Yamaha DX7 Faulty/Defectuous powerplug/internal transformer

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Mooger5
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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by Mooger5 » Fri Oct 28, 2016 8:08 pm

Yes, and today some if not most turntables still use it. Cartridges of the moving coil type output an even lower signal.
My pre and power amps had dedicated connectors for the separate ground wire that linked both chassis.

I now recall I got rid of the Oberheim Matrix 1000 humming noise by disconnecting the earth wire that went from the IEC inlet to the chassis. I later replaced the earth wire with a pair of power diodes wired in paralel but pointing at opposite directions. The chassis was still earthed for safety purposes and the diode loss attenuated the hum enough so it became inaudible. I learned this old trick on a hifi tube amps forum.
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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by Mooger5 » Fri Nov 04, 2016 1:24 am

Last evening some curious thing happened. I was installing a backlit lcd to the dX and had to open it up and turn it on to align the screen. The dx was plugged into the same outlet as the new tv. As I turned it on the tv turned itself off and turned back on as soon as the relay in the dx kicked in, as I could hear it.. I repeated the process and the same thing happened. Either those smart tvs are too sensitive or the dx is injecting some dc thump into th power line via the black wire. SO I disconnected the wire, turned the dx on and the tv remained on.
I´m not taking any riskes. The black wire will be removed.
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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by Werecow » Sun Nov 06, 2016 1:26 am

Mooger5 wrote:Either those smart tvs are too sensitive or the dx is injecting some dc thump into the power line via the black wire.
There's no return path for DC through the DX, so you can rule that out. Since you do apparently have the US version, I wonder if C11 & C12 in the DX are underrated at 250V, the same as C1. If they're arcing, that might have an adverse effect on the TV.

Of course, you could just have a finicky TV, too. :)

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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by Mooger5 » Mon Nov 07, 2016 5:05 pm

Here´s a pic. There are two additional components (I can´t read them. VDRs?) in series with the black wire. All appear to be fine. There is also the voltage selector, so this model must be "universal".
There is no return path for dc but as it´s b locked by the non polarized cap it creates a thump (peak). Couldn´t it go through the 10k resistor just begore the relay, to chassis ground and into the power line?
The tv is one of the latest samsungs. The problem might be the power line itself. Old house in the countryside.

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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by Werecow » Sun Dec 04, 2016 3:53 pm

Mooger5 wrote:Here´s a pic. There are two additional components (I can´t read them. VDRs?) in series with the black wire. All appear to be fine. There is also the voltage selector, so this model must be "universal".
There is no return path for dc but as it´s b locked by the non polarized cap it creates a thump (peak). Couldn´t it go through the 10k resistor just before the relay, to chassis ground and into the power line?
The tv is one of the latest samsungs. The problem might be the power line itself. Old house in the countryside.
Catching up today after a few weeks' absence..

According to the schematic that I have, those additional components (4.7nF Y-class caps C13 and C14) don't exist. A look at a TX81Z schematic from the same era clears up the confusion: they're very likely in series, connected between the black wire to the chassis and the junction of C11/C12.

Judging by your photo, C10 appears to be original. If so, and if it hasn't failed yet, it will. And judging by previous posts, it will fail in spectacular fashion :) I suspect that it's already bad, not performing the suppression function, and allowing the arcing that's upsetting your TV. Connecting the earth may just exacerbate the situation.

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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by Leo Castro » Mon Feb 06, 2017 3:44 pm

Hello everyone, this is my first post in the forum. A few days ago I bought a Dx7 mk1 and now I'm putting it in the best possible way. I already bought the screen with backlit, Ser7 eprom and a new volume potentiometer.
I see that the heatsink heats up a lot, but that seems to be common in all mk1s. I came to this post looking for information about the capacitor at the AC input. Mine was broken like the one in the photo. I changed it to a 0.47 uF and 270 volts, but now it seems that, although the switch turns off, the audio circuit seems to be still on and I can hear hiss on the headphones ... You think the capacitor value is the problem or is it due to a bad capacitor? Thanks to all for the help.
Greetings from Uruguay.

