis. Sure enough, there's no noise I can discern in the speakers (admittedly tiny old AudioSource things). I re-set bias and things seem to be humming along quite nicely.
I figure I'll use it in this configuration in my garage for a few days/weeks and see if anything weird happens. I'd still like to swap out the left channel bias pot to a new one, but I can wait a bit.
It sounds as though things are coming along nicely. Is your DC offset still a bit high on the one channel? If so there's a solution you could employ from the first gen PA-7 amps to tweak it back to zero.
I left the amp on for 20 minutes and took these measurements:
Left channel offset: +8.8mV
Right channel offset: -7.5mV
Left channel offset: +8.8mV
Right channel offset: -7.5mV
Oh that DC offset looks great now!
Let us know how it sounds once you finally trust it enough to connect it in your audio system. You should be relatively safe with the protection relays.
Let us know how it sounds once you finally trust it enough to connect it in your audio system. You should be relatively safe with the protection relays.
I ordered some new bias pots to replace the broken one. I replaced the right channel one too because I didn’t really care for the pot style I had in there. I tested it partially reassembled in my garage for a few days, then the other night I finally got up the courage to fully box it up. Everything seems to be working beautifully! My speakers are pretty power-hungry, the KEF Reference 107/2, and it drives them really well.
One thing I do notice is that, when I first power it on, for about the first minute while it’s warming up, there’s a bit of hiss in both speakers. I’m guessing that’s normal, but I’m curious to hear if your Nakamichi does the same thing. After that time, there’s absolutely no audible noise or hiss and it really just sings. I added some photos of the complete unit and a video too:
Nakamichi PA-7AII - Google Photos
Chamberman, I also just wanted to say thanks again for sticking through this whole process. It’s truly been epic, and it’s super gratifying to take something that I was pretty sure was beyond repair and making it work again. It never could have happened without your knowledge!
One thing I do notice is that, when I first power it on, for about the first minute while it’s warming up, there’s a bit of hiss in both speakers. I’m guessing that’s normal, but I’m curious to hear if your Nakamichi does the same thing. After that time, there’s absolutely no audible noise or hiss and it really just sings. I added some photos of the complete unit and a video too:
Nakamichi PA-7AII - Google Photos
Chamberman, I also just wanted to say thanks again for sticking through this whole process. It’s truly been epic, and it’s super gratifying to take something that I was pretty sure was beyond repair and making it work again. It never could have happened without your knowledge!
Fantastic! The story of this amp coming back to life and the two of you sticking with it is epic.
Congratulations to both of you.
Congratulations to both of you.
A bit late to respond we just got back home tonight. Tenelson we've been visiting your home state. We went to Cloudcroft and High Rolls this past week and stayed at a couple of different locations there in the mountains with friends. We had a really great time. Its very nice up there! Especially this time of year when its so hot here in Dallas.
That's great to hear that the PA-7II is working well for you and I'm glad my advice and guidance didn't make things worse. 😉
I'm still surprised nobody ever came in and began assisting as well. It was almost like we were working in a bubble. Sometimes that's a good thing though as it allows you to stay focused without getting hit with too many suggestions.
I checked out your pictures and that is one clean amp and it was definitely worth resurrecting. It seems like many times the amps that people want to repair are in as bad or worse shape cosmetically than the electronics. Its a great addition to your system for sure.
On the hiss,
I do not remember my PA-5 having any added noise at start up but I'll check it in the next few days and see. If its present in both channels and disappears simultaneously in both channels then I'd expect that its probably power supply related.
I connected my PA-5 this past weekend and checked it and its completely silent with my ears right up a foot from the speakers from cold start all the way until hot. If yours is only doing it at start up and only for a minute or so then its probably not worth messing with.
I just want to say I spent the past 3 hours ish reading this thread from beginning to end. Epic win!
I learned a lot through both of your trials and tribulations; troubleshooting is an art and you both performed at an "11". Some of these agonizing situations, such as the bit of solder identified and causing a inadvertent ground....really hit home troubleshooting my own gear (both DIY amps and amateur radio repairs). Most frustrating finding those oddities; it was a youtube collection of personal memories as you traversed this journey.
Bravo!!!
I learned a lot through both of your trials and tribulations; troubleshooting is an art and you both performed at an "11". Some of these agonizing situations, such as the bit of solder identified and causing a inadvertent ground....really hit home troubleshooting my own gear (both DIY amps and amateur radio repairs). Most frustrating finding those oddities; it was a youtube collection of personal memories as you traversed this journey.
Bravo!!!