Minolta Scan Dual III: "wavy" noise pattern??

Discussion in 'Digital Darkroom' started by michel_wilson, Nov 7, 2020.

  1. I've recently acquired a Minolta Scan Dual III (yesss, finally a film scanner). However, the scanner exhibits an odd "wavy" noise pattern, mainly noticable in dark areas. The following sample show it clearly. This was taken from a color slide, scanned using VueScan. And yes, I did allow the scanner to warm up, and I did use the "Calibrate" function in VueScan prior to scanning (several times, even).

    This is a 100% crop at full resolution with only basic sharpening applied:

    [​IMG]

    Depending on screen brightness, the noise is actually pretty noticeable. To show it more clearly, I've extracted the green channel (in this channel, the noise is most noticeable) and increased the contrast a lot using levels:

    [​IMG]

    Obviously, I would normally never do this, but it does serve to show that there appears to be some kind of issue. I've played around a bit with multi-sampling, which helps a bit, but the old adage of "garbage in, garbage out" applies here as well: if the individual scan is plagued by too much patterned noise, multi-sampling only does so much.

    Does anybody have a clue as to what is happening here? My gut feeling says something related with interference due to maybe the non-original power supply, or possibly some kind of voltage stabilisation internally not working properly to maybe old capacitors? Is this a known issue perhaps?
     
  2. I remember Minolta Scan Dual III from times that I had one. My guess is this phenomenon is somekind of moirepattern resulting from 2800dpi scan and film grain.

    My Scan Dual III had only USB1.1 connection, so newer Scan Dual IV proved much faster in operation.
     
  3. It could be grain, but I have a hard time believing that grain, which is essentially random noise, could generate such moire-like pattens. I think interference can only happen when two frequencies interact in some way, and the white noise of the grain should not give this pattern. I will try to scan and enhance a different kind of film sometime, to be very sure.

    I did notice slow scan speeds, the scanner was mechanically faster than the USB connection, which is a bit of a pity (it stopped and restarted several times during scanning). My system reported it as USB 2.0 high speed device, so it should be able to keep up, but apparently this is not the case.

    My current plan of attack is to first double-check with another type of film, if I can repeat the wavy pattern, and then to check the noise levels in the power supply, using an old scope I still have.
     
  4. It's too regular to be grain aliasing, but the real test would be to scan a dark ND gel filter.

    My guess would be ripple on the power supply. You say it's a replacement one? Is that an old bulky transformer type, or a smaller switch-mode supply? Either could have a faulty capacitor and the only difference would be the frequency of the ripple. But I'd expect there to be additional smoothing within the scanner.

    Check that the scanner is positioned well away from adjacent mains cables as well. It could simply be airborne/magnetic interference from a nearby device or cable.

    Another likely cause could be mechanical 'stutter'. In that the transport mechanism isn't stepping smoothly. A clean and lube would probably sort that out.
     
  5. It's a "Delta C7690-84202 24v 0.84a" power supply with UK plug, which seems to be originally used for some HP scanner/printer, oddly enough. It has to be switch mode, it's one of these plug-type supplies. I will measure it hopefully tonight to see the ripple output. If there's substantial ripple in the output, the question is then, what supply do I get to replace it? The connector seems to be an EIAJ-03 connector, 5.0mm OD, 1.7mm ID, which is some kind of not very standard Japanese connector ... but I can always do a bit of soldering of course.

    Wrt your idea of mechanical "stutter": the crop was taken from a slide in portrait orientation, so the wavy lines are perpendicular to the transport direction of the film, so that does not really make sense, right? I also unfortonately do not own ND gel filters, and it doesn't really make sense to buy something like that specifically for testing this. I will test with another film type.
     
  6. I'm not so sure. There could be some play in the transport mechanism that allows the slide to skew back and forth as it's stepped.

    My first suspect would still be electrical interference or PSU ripple, but check electrical cable proximity first.

    I had a similar effect suddenly appear on one of my scanners. It turned out to be a mains cable that had got routed underneath the scanner. I moved the cable away and the scans cleared to normal.

    FWIW, my source of small pieces of ND gel has been a book of Lee Filters sample swatches of lighting gels. These used to be given free on request, but in these more frugal times I think there's a small charge for them these days.
     
