DX7 II FD collapses to white noise on boot

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Burfix
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Re: DX7 II FD collapses to white noise on boot

Unread post by Burfix » Fri Jul 05, 2019 4:45 pm

I just asked my wife to turn it on whilst I held down the edit+16+32 button to see if I could enter the test mode at all. The welcome screen showed, then the message "Change Battery" then normal play mode was displayed (even though the patch names were scrambled). None of the buttons nor keys were responsive however. Powering up a 2nd time (today) a few seconds later and the display was garbled along with the white noise as in other times.
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Re: DX7 II FD collapses to white noise on boot

Unread post by EB5AGV » Sat Jul 06, 2019 7:27 am

Burfix wrote:
Fri Jul 05, 2019 3:41 pm
Question 1: To hold the IC's in a reset state, generally do you ground the reset pin or take to 5V?
Usually, RESET is LOW state (GND). I use a low value resistor (around 100 Ohm), connected to GND, to apply it to the RESET pin of the ICs, so the current is limited.

I wish you luck with this repair, it seems not really a simple one (I fix complex electronics for a living)

Please, keep us posted.

Jose
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Re: DX7 II FD collapses to white noise on boot

Unread post by Miks » Sun Jul 07, 2019 11:12 am

Burfix wrote:
Fri Jul 05, 2019 3:41 pm
Question 2: Are IC 15 and IC 16 factory loaded with program or do they get it from the ROM on startup?
Hi Nigel, IC16 (HD63B03YP - CMOS 8-bit single chip microprocessor) only has 256 bytes of RAM, no ROM aera (afaik).

IC15 (HD6805S1P - CMOS 8-bit single chip microprocessor) has 64 bytes of RAM and a 1100 byte user ROM aera. But I have no idea if the ROM area contains pre(mask)-programmed data or if it's left empty 8O
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Re: DX7 II FD collapses to white noise on boot

Unread post by Burfix » Sun Jul 21, 2019 9:04 am

Sorry for the long delay in updates: still waiting for my Hantek Oscilloscope to arrive from HK.
But in the meantime I bought a IC16 (HD63B03YP) but did't get it in yet.
I also ordered a handful of IC sockets to solder in to make the future easier.
I found a logic analyser and managed to get the PC to listen to it. I checked the logic lines between ICs 32, 34 and 37. The inputs to IC34 (YM2604) from IC 32 (YM3609) seem to be all logic 1 which I think means nothing is being sent to it. However, some changes in logic are happening on the DAC inputs to IC 37 from IC34. So my guess is that the YM2604 is putting junk on the lines. This would explain the noise output. I could say that maybe the YM2604 is broken as its powdery appearance suggests. And maybe I should start looking for some surplus stock somewhere - along with a couple of Unicorns and a Dodo!

But, that doesn't explain the display problems which indicate the machine is not getting past boot. So I hope the oscilloscope will tell me there are real power spikes somewhere.
In the meantime I bought a Tascam 38 - 8 track reel-reel recorder. A nice bit of kit but one channel isn't recording.

Never a dull day in the man-cave!

Happy Sunday everyone.

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Re: DX7 II FD collapses to white noise on boot

Unread post by Burfix » Wed Jul 24, 2019 9:33 pm

Hi there,

I’ve been checking out the reset cct. Can anyone take a look and let me know if anything looks abnormal please?
The upper numbers are voltages when the power is off and the lower ones are when powered on. So I think the battery side part is working ok. I think the voltages on pins 3 and 4 on IC 20 look wrong. This was done with a dc multimeter so I’ve no idea if they are oscillating. But my guess is they should be either near to zero or 5v. I did this check after seeing these mid voltages on the cpu reset pin. Hmmmmm! Thanks. N
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Re: DX7 II FD collapses to white noise on boot

Unread post by Burfix » Sat Jul 27, 2019 3:07 pm

...a better photo.... :roll:
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Re: DX7 II FD collapses to white noise on boot

Unread post by Burfix » Sat Jul 27, 2019 10:36 pm

Tonight measured the resistance to ground on pin 4 of IC 20. It was around 200 Ohms. I figured this is what was pulling the voltage so low as the resistance to ground from pin 10 (doing the same job) in IC 20 was almost 2 kOhm. I desoldered IC20 and checked the resistance from pin 4's hole to ground and it was the same (round 200 Ohms). So now I'm going to track back to IC16 - Pin 6, IC28 - Pin 28 and IC41 - Pin 11 as these all take the /RES signal from IC 20. Either its them or there is a short on the tracks. Will try to do the simplest mod-checks first :-).

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Re: DX7 II FD collapses to white noise on boot

Unread post by Burfix » Sat Aug 17, 2019 7:13 am

I’ve removed all ICs that take the reset signal from pin 4 of IC20, apart from the YM3609. If it is this that has the is pulling the reset voltage low then I’m screwed. I’ve used my contacts in the electronics industry to try and find one but it seems that they have all gone. I did manage to buy a YM2604 from HongKong. It looks genuine but still could be a fake. I didn’t try it yet and probably won’t need to.
Today I will pull the YM3609 and see if the resistance on the reset line increases. Still have a dim hope that it might be a shorted track problem. I’ve refitted the ICs with sockets and so far, I didn’t make anything worse. I also got a new IC20 but it made no difference. The Hantek oscilloscope that arrived showed me that the supply voltage is stable and that all the oscillator circuits are working ok. Happy Saturday!
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Re: DX7 II FD collapses to white noise on boot

Unread post by EB5AGV » Sat Aug 17, 2019 8:49 am

You are doing a very good job on this!

