Hello syntaur forum, this is my first post.
I am having trouble with my dx7…
I turned it on after a few months and it worked fine. I turned it off and a few hours later it wouldn’t turn on anymore. I opened it and the fuse is ok. Internal Battery is also ok. anyone have any ideas on what else to check? Or does anyone have a power supply circuit For sale? Thanks!
My DX7IID had the same issue. It turned out it was the larger, main capacitors in the power supply had gone bad. I replaced those and BINGO! Had it working in no time.
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How did you figure this out, I have a DVOM, but I’m not very good with component testing/soldering.
I did disconnect the board from the rest of the synth before checking diodes…
As long as the power is disconnected, you can try measuring the resistance across the main caps. If you get a low reading, they are most likely shot. Keep in mind that because they are still in-circuit, this isn’t a foolproof method. If I recall, they were a pretty common cap, so replacing them “just in case” wouldn’t hurt.
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One thing to always look for on electrolytic* capacitors, beginning with the big ones in a power supply board is the tops of the cylinders which should be flat, as when new. As they age they can start to swell and the round tops eventually bulge out which is a sure sign they are bad and need replacing. As they are all initially about the same age when installed, if you find one bulging, it would be common to find more than one bad and bulging. Also if any have gone extreme and started leaking, then definitely remove them and carefully clean the circuit board before installing new one(s). *Electrolytic caps are polarized having both a positive and negative lead which are clearly marked on the component casing, so always observe the proper orientation before removing one and when installing new ones. Ideally the pcb should have a + icon on one of the 2 holes to indicate where the positive lead goes. That may be either silkscreened on the component side or etched on the opposite circuit traces side which may be hidden until you remove the board for unsoldering and resoldering components. But with age and handling it’s possible that silkscreened lines can get scratched off.
BTW, I like to use a fine tip indelible marker to make helpful larger marks on a chassis or PCB to aid me during reinstalling and reassembly, especially if original printing is very tiny or for a small “minnow” easily lost in a large school of swimming markings and lines all over the area of focus. Taking snapshot pics for your own reference is a very helpful practice too. In the bad ol’ days before digital photography we had to scribble our own diagrams as we worked.
The filter caps on a power supply if not removed from the circuit are going to be loaded through the rectifier diodes to the transformer secondary side coil(s), which would have some resistance, yes? I’m not certain what a proper resistance for a good capacitor should be in circuit, do you know? 
You are right, the rest of the circuit will obviously impact what resistance measurement you get. In theory, a perfect capacitor should appear as an open circuit (infinite resistance), but that can’t possibly happen in reality. But the resistance should NOT be really low. Those caps aren’t the most expensive ones on the planet, so if you’re feeling brave, you could simply replace them to start with. Just make sure the power has been disconnected for a while before you go poking around in there. It’s been a couple of decades since I had to work on mine, so I’m sorry if my memory of it is a little vague.
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