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Repair a DX7 that resets itself
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Repair a DX7 that resets itself
widdly
I bought a crusty old DX7 recently. It has a problem with restarting itself every so often. It mostly comes back fine after reset but sometimes freezes. The presets don't get scrambled when it restarts. Other than that everything works fine.

The backup battery appears fine. I thought the 5v PSU might be dodgy but the caps look OK. I replaced the 5v regulator.

Any ideas on what might be wrong or where to test?
EATyourGUITAR
just follow the power supply. see where it goes. lots of times you will have doughter power supplies to drive the screen, preset storage, audio out. you might have a voltage regulator or a charge pump that is acting up but its not located in an obvious spot where the power comes into the unit. if you have a scope with 4 probes, you can see where the voltage drop (or spike) is happening first. put two probes on the dc out of the power supply and put the other two probes on the lcd power. thats how I would do it. you could also just replace every electrolytic capacitor in the whole keyboard and your probably all set.
Low-Gain
i got a DX-100 that does that too..
haven't opened it up yet to search out the problem.
Doesn't help that the case looks like it was in a fire as it's partially melted. *laughs*
widdly
Thanks EATyourGuitar. There are a couple of places on the board where the supply is regulated down using opamp buffered voltage dividers. I've got access to a scope for a few days which should be helpful in tracking down faults.

I'm currently replacing all the caps. I started with the bypass caps because I had those on hand. That didn't seem to fix it. Next I'll do all the electrolytic caps.

I was reading this.. http://www.oldcrows.net/~oldcrow/synth/tips.txt The bit about CMOS failure is interesting. If new caps don't work, I'll replace the CMOS parts. I've found sources for all the chips on the board except the ROM image.

If anyone has a copy of the DX7 Mk1 SER7 special edition rom I'd be interested.

BTW. The DX7 is really nice to work on. Remove 5 screw and the whole top of the synth opens on hinges. Everything is so easy to get at. I wish more synths were built like this.
ringstone
widdly wrote:

BTW. The DX7 is really nice to work on. Remove 5 screw and the whole top of the synth opens on hinges. Everything is so easy to get at. I wish more synths were built like this.


Not just the DX7, the Yamaha CS- series were built like this as well (at least the CS-15 and CS-30 anyway!)... no stacked circuit boards, everything laid out neatly... very nice.

Blair
widdly
I've swapped out all the electrolytic capacitors and the DX has been working perfectly. screaming goo yo

I've got it hooked up to my TX816 and it sounds awesome. Nine DX voices panned and detuned and I'm feeling like Vangelis. SlayerBadger!
RF
Congrats! For future reference - it's almost ALWAYS the electrolytics.
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