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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by placemat » Tue Feb 21, 2017 6:53 pm

Thanks to everyone for helping with this issue. I have successfully re soldiered a new capacitor onto my DX7's power board. All seems to be well.

Image
Last edited by placemat on Wed Mar 01, 2017 3:41 pm, edited 1 time in total.

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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by jacobchampion » Wed Mar 01, 2017 1:40 pm

Hi everyone,

I'm new here. Hello, hello...

I think I'm experiencing something similar to posters on this thread. I got my DX-7 home last night after a period of separation, it living in a quite dusty room it seems and I in someplace else :shock:

I switched it on and played a while, left it to sit and heard some clicking then a rather pungent smell. I turned it off and left it over night. Today, I've opened it up and checked the inside - where I discovered a lot of dust. I've cleaned it all off and the smell has gone but the capacitor (the white box that sits up tight to the taller box on the power board) looks to be maybe split and potentially damaged - see attached images.

http://imgur.com/a/7Y5kY

http://imgur.com/a/urKQJ

The power switch no longer seems to work but everything else is functioning as it should and it sounds wonderful as ever.

What would you experts recommend?

The smell has stopped and there's no smoke (there was a tiny bit before I cleaned the dust) so I assume this has been eradicated via cleaning.

Thanks in advance for your help :)

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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by placemat » Wed Mar 01, 2017 3:22 pm

jacobchampion wrote:
The power switch no longer seems to work but everything else is functioning as it should and it sounds wonderful as ever.

What would you experts recommend?

The smell has stopped and there's no smoke (there was a tiny bit before I cleaned the dust) so I assume this has been eradicated via cleaning.

Thanks in advance for your help :)
You capacitor has gone. It will work, but it is unprotected from power spikes, so don't use it until you have replaced the capacitor.

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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by jacobchampion » Wed Mar 01, 2017 3:56 pm

Thank you for your advice :)

Here's an image of the full power board:

http://imgur.com/a/nkTI5

Do you know if this is the correct part that I should invest in?



I'm not sure if it's an EU or UK version, though from reading the rest of the forum it doesn't seem like thats an issue voltage wise as long as the capacitor is able to withstand it.

Many thanks again.

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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by jacobchampion » Wed Mar 01, 2017 4:06 pm

Though the multiple language advice on the board hints at it being EU perhaps... :roll:

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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by placemat » Wed Mar 01, 2017 4:13 pm

jacobchampion wrote:Thought the multiple language advice on the board hints at it being EU perhaps... :roll:
Yes, that capacitor is spot on. If you look at the image I have posted a few posts previous above, you will see that it is the same capacitor that I have used.
There is no polarity, so you can soldier it in either way.

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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by jacobchampion » Wed Mar 01, 2017 5:08 pm

I'm not actually able to see the pics I'm afraid, the dropbox link seems to have expired? It's giving me a dead end at least.

Thanks for the polarity tip, might have missed that. The internet is such a wonder.

I've ordered the capacitor and a new soldering iron. This DX7's life will be saved at my fair hands with the help of youtube also 8-)

Thank you :)

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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by placemat » Wed Mar 01, 2017 6:20 pm

jacobchampion wrote:I'm not actually able to see the pics I'm afraid, the dropbox link seems to have expired? It's giving me a dead end at least.

I'm not sure if this works, but I have put all of the images on my Google+ page. It always is a ball ache to put images in forums!!

DX7 Repair Images

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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by jacobchampion » Wed Mar 01, 2017 7:30 pm

I got them, perfect!

So, it seems like it'll all need disconnecting for me to solder in the new capacitor. I've not used the ol' iron in a long time but sure I can get to grips on a few toasters, maybe a TV first.

8-)

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