Yamaha DX7IID Low output, no display

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valveandsound
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Yamaha DX7IID Low output, no display

Unread post by valveandsound » Tue Oct 01, 2019 4:00 pm

Greetings everyone!

I recently acquired a Yamaha DX7IID that has quite literally sat in a road case for the last 25 years. I pulled it out of the case and turned it on, to find that the backlight on the display did not work and the LCD text was very faint. Output was very very low from both outputs, as well as the headphone out.

I measured the backup battery and got 2.9 volts. In an effort of preventative maintenance, I replaced the battery and installed a battery holder. New battery reads 3.4 vDC with no power applied to the unit. I re-seated all molex and ribbon connectors on the pcbs, as well as replaced all the electrolytic capacitors in the switched mode supply and adjusted the PSU trimmer so that the unit puts out the correct 5vDC output.

When i booted up the synth next, all i got was faint black squares on the lcd and "88" on the 7 segment display. This led (no pun intended) me to believe that the LCD display or main processor has failed to initialize. I went through and confirmed all molex connections are properly installed and seated, no change.

Now, when i boot up the device, i get no text on the LCD screen at all and the 7 segment displays read "88".

So i scoped the address and data lines of the main CPU (HD63B03YP) after boot up. I get pulses from the address lines that look clean. I also get pulses from the data lines, however they do not seem like clean 5v logic pulses. They look, in a word, "craggly". not clean square wave pulses. I also scoped the pin inputs to the LCD screen (most of these are also data lines coincidentally) and i do get signal to the LCD.

My guess at this point is the LCD has gone bad. I went ahead and ordered a 40x2 OLED display that is compatible with the LCD driver IC.

I do not know why the unit won't boot up at this point, as I have exhausted what knowledge I have of digital stuff and micro controllers.

Does anyone know how to confirm the RAM and ROM ICs are good? Any thoughts as to what my next step should be? I think you all for your time and knowledge.
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valveandsound
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Re: Yamaha DX7IID Low output, no display

Unread post by valveandsound » Sun Oct 06, 2019 12:46 am

Ok, so update:

New LCD is installed, when i boot up the unit it is alive! It looks really cool with the blue LCD...

However at this point i still get no output. I did have the factory reset cartridge and loaded the factory presets into the unit.

I can cycle through them and they are all clearly the factory patches (Clavecin, Smoohbass, PipeOrgan, etc) but no output from the synth. I did check performance mode and made sure the volume was up. Where do i check next on these things? Guess I'll poke around the YA3 board for input.

Also, here is the other weird thing. Whenever I select the 1-32/33-64 button, or the SPLIT button, the backlight for the LCD turns off. Whenever i back out of those menus and go back to the SINGLE mode, the LCD comes back on. It's like the LCD backlight on/off is tied to certain buttons! Has anyone ever seen this or have a remedy for it? Thanks.
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Miks
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Re: Yamaha DX7IID Low output, no display

Unread post by Miks » Sun Oct 06, 2019 10:49 am

Hi valveandsound,

welcome to the forum (Y) - and sorry for the late reply :whistle:
valveandsound wrote:
Sun Oct 06, 2019 12:46 am
New LCD is installed, when i boot up the unit it is alive! It looks really cool with the blue LCD...
Yeah - that's why mine got a 'Cool Blue' too :D
However at this point i still get no output. I did have the factory reset cartridge and loaded the factory presets into the unit.

I can cycle through them and they are all clearly the factory patches (Clavecin, Smoohbass, PipeOrgan, etc) but no output from the synth. I did check performance mode and made sure the volume was up. Where do i check next on these things? Guess I'll poke around the YA3 board for input.
Have you checked if the headphone jack is 'dead' too? If that's the case it seems to be a kind of general problem. But first of all I recommend to check the solderings of ALL jacks (including MIDI) to see if some of them may have wiggled loose over time due being often used! The solder may have cracked and so lost contact... On my DX7II/D I've resoldered ALL the jacks to make sure they'll have proper contact from now on. If that doesn't help make sure that the volume slider is working well...

