D550 s/h buffer opamp replacement

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Re: D550 s/h buffer opamp replacement

Postby Mooger5 » Wed Apr 26, 2017 7:50 pm

Yep, the stock 5238 chip provides a wider bandwidth and a more even, less euphonic sound. This however accounts for more noise at the output, the type of noise you hear through the DX7 Mk1 headphones out. It´s more evident on some presets than others, though.
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Re: D550 s/h buffer opamp replacement

Postby jxalex » Thu Apr 27, 2017 2:10 pm

And when measuring noise floor (A-weighted), how many dBs differ the 5532 and 5238 ?
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Re: D550 s/h buffer opamp replacement

Postby Mooger5 » Thu Apr 27, 2017 4:36 pm

Didn´t do any measurements, jxalex. Should also have recorded some audio, that would have been useful...
The priority was to get rid of that crackling.
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Re: D550 s/h buffer opamp replacement

Postby Mooger5 » Wed May 03, 2017 9:31 pm

madtheory wrote:
Mooger5 wrote:Well I was about to post some boring good news: reworked the joints and the crackling stopped.

This is great news, for me :) I have a D50 here with the same issue. I had suspected the relays on the outputs. Plus broken sockets and sticky buttons. It was not loved.

So you've found that the joints were bad on the op-amp- but is it possible that the heat from the soldering iron gave a temporary fix to an internal open circuit in the chip? Doubtful I know, I can see from the pics that your soldering skill is perfect! Very nice job on SMD :)


So how´s it going? If you need any help with the troubleshooting just ask!

The muting is done by transistors, not relays BTW, and they can also be a source of noise. To rule them out and pretty much everything else you can check right away for crackling noise at TP4 or TP5.

I desoldered the chip with a hot air station, but if it´s a case of bad solder joints I think there+ s no need to remove the chip. You just have to use desoldering braid to remove the excess solder. You´ll then be able to rework the joints with the iron. Be sure to use lots of flux and clean in between steps with IPA.
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Re: D550 s/h buffer opamp replacement

Postby DigitalMetal » Tue Jul 18, 2017 4:01 pm

Hi,

I'm glad i've found this post i'm having "noise" problems with a D50 I picked up cheap with an intention to getting it back to its former glory and I wonder if any of you can help.

What i have is a high level hiss (like tape hiss) thats way louder than it should be, I've compared it to a mint D550 which is silent so i know this hiss is not normal.

the strange thing is that i appears to be coming from the digital domain as the hiss alters in intensity depending on the patch preset loaded, it goes from almost acceptable low level to incredibly high level.
this might be a silly question but it there a noise generator as part of the synth engine on a D50 that could be cranked up accidentally and then the preset saved over?
I guess the way to find out would be to do a factory patch dump reset.

Could it be faulty ROM chips? (I only see one present on the board).

Any futher thoughts anyone?

So far i've totally recapped the PSU, replaced the DAC IC, replaced all of the electrolytic caps on the board, replaced C60 and R13 and the hiss is still there.
Whats gets me is that if i listen at TP3 i hear the hiss and if i look at it on the scope i see it, If i use the scope to look at the output pins of the DAC (pins 23 and 25) i see the noise too albeit at lower level so as i say it surely has to be in the digital domain if the noise level alters with the patches?

Thanks !
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Re: D550 s/h buffer opamp replacement

Postby DigitalMetal » Tue Jul 18, 2017 5:06 pm

I've reloaded the Factory presets via a MIDI dump just to make sure i'ma t a good starting point and the noise is still there.
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Re: D550 s/h buffer opamp replacement

Postby madtheory » Tue Jul 18, 2017 7:31 pm

Are the noisy patches using the chorus? It's a standalone circuit.
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Re: D550 s/h buffer opamp replacement

Postby DigitalMetal » Tue Jul 18, 2017 7:53 pm

All the patches are noisy to some extent, i did edit a patch to turn the chorus and reverb right down to dry and the noise still exists but obviously lower level.
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Re: D550 s/h buffer opamp replacement

Postby Mooger5 » Tue Jul 18, 2017 10:20 pm

Is the noise present in PCM-only patches, VA-only patches, or both? Uper only, lower only, or both?

Tried adjusting VR1 for minimal distortion?
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Re: D550 s/h buffer opamp replacement

Postby DigitalMetal » Wed Jul 19, 2017 12:46 am

The noise is on everything straight out of the DAC, yes ive adjusted the VR1 but this noise is way above the level that this adjustment deals with.
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Re: D550 s/h buffer opamp replacement

Postby Mooger5 » Wed Jul 19, 2017 4:51 pm

The PCM ROMs and the Synthe chip are standalone circuits too. Together with the FX they all merge into the DAC. Editing a patch to use PCM only and/or Synthe only might help to isolate the noise source.
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Re: D550 s/h buffer opamp replacement

Postby Mooger5 » Thu Jul 20, 2017 8:43 pm

You could also scope the input ports of the DAC for noise.

It´s written elsewhere the three causes for DAC output noise are failure of the DAC itself which we can rule out since it´s a new part, the opamp used for the I/V output conversion and noise in the voltage reference circuit, external to the DAC. I haven´t checked, but it´s possible the opamp chip is the same IC12 for both functions...
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Re: D550 s/h buffer opamp replacement

Postby DigitalMetal » Thu Jul 20, 2017 8:55 pm

Thanks I havent had a chance to get it back on the bench again yet for more testing, but I did change the IC12 which didnt help (had to put a TL052 in for now as i couldnt get hold of a 5238 I was told this would be ok).

I've found some reading material about voltage reference noise that i'll take a look at:
http://www.ti.com/lit/an/slyt355/slyt355.pdf

I've made a recording of the various levels of noise depending on the preset loaded that i'll upload asap and i'll see if I can edit some very basic PCM vs Synth chip patches to compare.

Thanks very much for your help so far !
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Re: D550 s/h buffer opamp replacement

Postby DigitalMetal » Fri Jul 21, 2017 9:22 pm

Audio/video Demo of the noise
Watch on youtube.com
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Re: D550 s/h buffer opamp replacement

Postby Mooger5 » Fri Jul 21, 2017 10:38 pm

There´s a clue in one of the examples, I think: at 3:52 the noise, on its own is being effected by the Flanger. It could mean the fault lies before the Chorus chip.
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