Snagged a Free Yamaha DX-7 MkI

Yamaha DX-7 Synthesizer
Yamaha DX-7 Synthesizer

As I mentioned in my previous article about the Yamaha CS1x, I managed to acquire a used Free Yamaha DX-7 over the weekend in prestine condition. The main reason besides being a frequent customer of this particular used music store in Nagano, the Yamaha DX-7 was not functioning properly. The sales clerk couldn’t fix it so he thought it best to just get rid of it. What I particularly liked about it was the fact that it was in near mint condition. When I got home I opened it up and there wasn’t one scratch or dust bunny inside. It was virtually brand new and perhaps a bit strange to be in such great shape. Cosmetically on the outside it was in spit shined mint condition as well.

Currently I already have a working Yamaha DX-7 with the Grey Matter expansion board that I installed in it. The only problem is that cosmetically the DX-7 is really in bad shape being all dinged up and having some rust in some places. I thought at the very least I could transfer the insides of the working DX-7 to one that looked brand new I got over the weekend. I could then strip it for parts and use them when needed or simply sell the rest. The Yamaha DX-7 also came with the original case as shown in the photo which was cool.

So what it wrong with the new Yamaha DX-7? I am not sure yet. The problem is that when you turn it on, it becomes stuck with the “YAMAHA DX7 SYNTHESIZER” message in the display. Sometimes it says to insert the cartridge or that the cartridge protect is on. There is also no sound and the buttons do not respond at all. The battery in the DX-7 was recently replaced and I replaced it again just to make sure it was new. Later this week when I start transferring over the boards from my other DX-7 I plan to try and isolate the problem. At this point I am not sure what it is, but I’m sure I’ll eventually find the problem. If not, I hope to put together a near mint working Yamaha DX-7 and will likely part out the other one. We’ll see. I’ll update this article as I progress and will certainly post the problem and solution once identified. Stay tuned!

7 thoughts on “Snagged a Free Yamaha DX-7 MkI

  1. rich

    Check the power supplies for AC ripple. The voltage on DC would be 5VDC, and + and – 15VDC. Then switch the meter to AC and repeat. Acceptable amount of power supply ripple is 5 mV RMS. Anything like 50 mV RMS or more is a problem.

  2. Update #1

    I pulled out my other “fully functional” DX-7 and removed the main CPU board. I then put that working board into the newer DX-7 that is having the trouble. Presto! The newer DX-7 worked perfectly! I then took the non-working main board from the newer DX-7 and placed it into the older “working” DX-7 that I had. The problem was the same. Thus I can officially confirm that the issue lies somewhere on the main cpu board. It’s not the power supply, outputs, or any other aspect of the DX-7 I acquired last week.

    So what could be the problem be? I’ll be working on that this week hopefully if time permits. Off the top of my head it could be something with the battery OR some other issue with cracks or cold solder joints. I’m not sure how to diagnose this quite yet, but at least I know the problem is somewhere on that CPU board.

    Perhaps it could be the chips? Maybe a broken trace, or even a broken pin somewhere that isn’t getting any juice. Or perhaps that battery solder job both the guy before me and the solder job I did screwed things up. Somebody definitely tampered with the board before me because a battery holder was installed and a wire clamp fastened to the CPU board on the right side was removed. My other DX-7 is totally stock so I can see the difference. That actually may help in finding the problem having two boards to compare. We’ll see.

  3. Update #2

    I am getting closer to the problem. On my first DX-7 I had installed the optional Grey Matter expansion board. In order to do this, you have to pull the original IC 14 and IC 21 chips out of the DX-7 circuit board. I remember I had these two chips saved so I figured I would swap the IC chips to see if the issue might be related to corrupt Ram. I inserted at first both the spare IC 14 and IC 21 chips. Bingo! I now was able to get my functions and MIDI back. In fact everything worked beautifully with the exception of three problems.

    1. Output was erratic at best and not working right. Some keys would work and others wouldn’t. The output levels were faint most of the time, but noticeable enough to know that patches were different. ( Keep in mind ALL of the other boards in the DX-7 work 100%. The issue lies entirely on the main circuit board ).

    2. The tail ends of the patch names were garbled. This is different than the conventional garbled messages you get with a bad battery. The names were intact but just the last few character spaces were funky. I also tried a working RAM cartridge and it show those patches with a similar garbled ending so something was not quite right there. ( Midi transfers now work though along with all other brown function keys ).

    3. I ran the test procedure and got stuck right away after pushing “Yes” with an error message which said “V1.8 Error Ram IC”.

    So, I removed IC Chip 21 and replaced it with the previous one so that the identities would match. The IC 21 chip from my other DX-7 had a different ID. I then powered back up and the DX-7 continued to work with all functions and MIDI. This in turn led me to believe that the Ram Error was likely coming from the IC 14 chip. I then removed the IC 14 chip again and replaced it with the one previous. Bingo again. The DX-7 went back to it’s non-functioning troubled state when I first got it. That first IC 14 chip I now know doesn’t work and is corrupt.

    It gets confusing here, but I know for a fact that my first DX-7 spare IC 14 and IC 21 chips work perfectly. When they are inserted I get the Error Ram IC. This “could” be a problem with IC 20 down below which hasn’t been replaced and has a different ID then IC 21. For example

    First DX-7 Working Chips
    IC 14 = Toshiba TMM24148AP
    IC 21 = M5M5118P – 15L 526104
    IC 20 = Under my Grey Matter Board, can’t see but likely the same as IC 21

    Second DX-7 Chips
    IC 14 = Yamaha IG11464 D63-5G1 ( Not working or Severely Corrupt )
    IC 21 = M5M5118P – 15L 531112 Working
    IC 20 = M5M5118P – 15L 531112 Not tested

    Thus you can see there are some IC chip differences and depending on which ones are used, I get life back in the DX-7.

    Another possibility is that I still might have a battery issue. Not sure. I can now get a battery reading which is consistently showing 4.5V. Also when I power off the DX-7 and power back on, the voices are not retained. They need to be reloaded which is generally a sign of a bad or low battery.

    With that said, I need to find a way to eliminate the “Error Ram IC” message when starting up the test phase. My thinking is now pointing towards IC 20. As I mentioned I can confirm that my spare IC 14 and IC 21 chips work in the DX-7 and most is well, but IC 20 could still be corrupt. I suppose I could disassemble the Grey Matter Board and access my other IC 21 from my there and then I know I would have three working IC chips to replace on the newer DX-7. That way I could 100% confirm things with regards to the IC chips. We’ll see.

    So far though I would say my problem is definitely chip related sprinkled with a possible added battery issue. Since one IC 14 chip works and another doesn’t I can completely rule out that the battery alone is the problem. It has to be the chips in my book and hopefully I’ll be able to confirm that.


  4. Finally fixed the DX-7!! The IC chips were bad, particularly IC 20. I replaced them and now the DX-7 works and sounds excellent. Having another DX-7 helped in diagnosing the problem. I simply removed the chips from that board and testing them on the new one. They all worked, so I just had to get some new chips which I found on Ebay from another board that were being parted out. Thanks everyone!

      1. You can find replacement chips sometimes on Ebay. I actually saw a set last week on there. In my case I found a used DX-7 mother board that someone was selling on Ebay and bought that for a great price.

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