A DX7 Mystery

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A DX7 Mystery

Post by HappyFunTimes » Sun May 10, 2015 6:32 am

First, let me explain my motivation. My brother died in 2013, and to remember him I started learning to play synthesizers since he loved all forms of electronic music. I began with some cheap casios and yamahas, and then learned subtractive synthesis on a microkorg. Once I was a little better I decided to get a dx7 because my brother was born in 1983, and the 80s sounds were some he truly loved. I read that the dx7 was notoriously difficult to program, but I welcomed the challenge.

The first few months were not easy. In the same time frame, I learned basic music theory, and could improvise well enough on a piano enough to entertain non musicians. However the dx7 did not sound well when I played it. I started to have second thoughts about whether my time and energy directed at this machine would ever pay off. Or, maybe I was just not a musician. I stuck with it, and was finally beginning to witness the power at my hands. Yesterday as I was playing, it seemed the dx7 was in fact the time machine I had been looking for. I could do almost anything I wanted with it, and it all sounded like I was back in the good old days. Then I hear this "pop" sound, the display disappears, and silence.

f**k MY LIFE!

I switch off the power, and then back on, nothing. I opened the top end expecting, (hoping) to find a blown fuse, or something physically obvious that I could just replace. Everything looks normal. I know little about electronics. I used to refurbish computers, but all we did was basic troubleshooting. If we found a bad capacitor on a board we just swapped the board out. I could identify a capacitor or resistor, but I don't know what they do, and have never soldered anything.

I know a dx7 is relatively cheap and easy to come upon, but I am in no position to be buying a replacement. I never would have bought one if I hadn't been able to talk the seller down to $180 from $275, and even then it was an extravagance. So, either I fix it, or wait a good 6 months or more before I can buy another. In the meantime I'm starting to feel depressed. I hadn't realized how much happiness making music could bring. Being without that sucks. So please VSE community HELP ME fix this if it's possible.

A little more background on the specifics of my dx7, and the circumstances around my crash landing. I bought it from an old woman in the middle of nowhere who never really used it much. Cosmetically it's in near mint condition. I tested the battery, and it was well within range, but I don't think that has much bearing on the rest of the machine. The inside is clean, there are no lose screws or other bits to cause a short.

When it stopped working I was using the auto accompaniment of a Yamaha PSS480 to trigger the DX, and using the mod and pitch wheels. I started switching the MIDI channels on the PSS and switched on all of the accompaniment section instead of just bass, or rhythm. I thought perhaps the CPU may have been overwhelmed by the midi switching.

I don't have a multimeter to test the power, but I believe the power supply is ok. When I flip the power switch the display does not come up, nor do I get any sound from the keys, but there is the familiar noise in the headphones! If I flip the switch off, no noise. Also, the "pop" sound I heard when it went down was the same I heard every time I turned it on. Basically the speakers turning on. Other than this noise I have no indications to work with. I tried to enter the diagnostic routines via holding Function, 16, and 32. Nothing. I'm at a dead end with my troubleshooting.

Any and all help is appreciated. I need help diagnosing the issue, and with the actual repair. Even a nudge in the direction of where I might have luck finding more help would be useful. Thanks.
Last edited by HappyFunTimes on Sun Aug 09, 2015 12:59 am, edited 1 time in total.

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Re: A DX7 Mystery

Post by synthroom » Sun May 10, 2015 5:36 pm

It sounds like a power supply issue. Are you somewhere you can take for service to get it looked at?
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Re: A DX7 Mystery

Post by HappyFunTimes » Sun May 10, 2015 7:49 pm

I imagine there is a place here in Seattle, but I'm trying to keep the cost of this down. From what I've heard synth repair is usually pretty expensive if you have to pay for it.

I was thinking I might rule out a power supply problem since it is producing noise. Can something produce noise if it's power supply is damaged, but not completely inoperative? My first thought was a fuse had blown on the power supply, but it looked ok to my untrained eye.

I remember it used to sound like an old CRT tv when it was powering up.

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Re: A DX7 Mystery

Post by db0451 » Sun May 10, 2015 7:59 pm

I was thinking I might rule out a power supply problem since it is producing noise. Can something produce noise if it's power supply is damaged, but not completely inoperative? My first thought was a fuse had blown on the power supply, but it looked ok to my untrained eye.
of course, if the audio circuits run on separate secondary supplies compared to the CPU and other digital logic.

and the DX7 is a textbook example: it puts out +/-15 V for audio and other analogue circuits, +5 V for digital ICs and the screen and so on.

These separate secondary outputs most likely have their own fuses, voltage regulators, rectifiers, etc. – all things that can fail and knock out entire swathes of the circuit, without necessarily leaving any simplistic visual signs.

This may not be the exact cause in your situation, but it is a very real possibility in general.