  7. Ah that's a good point. If there's play in the mechanism I'm not sure what I could do to fix that. Lubricating it will probably not help much, but can't hurt to try it ...

    I've ordered a new power supply, some kind of generic supply that can do 24V, by Ansmann, which is a reputable German brand (at least, I think it is). Probably won't be made in Germany, but you'd hope they would do proper QA on something that carries their name. Let's hope that this will improve things.
    Ah, nice idea. I'm also still searching for the IT8 targets I'm supposed to have somewhere. I'm sure I used to have them. But no clue where they are hiding currently :D :D
     
  8. Ooooh, I hate to burst that bubble, but my Ansmann purchases (not many, admittedly) have been pretty poorly made. They work, but the standard of finish and materials aren't great, and basically they're just budget made in China stuff re-badged.
     
  9. If the Dual III is made like the Elite II (and I think the internal layout may be similar), when you pop it open, the transport/scanner/etc. is a piece that sits in front of and slightly over a board that includes the internal power supply along with some other components. There's a ribbon cable that connects the front piece to that card, plus power supply cables, and then a cable for the front buttons / door sensor.

    I've had that internal power supply stop working - I had more than one unit to test with, so I experimented a bit. I'd suggest reseating the cables. Also, the Scan Elite II had a ferrite band added to the ribbon cable (part 2888-1034-01) and it's in the service manual as needed for "Noise reduction". It's possible you're seeing internal interference, not external. That was a later part substitution on the Scan Elite II - there was another part there previously.

    This is a long way to say that the interference COULD be generated from inside the unit if moving it, trying a different adapter, and changing out the USB cable all fail to help.
     
  10. So, got the adapter today. I have to say build-wise it actually looks quite decent, sturdy construction and okayish finish. Definitely made in China, not in Japan, but good enough. Also, there's a connector that sorta fits...not quite, but good enough to make it turn on. That's about the end of the good news though, still wavy interference-like noise...
    Some tests I did:

    - fiddling/fudging with the tray (softly pushing it, or softly pulling it) does nothing at all to the noise pattern. I think this would eliminate anything caused by mechanical play, because then you would be able to see it
    - moving the scanner: no effect
    - reversing the AC plug also doesn't do anything ... little chance that this had effect, but easy to try
    - i've been scanning a bit of fully exposed trailer of positive film, and there's also a sticker on this part. There's also noise in the sticker, which is opaque, so it's not related to anything in the film itself, must be in the analog part of the circuit before the conversion to digital (or bits are being flipped, but that would look differently i think?)

    I'll open the thing up and start fiddling with cables at random. Also cheap to try, can't hurt you'd think, right? ;)
     
  11. So, cable-fiddling did nothing. It does seem to have ferrite around all the ribbon cables, have never seen anything like this before. It's hotglued to the PCB connectors it seems:
    [​IMG]

    But, what do we see on the left? Indeed:
    [​IMG]

    That looks somewhat suspicious. Unfortunately it looks like a surface-mount capacitor, I've never soldered SMD stuff before. Still, might be worth a try!
     
  12. Yep, that bulge indicates a dried out capacitor.

    It shouldn't be too fiddly to replace. Looks like there's bags of space around it, and you can probably fit a 'normal' radial-lead through-hole component in its place. Soldered onto the existing pads. I'm not suggesting you drill holes in the board!

    The ground lead will need a powerful soldering iron to release it. All that copper will suck the heat away before the solder melts. Probably easier to cut the ground lead from the old cappy. Re-soldering shouldn't need quite as much heat.
     
    Last edited: Nov 13, 2020
    PaulCoen likes this.
  13. Well, sad face.

    I changed the capacitor, which went pretty smoothly. Then, bit of a scare, as my computer did not see the USB device anymore. After I fully pushed in the USB connector, the scanner was detected again and my heart restarted. But: no joy, still the same noise pattern.

    Next, I noticed an odd solder blob next to the crystal under the (I think) CPU. You can actually spot it in the first photo! So, I set out to remove that blob, it seemed like it could cause such an issue. But, again no joy, same noise pattern.

    So, I'm out of options for this scanner I think. I'll take my loss I guess, and start looking for other likely candidates. At least I can keep this one and use it for parts.
     

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