D550 s/h buffer opamp replacement

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Re: D550 s/h buffer opamp replacement

Postby DigitalMetal » Fri Jul 21, 2017 10:44 pm

Yeah that's kinda what i've been suspecting all along and even more so now that ive kinda run out of things after the DAC to replace.
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Re: D550 s/h buffer opamp replacement

Postby Mooger5 » Fri Jul 21, 2017 10:57 pm

viewtopic.php?f=5&t=78276

This is interesting and useful. This user solved a problem very similar to yours. From what I could understand he replaced a lot of chips only to find a solder bridge in the Chorus chip was the culprit!
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Re: D550 s/h buffer opamp replacement

Postby DigitalMetal » Fri Jul 21, 2017 11:40 pm

Thanks very much, i'm going to have to go over it with a the microscope!
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Re: D550 s/h buffer opamp replacement

Postby Mooger5 » Tue Jul 25, 2017 11:31 am

Did you find anything? You know, some freezing spray over the zuspects might help.
Cheers.
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Re: D550 s/h buffer opamp replacement

Postby DigitalMetal » Tue Jul 25, 2017 6:14 pm

I've been doing a little bit of investigating and started my deconstructing some of the noisy presets, bear in mind these are the factory presets I downloaded from the Roland website and sent to my D50 via Midi dump.
I loaded up one of the particularly noisy patches and muted each partial in turn to isolate it, looking at the EQ some of the settings were with +10db of gain if i turned this down to zero the hiss was radically decreased, if i did this across all bands (basically flattened the EQ out) that was a drastic improvement.
Also tweaking the didgital output level down a bit (in the menus not the hardware slider) again drastically reduced the noise down to the point where its pretty acceptable.

So I'm obviously massively confused at this alone and whether there is just too much internal gain there somewhere before the EQ and main digital volume.

When i get a moment im going to try and compare the patch settings to that of a very quiet D550 i have access to.

But in the meantime ive checked for solder bridges on all the IC's with a microscope and found nothing, none of the IC's feel warm but i could try some freezer spray, i would have at least thought it would be ok when dead cold at first switch on if it were a heating problem though wouldn't it?
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Re: D550 s/h buffer opamp replacement

Postby Mooger5 » Tue Jul 25, 2017 8:15 pm

What I have in mind is the freezing will contract the joints thus improving whatever bad connection there is that might cause the problem. I didn´t find anything wrong in the Sample & Hold opamp either, yet the problem was there.

I´ve found something of a clue in the Service Manual: there were a number of revisions to the boards, each having had its own firmware updated. Repair techs are advised to install new EPROMs as the units turn up for servicing. It even says "the user will be underwhelmed if a wrong update is performed onto a specific revision", or something like that.
Even if the firmware is right, reburning the EPROM with the same .BIN file isn´t out of a "to do" list. I´ve researched how EPROMs work and like all capacitors they don´t hold their charges forever. For peace of mind at least, I´d compare the checksums.
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Re: D550 s/h buffer opamp replacement

Postby DigitalMetal » Tue Jul 25, 2017 8:37 pm

Ah ok I get it, I'll have a got with a freezer spray on the weekend.
Thanks for the infor about the EPROM, It did cross my mind that maybe someone had performed an upgrade seeing as it is on the latest version, It is soldered in rather than in a socket.
Do you know where i can get a .BIN file or do you literally bean try and extract the current one and re-burn it back again?
I might solder a socket to the board if im going to be doing some experimenting to save the PCB from the heat abuse.

Thanks!
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Re: D550 s/h buffer opamp replacement

Postby Mooger5 » Tue Jul 25, 2017 10:05 pm

Here http://dbwbp.com/index.php/9-misc/37-synth-eprom-dumps

There are several versions there. Scroll through the entire Roland list. Choose the appropriate one according to the serial number, may not necessarily be the latest.

If the checksums don´t match, the code in the EPROM is corrupt, and useless if extracted. Better erase the chip and burn a fresh file.
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Re: D550 s/h buffer opamp replacement

Postby DigitalMetal » Fri Jul 28, 2017 3:37 pm

Ok I burned two brand new Eproms, One with Version 2.10 and another with 2.20 using either of them still exhibits the same noise :(
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Re: D550 s/h buffer opamp replacement

Postby Mooger5 » Fri Jul 28, 2017 4:30 pm

That´s too bad. Sorry for leading you into all that trouble. At least we can rule a bad EPROM out. And your synth has an upgraded firmware, that is if it wasn´t already.

