Electronics > Repair

Roland D-50 noise issue with PCM sounds, SYNTHE sounds fine

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frisc0:
Hi, new here on EEVblog, found this forum while searching for a solution for my Roland D-50 synth and hoping to get some help getting this fixed. Not very technical but know the basics. I've posted the same issue earlier on the VintageSynth forum but no solution yet, now hoping to extend the help from this tech-crowd :).

The issue, copied over from VintageSynth forum:

My Roland D-50 has an issue playing patches that use PCM waveforms, but plays all SYNTHE-only patches just fine. PCM waveforms sometimes play fine for a few times and then start cracking or stop playing completely. Sometimes they only produce a 'click' and after hitting some keys it's like something is 'popping' open and the PCM waveform is played correctly again, but always with a cracking sound.

Chorus and Reverb chips are okay, since the problem only arises with PCM waveform patches and the SYNTHE sounds also go through these chips afaik and sound fine. That's why I think the DAC and everything towards the outputs is fine as well. So it's probably something before the DAC.

Now, where to look?

What I did so far:
- replaced the battery
- reloaded the factory patches via MIDI
- ToneRAM reset via key 0 + Data Transfer
- performed the internal tests, multiple times, memory is (appears) ok
- unplugged and replugged all connectors
- checked mainboard for any visible issues to components
- recalibrated the MSB trimpot via procedure in service manual
- changed voltage on the mains transformer from 220V to 240V (230V in my country)
- changed all capacitors on both PSU and mainboard (...)

I have the schematics and tried to back trace down from the DAC. I also suspected the PCM ROMs, but my guess would be if they're faulty, they would never produce a waveform, and the 'popping open' in my mind would more suggest an analogue component, like caps? Note I did not replace the SMD caps, but did measure them (in circuit) and they seem to be ok (all have same resistance and charge/decharge similarly).

One thing I forgot to mention: it appears that Reverb presets 17-32 influence the cracking patches, some reverbs in this range even end up in a sort of feedback loop resulting in lots of digital noise. Could that be a RAM issue (either tone RAM or working RAM)? Is there any way I can test that without replacing all RAM ICs?

I did some further measurements on the mainboard, on the resistor arrays (in circuit). Most RA's are measuring values that make sense (15K-ish) but on RA10 (next to the CPU) 2 of the 8 resistors measure 10K instead of 15K ohms. RA10 is attached to the higher address lines (Ah). Afaik, these RA's are used as pull-up resistors to make sure to always get a clear 0 (ground) or 1 (5V). Could these 10K resistors (if it's a correct measurement, being in circuit) result in random ROM/RAM locations being addressed and read and as a result, the distorted PCM patches?

To further narrow down the issue, I've set up 3 basic patches with only 1 partial, PCM only (structure 6):

1. PCM waveform 1, Marimba
2. PCM waveform 32, Breath
3. PCM waveform 100, Loop24

Tone 1 always sounds clear. Tones 2 and 3 are mostly distorted, however!

When I switch off and on the D-50, the first 3 or 4 key presses for tones 2 and 3 sound clear and only after 4-6 key presses they start cracking and sometimes failing completely (only clicking/popping).

Interestingly, using tone 3 (which is looped), after switching off and on the D-50, if pressed and hold for a long time, the sound remains clear without distortion! Only after triggering this loop tone multiple times it starts cracking.

In some cases, switching from tones 2 or 3 back to 1, this tone also sounds distorted for the first 2-3 keypresses and then sounds clear again and remains clear.

My conclusion from this behaviour: PCM ROM IC2 is ok, it can deliver the correct data. Something in between is failing. I'm suspecting noisy addresslines in the higher area's of the memory map, therefore affecting only the waveforms from nr 32 and higher... does this make sense and if so, where to look?
Maybe the pull-up resistor arrays on the addresslines? I've measured these before and saw some lower resistance values (10K instead of 15K), but that would end up with a solid 5V as well, right?
Could also be the 0,1uF smd decoupling/bypass capacitors near the memory ICs that I haven't replaced yet.

I'd love to get this awesome synth going again, so all help is greatly appreciated!

floobydust:
Without a schematic there's not much help we can offer. The synth might have a different path for PCM sound generation. Roland got cute with their designs to keep speed up and cost low.
I would heat/cool some of the usual culprits. Take a can of freeze spray and cool some memory IC's, or a heat gun and warm them to see what aggravates the problem.
PCM sounds might always use the reverb chain and bad RAM there would corrupt the reverb tails, which can be what you are hearing.

frisc0:
Thanks for your reply floobydust, the schematics can be found here: http://www.synfo.nl/servicemanuals/Roland/D-50_SERVICE_NOTES.pdf

As far as I can tell all sound data comes from the Synthe IC and leads through the Reverb and Chorus circuits. If these circuits/ram are bad, it should affect the non-PCM patches as well I would guess? Btw I think these circuits are always engaged, can't switch them off in a patch only set the level to zero.

The symptoms (crackling starting after a few key presses) seem to point towards some analogue component 'saturating/overloading', rather than a digital domain component? Anyway, I'll check with your suggestion on cooling/heating to see how that affects the issue, thanks.

floobydust:
You have to be a sleuth and try things to really confirm where the problem is.
Some PCM patches might set the reverb to different settings.
I would suspect a bad DRAMIC uPD41416C-12 16Kx4 (4416) if the problem is the reverb, in the digital domain. If they're in sockets you can swap positions to see if a problem moves. Or just heat them up a little and if bad it will get much worse.
During the release, the reverb data will be cycling down to all 0's but during sustain portion reverb is not involved other than loading.

To fix these I usually start by inspecting the solder joints, keyboards get banged around and it's common to have fractured joints at I/O jacks and buttons etc. Use a magnifier and bright light to carefully look around. You can tap or press the boards with a plastic pen (while holding a note) and see if a bad connection shows up.

If you think it's an analog problem, ESD hits at the output jacks does damage the muting transistors Q1-Q4 so you might work backwards from the output back. But the problem is in both channels? Does the volume control affect the problem, headphones the same?

frisc0:
Thanks floobydust for these leads and thinking along, I already did some detective work on this board looking with a flashlight in detail at all components and soldering joints, do not see anything suspicious. Unfortunately, the DRAM ICs are not in sockets, so I'll try the heating you suggested.

The 3 test patches I've set up are build-up from an initial/empty patch, but I'll check what that does with reverb settings. Those reverb settings (as well as other patch data) is stored in the (battery-powered) Tone Ram IC. However, since SYNTHE patches that use reverb sound just fine, I don't think it's in this circuitry.

The interesting behaviour I see consistently is that after power on it takes 3 or 4 keypresses before test-patch 2 (breath) or 3 (loop) starts crackling. It's always clear the first 3 key presses. And patch 3 which is looped, also starts crackling after 3 or 4 keypresses, but being activated and hold for a long time the first few keypresses after power-on, the sound remains clear during the hold-period. So it definitely is related to how often the patch is triggered and not how long the sounds is produced.

I suspect that with every keypress, the PCM data for that waveform is being re-read from the PCM-ROM(s) and the distortion starts only after 3 or 4 'reads'. Since this behaviour can be reproduced consistently, I suspect 'something' is overloading during this process, but I can't pinpoint it to a component... should I replace the SMD decoupling capacitors next to the PCM ROM ICs?

I don't think it's in the output circuitry, since the SYNTHE patches sound fine and go through the exact same route. Also ,there is no difference in the upper and lower channels or headphone vs output jacks.

I did see some forum posts on muting transistors being faulty, but that would also affect the SYNTHE patches, right?

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