The first thing to do is to get a voltmeter and probe the voltages across the secondary outputs of the PSU + the corresponding inputs of important components like the main CPU, memory and sound chips, display, etc. Simply looking for visually blown fuses or whatever is unlikely to be useful.

Good luck. It’s unlikely that any proprietary/near-irreplaceable IC has been fried; usually, the PSU shoulders the damage way before that can happen. Most likely, with some proper diagnostic techniques and maybe some inexpensive parts, it can be repaired.

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Re: A DX7 Mystery

Post by HappyFunTimes » Sun May 10, 2015 8:55 pm

db0451 wrote:of course, if the audio circuits run on separate secondary supplies compared to the CPU and other digital logic.

and the DX7 is a textbook example: it puts out +/-15 V for audio and other analogue circuits, +5 V for digital ICs and the screen and so on.

These separate secondary outputs most likely have their own fuses, voltage regulators, rectifiers, etc. – all things that can fail and knock out entire swathes of the circuit, without necessarily leaving any simplistic visual signs.

This may not be the exact cause in your situation, but it is a very real possibility in general.

The first thing to do is to get a voltmeter and probe the voltages across the secondary outputs of the PSU + the corresponding inputs of important components like the main CPU, memory and sound chips, display, etc. Simply looking for visually blown fuses or whatever is unlikely to be useful.

Good luck. It’s unlikely that any proprietary/near-irreplaceable IC has been fried; usually, the PSU shoulders the damage way before that can happen. Most likely, with some proper diagnostic techniques and maybe some inexpensive parts, it can be repaired.
This makes a lot of sense. Especially why there is audio noise, but no other signs of power. There is a secondary output with fuses, and what not. Visual inspection was fruitless. I will pick up a volt meter later today. Is that the same thing as a multi meter? I will probably go for the cheapest model unless there is a real difference in performance. I read somewhere that you have to be careful when probing around inside to avoid damaging anything. Any suggestions on basic precautions? Thanks.

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Re: A DX7 Mystery

Post by db0451 » Sun May 10, 2015 11:27 pm

Yes, a multimeter generally has several functions, and voltmeter is usually one of them.

precautions: a properly constructed meter for voltage should be protected against shorting the subject, if you accidentally bridge two antagonistic pins – but it remains wise to make careful choices at all times, of course! If you use a meter in current-measuring mode (which should not be needed here), there is much less ability for the meter to protect against shorts. and of course, accidental slippage of the probes into/between bits of metal where they shouldn’t be… is something definitely to avoid. (I’ve never blown anything up, but I’ve gotten close!)

Some of this is from a good post here – http://electronics.stackexchange.com/qu ... multimeter – and rather than quoting it all again, I’ll just emphasise this:
Also consider personal safety if you're working at high voltages and make sure any probes you use have an appropriate voltage rating and reduce the chance of your hands slipping onto dangerous voltages.
This probably makes it sound a lot more dangerous/scary than it really is, 99.99% of the time! :D but it is always worth being as safe as possible.

Anyway, speaking of knowing what to probe: grab the schematic – http://yates.ca/dx7/Schematics%20&%20PC ... 0Tiled.pdf – which should provide all info you’ll need about what’s what. As nothing seems to be happening, it points at a fairly early absent function such as the CPU, RAM, ROM, etc. So, again, the best things to check are the PSU and whether it is reaching important ICs, based on the power output/input pins.

I’m completely self-taught on all of this, so I can’t claim to be an authority, but I hope it helps anyway.

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Re: A DX7 Mystery

Post by Peopleunderthesun » Mon May 11, 2015 4:32 am

Pm me if you haven't already isolated the problem. Im in Tacoma, Im inexpensive and I fix synthesizers all day every day :dancer:

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Re: A DX7 Mystery

Post by HappyFunTimes » Tue May 12, 2015 3:26 am

db0451 wrote:Yes, a multimeter generally has several functions, and voltmeter is usually one of them.

precautions: a properly constructed meter for voltage should be protected against shorting the subject, if you accidentally bridge two antagonistic pins – but it remains wise to make careful choices at all times, of course! If you use a meter in current-measuring mode (which should not be needed here), there is much less ability for the meter to protect against shorts. and of course, accidental slippage of the probes into/between bits of metal where they shouldn’t be… is something definitely to avoid. (I’ve never blown anything up, but I’ve gotten close!)

Some of this is from a good post here – http://electronics.stackexchange.com/qu ... multimeter – and rather than quoting it all again, I’ll just emphasise this:
Also consider personal safety if you're working at high voltages and make sure any probes you use have an appropriate voltage rating and reduce the chance of your hands slipping onto dangerous voltages.
This probably makes it sound a lot more dangerous/scary than it really is, 99.99% of the time! :D but it is always worth being as safe as possible.

