grinding KSS-240a laser assembly

I think that it is highly unlikely that anything is happening to the size of any gear wheels. The (very brief) sound that can be heard on your clip, sounds like the motor pinion slipping against the intermediate drive gear. On a typical mech that uses any of the KSS series lasers, the most common cause of this is bad contacts on the laser 'home' switch, normally located on the spindle / sled motor connection board, right underneath the turntable. When the contacts don't make very well, the laser homes as normal, but no signal is generated when it gets there, to tell the system control micro to shut off the drive to the motor, so it keeps running, and the gears slip against one another. After a while, the vibration caused by this often gets the switch contacts to make again briefly, which then causes the drive to be cut. My first move would be to clean the switch.

Arfa

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This sounds like a better theory than mine. lol

The sound seems to more occur when the Sega CD stops an audio track and goes to read data, so I guess it needs to return home to read the TOC to find the data it wants.

Thing is, it happens ONLY when warm. When the machine is cold, it never has this issue.

I'll give it a cleaning, though it's hard to get to that home switch (i know where it is).. Hopefully getting the assembly out will be easier than I fear..

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An update: I watched the laser move, and when it screeches, it's not even near the home switch.. The motor spins, and the cogwheel does move, but misses on the other gear (it never slips on the sled)...

There's a decent amount of play, because if I manually turn the cogwheel, it does sometime miss too..

Do you need to find out why there's free play. There may be enough free play for it slip or jump gears if something is binding, but not otherwise. Does the sled move freely on the rails?

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Thinking about it, there was an issue with some Sony decks (which this almost certainly is) with two of the gears being right at either end of their size tolerance specifications, and there was a mod kit for it with two new gears. There was a service bulletin about it, as I recall, but I wouldn't know where to lay hands on it now. If you have a local hifi repair shop, they will have scrap Sony decks coming out of their ears, as many warranty replacement lasers were supplied by Sony as whole KSM series decks. You should be able to get a scrap deck from them, which may not be exactly the same, but in general, the gears in the sled drive train are, so you can rob and transplant them into your deck. The other alternative is a bit of a 'bodge'. You can slacken the screws which secure the sled motor, then push a thin sliver of plastic sheet under the side of the motor closest to the intermediate gear. When you tighten the screws back up, the motor will have a very slight 'tilt' towards the intermediate gear, which will improve the meshing. Not ideal, but I have done similar things in the past, when parts have not been available, or have been prohibitively expensive.

Arfa

I will check at a local repair center and see if they have one..

Btw, I took the "drive" out, and noticed the home switch did have a cold solder joint, i fixed it, but no change..

I have an old Sony, and peeking in, it has a KSS 210B drive. Can that work? Or is it not compatible?

The '210 laser is completely incompatible with the '240 - different connectors for one thing. For sure, the 240 will fit on the 210 deck - the body shape of both lasers is the same. However, the decks themselves are quite likely to be different. They may have the mounting rubbers in slightly different places, or the actual shape of the metal deck may be subtly different, for instance. They may have different shaft lengths on the spindle motor, and differently designed turntables, which have to match the disc clamp. There are a number of variations of these decks, many of which look the same at a first glance, but are different when you dig in a bit deeper. However, most of then share the same drive chain gears for the sled, so you may be able to cull parts that will do the job, off that '210 deck that you have found.

Arfa

Ok.

The Sega CD's desk doesn't have any rubber at all, it's hinged at the back, and at the front, as a piece that attaches to a gear that's run on the tray, so when the tray closes, the desk pops up into position. The spot where rubber pieces go at the 4 corners, is actually some brass colored bolt, which is secured to a fairly large PCB at the bottom.. The only cable going to the Sega CD's motherboard, is a big flat ribbon on the right side of the PCB on the deck.

I will check all my old CD-players for gears that fit.

Btw, side question: I noticed when the 240a is reading a music CD, it is 1x as it should be.. But when it reads data, it does spin the CD faster, but not quite 2x.. Sega claims it's 1x read only, and from what I see, the 240a is 1x only. So why does it spin faster for data?

