I recently bought a Atlas VO-1000 and a drop in decoder. I ran the loco first for a bit on DC and it performed great. Had slow speeds and was quiet. I took apart the loco and installed the decoder per the included instructions. The loco does work on DCC now but has issues. I have to turn the throttle up quite a bit to get it to move, I can then throttle down some and it stays going slower. There is also a zipping sound made when it is in motion. I'm pretty sure there is something I've done wrong on re assembly but cannot figure it out. I've took it apart and inspected everything several times to try and see where the issue is. Each time I reassemble I cannot clear the no slow speed start or the ziping noise. On the plus side this thing has stellar electrical pickup. Hasn't stopped once at all due to dirty wheels or track.
'Zipping' might be something hitting the drivetrain from the motor to the worm. Does the model have wires that may be in contact with a moving part? Low-speed clicking is usually a lower drivetrain issue where the gears are turning slower, but high-frequency sounds are usually from where parts are moving faster. It could also be a decoder setting. Try changing the CV for starting voltage, maybe the default is too high.
Those loco like other Atlas locos have a history of the drive cogs slipping on the motor shafts. Easy fix. Just use CA adhesive to glue cog onto shaft.
Do you have a photo of the cog? I think I know what you mean but want to be sure. I've had the loco apart a dozen times now to no avail. However I haven't taken a part the trucks, is the cog in there?
Has anyone noticed that these units have trouble negotiating curves? I just got a couple and they are supposed to be able to take 9" radius curves, but they seem to want to jump on 9 3/4" curves. Testing the trucks, it almost seems like they might be hitting the fuel tank or the front sill step frame.
I think mine is the one made in Korea. It is Atlas #51009 and has made in Korea on the box inserts. What is bad is it ran well with little noise before I took it apart to put the decoder in.
Mine is also quiet at low speeds but starts to growl/buzz more as speed increases. IIRC, the reason the older Korean locos were noisier was because the worms rode a little too low and too deep into the gear atop the truck tower. Not positive on that but that rings a bell. I do remember that Atlas developed a way to fix the noise using some of the drive components from the later Chinese versions which, IIRC, included replacing the motor saddle and at least the worm blocks if not the entire worm. Last I checked, The instructions were still online but at least some of the parts to do the upgrade were out-of-stock.