One thing: does your synth have the shielded cables like all D50s I´ve seen? They look like wrapped in aluminium foil...

Another thing: the D50 and D550 mainboards are swappable. Did you try this, so to rule out the noise is coming from external sources...

Other than those, I can only think of reworking the joints. Not removing the ICs; just applying a generous amount of solder to the IC pins and then soak the desoldering braid with the excess solder by dragging it with the iron along the "combs". After that resolder the joints if needed. I´ll post a video that shows the idea much better than I can tell.
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Re: D550 s/h buffer opamp replacement

Postby Mooger5 » Fri Jul 28, 2017 4:45 pm

Here. It´s not of course the same task performed here. just a similar process of dragging the braid along the IC pins with the help of the sorderng iron. "Applying a generous amount of solder" just before doing that just helps the old solder to soak in. Like Chip-Quik.

Watch on youtube.com


I´ve watched this video a considerable number of times. It´s quite motivating. SMD is not as scary to work on.
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Re: D550 s/h buffer opamp replacement

Postby DigitalMetal » Fri Jul 28, 2017 4:54 pm

No problem, its all good as like you said it's ruling things out.
It was already on V2.20 so not an upgrade but worth checking still.

I'm pretty good at SMD soldering so wont need to drag solder the pins i can do them individually with a needle tipped iron and my eyes and check with a microscope which ive done before with memory chips and other tiny connections.

I did consider reflowing all the joints but wanted to rule out more obvious thing first and its getting the point where im running out of alternatives so maybe i'll do it over the weekend.

It might be that excuse ive been waiting for to buy a hot air station even but ive always got by with an iron so far.

Only the keyboard cable is foil wrapped, have you seen D50's with more than that wrapped?
The display cables have a large ferrite ring around them too but thats it as far as shielding goes.

Thanks again for your help!
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Re: D550 s/h buffer opamp replacement

Postby Mooger5 » Fri Jul 28, 2017 5:35 pm

I bought a rebranded Ayoue station reviewed on the EEV blog. Ultra-cheap but it works great! I just use it for SMD desoldering, and occasional heatshrinking cable sleeves.

Yeah, on the D50s I´ve seen only the cables from kbd scanner are shielded. I guess swapping boards isnt necessary.

Well best of luck.

One other thing: if the presence of noise can be traced at the DAC´s analog outputs, what about the digital inputs? It´s a parallel DAC so there should be quite a number of ports, like 12 or 16, carrying data from the FX and Synthe ICs. Did you find anything other than pulse trains on them?
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Re: D550 s/h buffer opamp replacement

Postby DigitalMetal » Sun Jul 30, 2017 8:34 pm

Ok,
The only thing that seem to have a direct affect are the digital volume control (in the patch programming not the volume slider on the front panel) and the EQ.
I can turn the digital volume right down and the synth becomes totally silent so the noise source but be before this, I can also adjust the EQ and either boost or attenuate the noise with it, one thing that seems a little strange which may point toward the cause of the issue is that if i sweep the EQ gain up and down it sounds like it stair steps/zippers though the parameters with very slight thumps, maybe this is expected and normal i just dont know but im wondering if its the EQ that is faulty but the trouble is i have no idea where in the DSP the EQ is, not the digital volume for that matter so its really hard to nail down any further than that.

I freezer sprayed everything to no avail, I reflowed all the solder joints on all of the main IC's (CPU, Chorus, Reverb) i'll do everything else i can when my hot air station arrives perhaps but im not sure it'll help.
Ive confirmed every IC has a ground on the pins they should have and checked for any shorts on the IC pins.

The DAC does have multiple pins carrying digital signals and looking at them on the scope they seem pretty uniform traces but i dont really know what im looking at here as im not at all familiar with digital electronics.

Image

http://imgur.com/a/DeBlU
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Re: D550 s/h buffer opamp replacement

Postby Mooger5 » Mon Jul 31, 2017 12:44 am

Did you run the Test Mode as described on page 12? If there´s a serious problem with the reverb and chorus ICs, it´s supposed to show.
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