See if the muting circuitry around the transistors T3, T4, T5, T6 & T7 works as it should - it prevents the outputs from giving a loud 'Plopp' sound when turning on the synth. The impulse for that muting circuitry is generated by the system reset part of the synth's circuitry. You DO have the schematic, haven't you...?
Also, here is the other weird thing. Whenever I select the 1-32/33-64 button, or the SPLIT button, the backlight for the LCD turns off. Whenever i back out of those menus and go back to the SINGLE mode, the LCD comes back on. It's like the LCD backlight on/off is tied to certain buttons! Has anyone ever seen this or have a remedy for it?
Hmmm that's strange! 8O I never came across such a behaviour. :? Have you checked the psu's voltages to see if their values are within their specs or if they're stressed by ripple? :think:
My (key related) gear (in alphabetical order):
Ferrofish: B4000+
Kawai: K1 II
Korg: M1 (up'd to EX, w/ 'Cool Blue'), M1REX (w/ 'Cool Blue'), Poly-800
Roland: D-50, D-110, D-550, PG-1000
Yamaha: AW1600, DX7IID (w/ 'Cool Blue'), EX5R, RM50, RX5, SY99 (w/ 'Cool Blue'), TG500,
TX-802, YMM2
Sector101: SYEMB05 (5x), SYEMB06 (3x), EXFLM2 (1kit), MCD Sweet16 (1x), DataBlade32 (1x),
WaveBlade 8MB Card (1x) & 1x Programmer Unit for WaveBlade
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valveandsound
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Re: Yamaha DX7IID Low output, no display

Unread post by valveandsound » Wed Oct 09, 2019 4:18 am

Hello! I have done more exploring and have come back with more information:

I pulled my scope out and started working from the output of the synth backwards. I get no signal input into the YA3 board (the analog preamp and volume control/VCA at the output) however i DO get healthy pulses going to all 16 bits of the PCM54HP DAC. Voltage rails are good at +/- 12vDC and +5vDC, but no output from the DAC. Guess i have a bum chip! going to order another one and socket it for good measure.

Miks,

Thanks for chiming in! I appreciate you sharing your knowledge. I do have the schematic, and had discovered a few faulty CMOS chips on the main board. At this point i have:

1.Completely replaced all the electrolytic capacitors in the synth.
2. Replaced the coin cell battery and installed a battery holder.
3. Reflowed as many solder joints on the main board that I can see!
4. vigorously cleaned the push button switches with contact cleaner
5. Cleaned and lubricated the volume and data slider contacts with Deoxit F5 faderlube.
6. Replaced LCD screen with blue 2X40 OLED

I have also socketed the RAM chips and all the logic chips i have replaced, and bought/installed the latest firmware EEPROM (v1.09 I believe?). I did trace out the connections coming to and from the RAM, ROM and main CPU and did continuity tests. I can confirm that everything on the address and data bus are connected to the things they are supposed to be!

Hopefully a replacement DAC will solve the final issues. This is my first real "deep dive" into a synth but this DX7 is very special to me - It's pretty much all the sounds from my childhood and Im determined to make it 100% and last another 20-30 years!

Will update when the DAC chip arrives.
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Miks
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Re: Yamaha DX7IID Low output, no display

Unread post by Miks » Wed Oct 09, 2019 11:01 am

So it seems that you probably have have found the culprit. I'll cross my fingers that after replacing the DAC the synth will come back to life. :pray:

BTW: the latest fw version is v.1.9

Please keep us updated on how you go on with the restoration. (Y)
Last edited by Miks on Wed Oct 09, 2019 3:37 pm, edited 1 time in total.
Reason: corrected a misspelling - thanks para!
My (key related) gear (in alphabetical order):
Ferrofish: B4000+
Kawai: K1 II
Korg: M1 (up'd to EX, w/ 'Cool Blue'), M1REX (w/ 'Cool Blue'), Poly-800
Roland: D-50, D-110, D-550, PG-1000
Yamaha: AW1600, DX7IID (w/ 'Cool Blue'), EX5R, RM50, RX5, SY99 (w/ 'Cool Blue'), TG500,
TX-802, YMM2
Sector101: SYEMB05 (5x), SYEMB06 (3x), EXFLM2 (1kit), MCD Sweet16 (1x), DataBlade32 (1x),
WaveBlade 8MB Card (1x) & 1x Programmer Unit for WaveBlade
User avatar United States of America
valveandsound
Posts: 5
Joined: Tue Oct 01, 2019 3:29 pm

Re: Yamaha DX7IID Low output, no display

Unread post by valveandsound » Thu Oct 10, 2019 4:31 am

Miks,

Thanks for clarifying about the firmware. I do indeed have version 1.9 - a typo on my part

DAC should be here in a few days!
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