Anyway, speaking of knowing what to probe: grab the schematic – http://yates.ca/dx7/Schematics%20&%20PC ... 0Tiled.pdf – which should provide all info you’ll need about what’s what. As nothing seems to be happening, it points at a fairly early absent function such as the CPU, RAM, ROM, etc. So, again, the best things to check are the PSU and whether it is reaching important ICs, based on the power output/input pins.

I’m completely self-taught on all of this, so I can’t claim to be an authority, but I hope it helps anyway.
Thank you so much for all of this information. I have to wait for my multimeter to arrive in the mail since the ones at the hardware store were too expensive. But, that gives me some time to learn electrical symbols in order to interpret that schematic. I will update once I'm able to run the tests.

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Re: A DX7 Mystery

Post by rhino » Tue May 12, 2015 6:01 pm

Not insulting you, nor flaming you. Just offering my thoughts: If this synth has sentimental value to you, please don't practice on it for your first time. The DX7 is a complex digital computer, filled with sensitive microchips that can be killed with one slip of the probes. Find a shop that has a tech(s) that know this dinosaur. Yeah, it'll cost some, but chances are you woun't end with a brick.
When the wise man points to the stars, the fool looks at the finger.
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Re: A DX7 Mystery

Post by HappyFunTimes » Tue May 12, 2015 8:58 pm

I felt pretty confident there for awhile. Now I'm scared. I guess that's good. I will be proceed slower, and with more caution. I know an electrical engineer who can give me some help, but he won't be around for another month. If the problem isn't in the secondary power supply, I'm going to wait until he's around. Thanks for the advice.

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Re: A DX7 Mystery

Post by HappyFunTimes » Thu May 14, 2015 9:08 am

I found this on a similar music forum. :

Be careful with first generation MIDI instruments.

I hooked up my brand new Kurzweil MIDIBoard to my Memorymoog Plus years ago. I inadvertantly left polyAT on, which can generate a torrent of MIDI traffic. After some playing from the MIDIBoard, the Memorymoog locked up. After power cycle I discovered several Memorymoog patches had been corrupted. The dense MIDI traffic overran the cpu stack, the firmware didn't have a stack pointer watchdog, and the cpu put the stack data into memory where the patches were. Ouch!

-The Real MC

I'm concerned that my DX did something similar. I was sending midi messages normally, and all was fine. Then I started sending a lot, and it was lights out.

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Re: A DX7 Mystery

Post by db0451 » Thu May 14, 2015 7:40 pm

yes, but (A) your OP does not say you were sending MIDI into the DX7 from another device, and (B) anyway you have more than just corrupted RAM, otherwise it would still display the boot screen and etc.

I have never heard of a DX7 or any other synth by Yamaha crashing due to excessive MIDI traffic, and this is a non-sequitur if said traffic is internal only, and it would not be able to corrupt the core firmware and whatnot anyway.

I mean, maybe... but probably not.

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Re: A DX7 Mystery

Post by Jabberwalky » Fri May 15, 2015 2:28 pm

I'm with Rhino. There are so many chips inside that will die from simple static shocks. There are also a ton of power connecters, which can be easily installed incorrectly. It may have been all digital, but a simple computer chip it is not.

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Re: A DX7 Mystery

Post by HappyFunTimes » Sun Aug 09, 2015 12:59 am

Thanks to everyone for the help, especially db0451. The problem has been solved, and I will give a brief description for anyone else who may encounter the same situation.

The key in the diagnosis was the audio noise. Although there was no sound being produced, nor LED, or LCD display the speakers would make a pop when I flipped the power switch. As db had mentioned the DX7 has a secondary psu, and the +5V AC end controls all the digital circuitry whereas the DC end controls the audio circuits.

We used a multimeter to probe the AC side of the secondary psu. This showed all of the leads to be producing the correct voltage, so there was concern the problem was farther downstream. However, when any type of load was placed on it, (we used a resistor) the voltage would drop to nothing.

My friend determined that the LM7805 +5V voltage regulator was failing when placed under load. I bought one for 11 cents. We soldered it in, and the DX7 is working like a champ.

If anyone else has a similar problem I hope this is helpful. Message me, and I will try to answer any questions I can.

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Re: A DX7 Mystery

Post by db0451 » Mon Nov 30, 2015 12:04 pm

Just noticed this now - a bit late! Glad to hear you got it sorted and that I was able to contribute.

Though I am pretty sure the digital circuits really want +5 V DC. ;)

I have a similar issue with a PSU where one rail is reduced to ~1/10th of its rated output when loaded by one of the 3 boards. However, I replaced that vreg and a bunch of related capacitors, with no change, so I think it's the PCB that has the problem in this case. Be thankful you didn't have to hunt across an entire board to find an esoteric problem! ;)

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