Nakamichi AV-10 Troubleshooting

audiomodder

Standard Member
I've really loved this AV-10 ever since I found it and I did some button repairs on it and it worked great for a while until last week when the standby light comes on but it won't turn on. I've checked the amplified IC stages for shorts and cannot find anything wrong. Capacitors visually look fine and my speakers are still good as well. I never even power the thing over 10 watts (I use it mostly for its spiffy optical inputs). But when I opened it there were dustbunnies galore on the power supply section-- so heads up! Sucking it away with a vacuum cleaner once every 10 years might be a good idea...Just wish there was a Service Manual I could buy somewhere :'(

The power amp pcb actually disassembled completely from the unit which is cool...for easy replacement I'm sure. but with that disconnected it still doesn't turn on (standby light still there)...so I'm a little baffled. Any ideas much appreciated and I can take pics too if that will help describe things better.

Thanks in advance!
 

audiomodder

Standard Member
Apparently the two main power transformers are shorted on the primary side :(

I'm located in Northern California, anyone want to pick this up for parts? I hate to see such a beautiful amp go to the dump :/

Most everything works I believe- all the bridge rectifiers tested out fine coming from the secodaries. Some inputs need their ground input frame resoldered, but that's not very difficult. Was a beautiful amp when it was goin. If you have a broken one with good transformers you should grab this from me and give it life.

Till next time,

-Audiomodder
 

TheRealIzzy

Standard Member
I have an AV-10 and love it! The left channel cut out on me. I've tested both A and B speakers, switched wiring, and speakers. It's confirmed. :-(

I'm tempted to open it up and see what I can find but my electronics knowledge is very limited. I have a multi-tester, a soldering iron, and the will to bring it back to life! :)

Do you still have your AV-10? I know it's been a couple of months but I just joined today. I also live in the Bay Area.
 

audiomodder

Standard Member
Yep! I still have it. email me at email address for your own safety and you can come get it!
 
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noiseboy72

Distinguished Member
The primary windings will have a very low reading if you put a meter across them. they are also one of the most reliable components within the amp. Unless you have had a huge power spike, it is highly unlikley these would fail as you suggest.

I would suspect a capacitor or other active component within the power supply protection circuit. If the standby lamp comes on, then the PSU is working to some extent.
 

audiomodder

Standard Member
noieboy,
Thanks for posting to help diagnose. I actually spend a good amount of time on this before coming to the conclusion that the transformers were blown. It's like so:

1) the standby light is produced from a tiny 15VAC transformer on the main motherboard, this transformer is fine.

2) when the power button is pressed, a relay switches from the 15VAC (converted to +12VDC) to apply AC power to the mains (2 giant transformers) which power everything else on the motherboard.

3) When applying AC to the mains with nothing connected on the other side, fuses smoke instantly-- on both transformers

4) The bridge rectifiers/caps on the motherboard check out A-OK

I had a slim PSTwo connected the extra AC jack on the back when it blew. I recommend to people NOT TO CONNECT ANYTHING to these AC outjacks if you can help it because there is NO fuse on the motherboard for them. There was supposed to be one judging by the printing on the mobo, but it was replacesdwith a solid jumper. My guess is there was a small power spike and since the PSTwo supplies are ****** (there was a recall because they caught fire on people), it did something to kill my AV-10.

It is a little strange, I'll admit, but that's why I came to the conclusion it was that.

Thanks for your suggestions,
-Audiomodder
 
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noiseboy72

Distinguished Member
Interesting. Is there any form of voltage selection switch? This are known to fail and go short circuit.

It could be the transformers, but I would check for shorts etc. I have only ever replaced one due to a short circuit and that was caused by some idiot PAT testing it with a high voltage tester causing the insulation to break down.

Most of the time they fail open circuit if abused. I guess if it overheated or was dropped, the windings could short and fail.
 

audiomodder

Standard Member
Nope, no voltage switch. US AC plug (120v) is wired right into the case. From what I understand it *has* to be the transformers since giving them AC when they are connected to nothing (on the secondary side or primary side) blows the AC fuse. They are both internally shorted and, well, it doesn't look fixable (the coils are even gelled together with some sort of adhesive too).

Fortunately TheRealIzzy has that other broken one and I think putting the two together will work to get one working.
 

desm0ndk

Novice Member
Hi, you guys seem to know your electronics. I have an AV-10 that won't power up. Originally the stand-by led lit up, but still it wouldn't power up. Any ideas on how to start troubleshooting. Is it likely to be the power supply?
 

audiomodder

Standard Member
Hi, you guys seem to know your electronics. I have an AV-10 that won't power up. Originally the stand-by led lit up, but still it wouldn't power up. Any ideas on how to start troubleshooting. Is it likely to be the power supply?

You just have to start by checking the voltages at that large transformer. It really needs an extra fuse for the built in AC outlet.

My New Repair Log 3/7/2022

I know it's been forever, but I got another AV-10 a couple years ago, and this weekend I finally took the time to repair it.

One thing I learned is that the tone on/off switch really shouldn't be messed with. Mine broke from the previous owner so it wouldn't latch on, so tone controls didn't work :(

Dissected the whole front panel (very tricky, old brittle plastic), disassembled the broken switch and put it under my microscope. The switches are completely irreplaceable, unless you have another broken Nak.

The very thin latching plastic had broken off the shaft and it was not fixable.. so I ended up looking at the service manual and basically shorted the connection points to make tone controls always active.

Maybe some day Ill add in an ADG1436 (high voltage analog multiplexer) to restore the OEM function... but honestly, I'm not sure what advantage there is to turning off the tone controls.

I repaired the remote using some MG chemicals conductive ink pen (awesome stuff)... the whole unit works like a charm! :)

One thing I don't remember, is if the fan is supposed to be on all the time. Its a bit annoying at low volume.

The fan (CN305 plug) is a 2-wire interface, it's not even PWM. It's triggered by a thermistor (TH301) so if I can't find the part number online (oddly, it's only listed in the schematic image) and while it ends in 471 ( which would make me believe it's a 470 ohm thermistor), mine measures 47ohms instead.

So one of these days I'd like to add a proper lefit fan control using an MCU and PWM with a UART thermistor. I have all the parts and all the code ready to go from another project.

Anyway, I love this amp, and I'm very happy for now. Hopefully this repair log is interesting to somebody.
I also have those pictures and a PDF of the full service manual in case anyone needs that.
 

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