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 » Wed Oct 12, 2016 11:29 pm

The mains filter board on mine should have been the european model and is the american model instead, the same as yours and you´re in the UK. The black wire attached to the chassis is what tells it. As mine has a captive two prong power cord, not connected to mains earth, it lights up a voltage tester pen anywhere in the chassis. Detaching the black wire it reads 50VAC to chassis. I found the line out ground isn´t isolated from the chassis ground though, so if they were connected to mains earth they would make a ground loop, I think. Puzzled by the american model as they have three pronged cords there. Don´t they have grounds loops with this arrangement? 60 Hz hum?
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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by placemat » Wed Oct 12, 2016 11:49 pm

Werecow wrote:The cap is shown as 0.022μF, 250V on the schematic for both US and UK models. The 250V part leaves almost no safety margin if you have 230V mains, so replace with a higher voltage part in that case.

A safety-certified X or Y class cap isn't necessary in this application, but if it does fail, you'll still be able to turn off the synth with the switch (X and Y class won't fail shorted), and the cap won't turn into a flamethrower. :o
I showed this thread to a friend of mine who is a very good electrician. He advised to go up to a 300w capacitor instead of the 250w? He also said to find out which material type of capacitor is required. There are ceramic, plastic, even paper that I've seen! Does anyone know which type it is? I'll also ask yamaha if they know.
Thanks.

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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by Mooger5 » Thu Oct 13, 2016 12:26 am

Well I notice the cap is not across Live and Neutral. It´s in series with the live wire. And in paralel with the power switch so its function is to absorb current spikes when the unit is powered up, I believe. Anyway the higher the voltage rating the better. Try 300 or 400V. Ceramic.
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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by Werecow » Thu Oct 13, 2016 2:52 am

placemat wrote:
Werecow wrote:The cap is shown as 0.022μF, 250V on the schematic for both US and UK models. The 250V part leaves almost no safety margin if you have 230V mains, so replace with a higher voltage part in that case.

A safety-certified X or Y class cap isn't necessary in this application, but if it does fail, you'll still be able to turn off the synth with the switch (X and Y class won't fail shorted), and the cap won't turn into a flamethrower. :o
I showed this thread to a friend of mine who is a very good electrician. He advised to go up to a 300w capacitor instead of the 250w? He also said to find out which material type of capacitor is required. There are ceramic, plastic, even paper that I've seen! Does anyone know which type it is? I'll also ask yamaha if they know.
Thanks.
As I said, the original part was under-rated -- a 250V cap on 230V mains leaves no safety margin and is just asking for trouble, since the full mains voltage is applied across it when the switch is turned off.

Use a film capacitor, at least 300V (not 300w), possibly 400V. Film caps are "self-healing" and thus more durable than ceramics, and their normal failure mode is to open, vs. shorting for ceramic caps. Something like this should do the job: https://www.amazon.co.uk/Capacitor-0-02 ... 022uF+400V Verify that lead spacing on the cap is at least a close match to the hole spacing on the board, to be sure the cap will fit without making a mess of the leads.

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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by Werecow » Thu Oct 13, 2016 3:14 am

Mooger5 wrote:The mains filter board on mine should have been the european model and is the american model instead, the same as yours and you´re in the UK. The black wire attached to the chassis is what tells it. As mine has a captive two prong power cord, not connected to mains earth, it lights up a voltage tester pen anywhere in the chassis. Detaching the black wire it reads 50VAC to chassis. I found the line out ground isn´t isolated from the chassis ground though, so if they were connected to mains earth they would make a ground loop, I think. Puzzled by the american model as they have three pronged cords there. Don´t they have grounds loops with this arrangement? 60 Hz hum?
The U.S. model also has a captive two-wire power cord.

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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by placemat » Thu Oct 13, 2016 9:47 am

Thank you Mooger and Werecow for the good info. I'll check out that amazon link. My friend said he'd do the job for me, (seeing as I get my watts & volts mixed up!) :oops:

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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by placemat » Thu Oct 13, 2016 9:52 am

UPDATE: just bought a couple of the capacitors for amazon as advised. Thanks again!

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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by Mooger5 » Thu Oct 13, 2016 12:53 pm

Werecow wrote:
The U.S. model also has a captive two-wire power cord.
Thanks. It does show in the circuit diagram. What do you think of the voltage tester thing?
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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by Mooger5 » Thu Oct 13, 2016 1:22 pm

placemat wrote:Thank you Mooger and Werecow for the good info. I'll check out that amazon link. My friend said he'd do the job for me, (seeing as I get my watts & volts mixed up!) :oops:
No problem, although I wasn´t of much help. In fact I need some help with my Dx too. Would you please ask your friend to unscrew the lid using a voltage tester/ checker and see if it lights up?? It´s one like this. Have to show a picture as I don´t know the correct name.

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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by placemat » Thu Oct 13, 2016 2:23 pm

Mooger5 wrote:
placemat wrote:Thank you Mooger and Werecow for the good info. I'll check out that amazon link. My friend said he'd do the job for me, (seeing as I get my watts & volts mixed up!) :oops:
No problem, although I wasn´t of much help. In fact I need some help with my Dx too. Would you please ask your friend to unscrew the lid using a voltage tester/ checker and see if it lights up?? It´s one like this. Have to show a picture as I don´t know the correct name.

Image
Yes I will. I've got one of those electrical screwdrivers. I'll keep you posted!

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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by Werecow » Thu Oct 13, 2016 11:38 pm

placemat wrote:Thank you Mooger and Werecow for the good info. I'll check out that amazon link. My friend said he'd do the job for me, (seeing as I get my watts & volts mixed up!) :oops:
You don't need to know watts from volts to solder. ;-) This might be a good learning experience, though, since it's a simple job, and you have an experienced person there who can look over your shoulder.

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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by placemat » Fri Oct 14, 2016 12:42 pm

Werecow wrote:
placemat wrote:Thank you Mooger and Werecow for the good info. I'll check out that amazon link. My friend said he'd do the job for me, (seeing as I get my watts & volts mixed up!) :oops:
You don't need to know watts from volts to solder. ;-) This might be a good learning experience, though, since it's a simple job, and you have an experienced person there who can look over your shoulder.
Thank you! I did successfully resoldier a new backup battery on both my DX7 & DX7s so it should be OK. :shock:

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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by Werecow » Tue Oct 25, 2016 2:02 am

Mooger5 wrote:
Werecow wrote:
The U.S. model also has a captive two-wire power cord.
Thanks. It does show in the circuit diagram. What do you think of the voltage tester thing?
Mooger -- sorry I missed seeing your post before.

The AC tester lighting up on an unearthed chassis is perfectly normal, due to the small leakage current through those X-class caps. You could earth the chassis (as I do on all my personal gear). An additional wire daisy-chaining all the gear together minimizes possible ground loop problems.

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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by Mooger5 » Tue Oct 25, 2016 10:48 pm

That reminds of my old stereo system; there was an extra wire for a shorter ground path between the preamp and the power amp. Noisefloor was almost imperceptible. Thanks.
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Re: Yamaha DX7 Faulty/Defectuous powerplug/internal transfor

Post by cornutt » Fri Oct 28, 2016 6:38 pm

Back in the day, most phonographs had a separate ground wire that you connected to a terminal screw on the amp. Because the output level of a phono cartridge is so low, it was pretty much essential to get noise and hum down to bearable levels.
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