Yamaha DX7 Battery replacement issue

pdman1 · Posted: Sun Mar 03, 2013 4:10 pm Post subject:

No not to ground.
Measure across D4. Put one probe on one end of D4 and the other probe on the other end of D4.
Then switch the probes to measure D4 again. What resistance do you measure?

Gothboy · Posted: Mon Mar 04, 2013 4:11 pm Post subject:

pdman1 · Posted: Wed Mar 06, 2013 4:58 pm Post subject:

You need to use the lowest ohms scale usually 200 or 400 ohms. Sounds like you used the 20 MegOhm scale. Measure again.

Gothboy · Posted: Thu Mar 07, 2013 10:49 am Post subject:

pdman1 · Posted: Thu Mar 07, 2013 4:48 pm Post subject:

Doesn't make sense. You don't get a voltage drop across D4 when measuring voltage. Does you meter have a diode setting? Use this to measure across the diode.

I would desolder what you did and look for open or shorted tracks, pads, etc. Then carefully install your battery holder. Such a simple procedure that is not making sense.

Gothboy · Posted: Tue Mar 19, 2013 8:38 am Post subject:

I'm at it again...removed the battery and cleaned up the area on the backside of the board where the battery goes. I'm attaching some photos of the front and back. You can clearly see where the holder goes and the + and - contacts on the front. My question is in regards to the area on the back where the + goes it's a strip on the right(Looks a bit like a landing strip) is the + supposed to be touching this or is it just for solder and the + tip is in the solder?

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pdman1 · Posted: Wed Mar 20, 2013 5:12 pm Post subject:

You need to clean up the backside of the PCB. Use 1t least 90% alcohol and an Xacto to scrape off all that gunk.
Also looking at the top component side battery minus terminal, you need to make sure you have continuity between the top pad and the bottom PCB pad. It looks like you may have destroyed the via or feed through of the Pad. This via connects the top pad to the bottom pad.

Gothboy · Posted: Sun Mar 24, 2013 10:52 am Post subject:

Cleaned the back of the board, went out and got new rosin core solder. Went at it again and did a re-solder. Still has the same problem so it's off to HiTech I'm afraid. It's apparently just beyond me at this point. Sad

pdman1 · Posted: Mon Mar 25, 2013 5:11 pm Post subject:

OK

as an option, I repair Synths here on the North Shore. I know Lawrence is closer but if they can't do anything let me know.

planetqwerty · Joined: Jan 17, 2016 Posts: 1 Location: UK

According to the YouTube clip titled "YAMAHA DX7 Battery Repair Tips #1" the battery should not be pressed hard against the board, otherwise it'll make contact with other connections. http://youtu.be/uGIaSzyLFb0?t=2m15s

The author suggests that, when soldering it to the board, you use a bit of card as a spacer. I guess it can be removed afterwards.

markrune · Joined: Oct 23, 2017 Posts: 3 Location: United Kingdom

OK I have been bashing my head against a brick wall on this one. After many years of use my DX7 started loosing its internal memories. Running the battery check it was showing it was at 4.6v , which is good for a 3o+ year old 3 volt battery. So looked like the D4 Diode had gone.

So I replaced with an IN4005 diode, which I was told is a replacement for the original ON95. So new battery holder and battery attached and I get 3.2v from battery to Diode, across diode I get 2.6v and from D4 to IC 19,20and 21 Pin 24 I also get 2.6v Smile

so all should be good.

Power on DX7 and get message Check Battery and pressing check function it is showing 0.7V so what is wrong. Do I need a different type of diode or has something else gone amiss ? Please help have been trying to resolve for three days now with no luck

Many thanks in advance if anyone can help

electrotech · Joined: Apr 24, 2013 Posts: 45 Location: Ayrshire Scotland

Hello Mark,

According to the circuit diagram, the original diode used in the DX7 is an OA95. This is a germanium signal diode and is used because it has a low forward voltage drop (~0.2V) compared to a silicon diode (~0.6V).
A 1N4005 is a silicon power rectifier and is not the ideal replacement...
Also, when soldering in germanium devices it's a good idea to use heat sinks on the leads as they are very easily damaged by heat from the soldering iron. (small croc-clips will do)
If you'd like some OA95s send me a PM and I'll send you some in the post.
I'm not that familiar with the DX7, but I would check the 'VB' line (battery voltage) is reaching the part of the circuit that reads the voltage as the in-built voltage check might be faulty.

Hope this helps.

Andy

markrune · Joined: Oct 23, 2017 Posts: 3 Location: United Kingdom

Andy, thanks for confirming my suspicions, I managed to come to the same conclusion that the Diode was the problem, actually it works as far as memory backup is concerned Smile

but when turning on the machine or running the power test, it is not showing the value that is actually being produced , when measured by a volt meter. I have located a couple of Germanium 0A35's on line, so will swap it out once arrived.

I guess for the last 25 years I was under the impression that the battery was better than it actually was, so wont hurt if for a week or so, it looks like there is an issue, when there is not.

This morning was the first morning for several weeks, that I turned on the DX7 and none of the internal memories were corrupted.

markrune · Joined: Oct 23, 2017 Posts: 3 Location: United Kingdom

OK so I have replaced diode with a germanium OA95 and no difference, all the voltages seem to be right

battery to cathode side of D4 = 3.2V

battery to anode of D4 =2.6v

anode D4 to pin 24 of ICs 19,20,21 = 2.6v

The synth seems to be holding memory but keeps displaying 0.7v on battery check and check battery on power up. Its bloody annoying, but the synth works fine. Also tried a Shoctkey Diode with exactly the same results.

any ideas or suggestions any one

dearyou666 · Joined: Aug 21, 2020 Posts: 1 Location: america

I'm having this issue so i'm going to unearth this post. I'm getting 5v from the internal diagnostic. Memory is corrupt if I leave it off overnight. I haven't taken a look at D4 yet but Markrune/Gothboy if you ever figured it out I'd love to know what happened.

grantb3 · Joined: Jul 31, 2009 Posts: 29 Location: USA

I'm also having that problem where the battery voltage reads too high, yet there is memory corruption. Not sure I have a replacement germanium that will work. The other possibility, though remote, is that the A/D and Sub CPU are getting the wrong value. One might need to look at that section. I don't see how bad RAM could cause an extra high voltage on the memory test, but I will check out all the avenues.