As an electronic repairman for decades, I can tell you I have found all kind of uncommon problems. Don't take for granted that the PCB itself is good. I have seen cases of internally shorted traces (on multi-layer PCBs) and also different leakage resistances even on two-layer ones. So, in case you don't find the cause of the pull-down resistance on an specific IC, you could try to isolate the traces which go to those IC pins. Sometimes you need to cut them (do it on a place which can be easily redone). The idea is to isolate the fault on one of the two trace splits and continue until you find the faulty area. Then you need to isolate the leaky trace, both sides, and replace it with external wiring.

I hope this explanation makes some sense to you :wink:

Jose
Jose Gavila

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Re: DX7 II FD collapses to white noise on boot

Unread post by Burfix » Sat Aug 17, 2019 9:12 am

Jose,

Thanks for your inspiring words. Honestly, I'm a bit scared about disturbing the YM3609 by removing, especially if it could still be OK internally. So I think I'll only cut some tracks as you suggest. Thanks for the hint about re-wiring to bridge the cuts.
Regards

Nigel
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Re: DX7 II FD collapses to white noise on boot

Unread post by Burfix » Sat Aug 17, 2019 9:21 am

Just a note on the use of de-soldering tools.

I bought a cheap electrically operated solder vacuum tool. I thought it would provide a constant vacuum flow but instead it was basically a plunger type but actuated with a solenoid. So you get the pin hot and then press a button to energise the solenoid to suck off the solder. When you release the button, a spring takes over and blows the air and some of the molten solder back out.

The first time I used this was to remove IC 15. I was making good progress along the top row of pins and when finished, I looked down at the bottom of the board and noticed that I had sprayed a large area with solder. It looked like one of my kids had attacked it with some Christmas glitter spray. Some gentle wire brushing and use of IPA removed it all but it was a good lesson and didn't I feel stupid for a while! :lol: :roll:
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Re: DX7 II FD collapses to white noise on boot

Unread post by Burfix » Sun Aug 18, 2019 6:17 pm

Hello.

I’ve cut some tracks and externally rewired the reset cct. Now it runs with just under 5v and the resistance to ground is about 60k. This is similar to the other output of IC20 on pin 10. So good progress I think. It doesn’t crash now and the display is stable. I get the “change battery” message every time on power up and after starting, There is no response from any of the buttons. Can’t even enter system reset mode. I disconnected the battery and grounded the +ve connection on the board until zero volts was measured. But this didn’t change anything. There is no white noise like before so I think the chips are being reset properly now.
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Re: DX7 II FD collapses to white noise on boot

Unread post by Burfix » Tue Aug 20, 2019 1:13 pm

The buttons are unresponsive and I’m getting 0.8 v on the middle 5 pins of the connector feeding IC 10 (cn7) So I think there maybe another short. The key Bed cct checks out ok by itself. I’m thinking about pulling IC 10 and rewiring it externally to check it.
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Re: DX7 II FD collapses to white noise on boot

Unread post by EB5AGV » Tue Aug 20, 2019 3:38 pm

Burfix wrote:
Tue Aug 20, 2019 1:13 pm
The buttons are unresponsive and I’m getting 0.8 v on the middle 5 pins of the connector feeding IC 10 (cn7) So I think there maybe another short. The key Bed cct checks out ok by itself. I’m thinking about pulling IC 10 and rewiring it externally to check it.
Have you checked capacitors for shorts?. It may be tricky to find, but sometimes electrolytics and even ceramics short or get to a very low resistance faulty condition.

Jose
Jose Gavila

Yamaha DX7, TX81Z, DX11, 5x SY77, TG77, SPX990, SY85, A3000, AN1x, EX5, EX5R, EX7, MOTIF RACK XS, DGX660, Genos (and 1992' FJ1200 :mrgreen: )

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Re: DX7 II FD collapses to white noise on boot

Unread post by Burfix » Sat Aug 24, 2019 12:28 pm

Hello,

Thanks again Jose. I'd already been around and changed most of the discs and electrolytic caps. I had a good hard stare at some of my soldering of them last night and thought it looked like my old soldering iron had not been getting hot enough. So I used my new adjustable one to re-flow all my replacements. I also did a few more 22nf caps and the poly-film (yellow) one.

This resulted in the KBD working but with very low volume. I changed the ROM for my new 1.9 one from EBay and this allowed me to do an Edit+16+32 reset.

I was then able to run patches from the only cartridge I have.

Everything looked and sounded ok apart from all B keys would not play. Referring to the schematic: All B keys are connected to pin 13 of IC 1. I reflowed this IC again but no change.

So I removed both of these decoders (ICs 1 and 4) to check the board for shorts and other mischief with the ICs not there. All seemed normal.

I soldered in some sockets and replaced the ICs. Same behaviour as before.

I swapped ICs 1 and 4 (both TC40H138) and the problem moved to the D# keys. The D# keys correspond to pin 13 on IC 4.
This tells me that the TC40H138 is at fault as the problem moved with the chip. So I have replacements on order.

Starting to feel more positive. Just have to wait a week for components :roll:

Regards

Nigel
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