[sdiy] Re: converting DX7II power supply

[Batz Goodfortune]

Y-ellow Riccardo 'n' all.
         The subject says "re:" So I've probably missed something 
important. I'm somewhat medicated at this point so please forgive me. 
(We're talking prescription drugs here OK! Got that? There's nothing fun 
about them. I was born insane.)

At 08:41 AM 5/3/02 +0200, riccardo wrote:

>I have a DX7II with US/Canadian power supply board, and I need to convert
>it to 220V.
>There's no indication on how to perform this on the board, but I know that
>european models give the possibility to switch from 110 to 220 Volts.
>Anyone who knows if this can be done for the US power supply board?

 From what I remember, the US/canadian market was sufficiently large to 
warrant it's own model. I think you can lump Japan it self in that group. 
I'd have to look on the service manual which I'll try to find a bit later 
on. However, for the rest of the world which was divided into many 
voltages, another version of the powersupply was made with a voltage 
selector switch. That would be Europe, Australia, New Zealand etc. I know 
mine had a voltage switch but I can't remember where it was located. If you 
don't have the switch, it's probably a fixed voltage for the US market.

I should point out that it may not have been a switch as such. Sometimes 
they used a kind of plug thing that looked like a rotary switch which you 
pull out and plug into a different position.

That's the bad news. The good news is that A DX7 doesn't chew a real lot of 
juice. We're not talking about a small power sub-station like my ol' Ampex 
deck had. So the best thing to do is to find a step-down transformer and 
plug the DX into that. 220 ~ 240 in ---> 110 ~ 120 out. If you can find a 
transformer small enough that can supply the required Is, it might be 
feasible to mount it within the DX it self. I can't remember how much spare 
space there is in the II. If not, cut your losses and get a bigger one and 
some US power outlet sockets, box it all up and then you have a 110 supply 
for any other US gear you might want to run.

It's not as bad as it sounds. Just takes a bit of planning. I now have 4 
sources of 110 ~ 120. A bloody great big one under my bench. (There's that 
word again.) But I also run several smaller ones. In particular, I have a 
2U 19" rack enclosure at the bottom of my main audio rack. It's chock full 
of transformers. Mainly to replace a gzillion unreliable plug-packs. (Wall 
warts)   But it also contains a beautifully sexy, chunky flat pack auto 
transformer which supplies a number of US mains voltage devices in that 
rack. Supplies about 3 ~ 4 amps.

It's probably the heaviest 2U rack box on the planet. It's solid packed 
with transformers. I guess you could make it heavier filling it with lead 
but you get the idea.

I have no idea where you'd find such a transformer, (auto or isolating) in 
Italy but here we have the afore mentioned "Robbies" and there's always 
plenty to chose from. If you have somewhere there that scraps this stuff 
you can save yourself a veritable bucket-load of cash. New transformers are 
always expensive. Especially if they know you really really need one.

The only other thing you could conceivably do is replace the DX7s 
transformer.  Unusual as this may seem, you can still buy these from 
Yamaha. You need a spare parts friendly dealer and a part number but Yamaha 
are usually pretty good on parts going back decades. But! It'll probably be 
costly. You never know though. At one time I use to buy pots and knobs as 
spare parts through Yamaha to use in projects because I could buy a 
pot/knob pair cheaper as a spare part than I could buy just a crappy pot 
off the shelf here. And they were really nice little pots too. But I digress.

Oh bugger it. Let me just pull the service manual out.
OK. I should go back and start this Email again but it was typed at 
considerable effort so you can just jolly well read it. But here's the 
definitive answer.

The Jap, Canadian and US models are as mentioned above. All fixed. The rest 
of the world gets a switch. You can select a bunch of voltages. At first 
the PSU looks a bit esoteric but it's totally straight forward. The rails 
are +15, -15 and +5. Separate analogue and digital grounds. The transformer 
therefore has 2  secondaries. In effect, 2 transformers. One feeds the 5 
volts (High current) and one feeds the +/-15. Not a whole hill of beans.

The voltage feeding the regulators have to be a minimum of 2 volts above 
the regulation voltage. That makes the rectified voltage +/-17 and +7. But! 
While it doesn't give actual secondary voltages, usually the secondary that 
feeds the 5 volt line needs to be 10 volts. You wouldn't want a drop-out on 
the digital supply now would you?

So, In effect you could replace this one transformer with 2 smaller ones if 
you can make them fit. There are two problems with this.

1) is that I have little idea what current would be required. However the 
fuses are 0.75amp on the 15 volt rails and 2 amps on the 5 volt. and...
2) I'm reading all this from a DX7 Mk 1 Manual. Doh!

If it's a DX7 II FD you've probably got an extra 12 volt supply in there 
for the floppy but other than that it's probably the same. But even here on 
this manual there are anomalies between the schematic and the board layout.

Essentially though you need two windings because you need to keep the 
digital and analogue grounds separate.

Another thing you COULD do if you REALLY felt the urge is to supply the 5 
volts from a small switch mode PSU. However don't get tempted to run the 
analogue supply from +/-12 volts. These things need the full 15 because of 
the weird way they've done the audio outputs. I guess they thought they 
were clever at the time but when we look at stuff today we tend to shudder 
at this stuff. Then you hear people saying things like. "Jeez! No wonder 
they're so noisy!"

There is a president for this approach by the way. I have a TX7, the weird 
sloping semi-table top version of the DX. It had a switch mode in it 
originally. Custom built for the unit of course. However one day while I 
was sitting here happly ignoring it, the bloody thing decided to burst into 
flames. (I'm not joking) The PSU was toast. I actually found out what was 
wrong with it but I figured I'd be adventurous. I was building up a 
computer system at the time, there was some spare room in the case. it was 
a 19" rack box. Perfect! I mounted a small +/-15 volt linear supply in the 
box somewhere and then derived it's 5 volt supply direct from the 
computer's PSU. (This is how I know +/-12 just doesn't cut it. Computer 
comes on, TX7 comes on. Easy. And is till working perfectly to this day. 
(Knock on wood.)

So now you have a ship-load of options. If you can follow the deranged 
rantings of a former DX7 owner armed with a DX7 service manual. Personally 
I'd go the Step down route. It saves a lot of logistics. But if you're 
adventurous you might like to look at those other alternatives.

Hope this helps.

Be absolutely icebox.

  _ __        _       ____Happiness is a warm penguin____
| "_ \      | |
| |_)/  __ _| |_ ____       ALL ELECTRIC KITCHEN
|  _ \ / _` | __|___ |  Geek music by geeks for geeks
| |_) | (_| | |_  / /
|_,__/ \__,_|\__|/ /    Bullshit --> http://all-electric.com
                 / ,__   Music -----> http://mp3.com/electrickitchen
Goodfortune    |_____|