The actual metal deck likely still has the corner rubbers, as these form the primary shock-resistant suspension system for it. The hinged plastic tray that is worked from the lifter gear driven off the drawer gears, is just a carrier for the deck. It is a common arrangement. It is also common for the rubbers to collapse over time, which can lead to a very tap-sensitive system, which might lead you to suspect a bad laser. The laser itself will connect to that sub pcb on the '210 deck'd machine that you have found. The connections to a '210 are two multi-pin sockets with conventional wires. The '240 has a single narrow-ish white ribbon.

I've no idea, really. I have seen 'conventional' audio lasers such as the KSS series, being run at higher than x1 in 'specialist' audio applications, so it seems that they are capable of it. The decks in personal CD players often run the disc at x2 or more in an effort to keep the bit bucket full for anti-shock purposes, but since the Sega is not subject to physical shock (!) that's obviously not the reason. Might be something to do with keeping a larger buffer of data available though, due to processing speed or something ?

Arfa

Hello,

One other thought that others have not mentioned. I've seen a failure of the clock circuit being fed to the mechanism fail and cause the laser sled to go flying to the outside if the disc and the sled drive will keep going. If left in this condition, it will actually grind the rack on the laser to bits.

I've never had one that was intermittent, but if the clock was beginning to fail, I guess it might do it for a short burst. If this is the case, then the sled is likely to be at the extreme outside edge of the disc.

The machine that I've seen this on it the Arcam Alpha 8, and the Alpha

8SE CD players. A surface mount transistor on the DAC board clock circuit was running to hot and would fail. When it did, the sled drive would force the laser to the extreme outside and make a horrible ratcheting noise. There was a production change to prevent this. I've NO IDEA if this is possible on your Sega player.

Regards, Tim Schwartz Bristol Electronics

After playing with the game console last night (and falling asleep, lol), I noticed a trend... It's grinding gears isn't so intermittent as I thought.

Batman and Robbin: When going from the title screen to the Options menu, it grinds gears. Also when going from Options to Title Screen, it also grinds gears. Sonic CD: In Time Trial screen, selecting a level grinds gears as soon as it starts to load a level.

Oddly, all three times it grinds gears on "command", was when it was playing a CD audio redbook track, and went to reading data (on track "1")..

This is leading me to confusion, because home switch is fine, i tested it, resoldered it, cleaned it, and it seems that it's the action of throwing the laser closer to the spindle... (the start of the movement, not the "stopping")..

Well, I guess it's just when the thing needs to move the sled somewhere else in a hurry. The play in the gears allows them to slip momentarily as the motor tries to accelerate the optical block. I wouldn't say that it is any kind of real 'mystery' to lose sleep over. Just find some replacement gears, or tilt the motor, and I'm sure it will be fine.

Arfa

Hello Tim. Yes, I've seen a similar problem to that as well, but I can't remember what it was on. As I recall, it chewed the teeth right off the motor pinion. From what I can gather, this one doesn't seem to do it at either end of the sled. Seems to be more when it starts to move the sled from any position, so probably a mechanical issue.

Arfa

Ahh, that did it.. When the Sega CD is tilted slightly clockwise, the grinding stops completely..

I will hunt gears then..

Thanks!

On Sat, 09 Feb 2008 13:24:12 GMT, Tim Schwartz put finger to keyboard and composed:

Wasn't the KSS-240A also susceptible to oscillation problems and wasn't there a kit that fixed this?

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.

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This isn't an oscillation problem, and it seems SEGA put brass weights on the assembly already.

Yes, seems to be an unrelated problem though.

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It's probably too late and Arfa won't see this, but I brought my whole Sega CD to a repair shop, and he was able to reproduce the problem when COLD.

With a scope, he discovered that the CD-ROM drive is in "seek" mode when playing video games, hence the disc spins faster. Possibly to boost transfer rate for the games "Full Motion Video".

Also, he discovered the grinding will ONLY occur when the drive is in it's "1.5x" mode, as when playing music CD's, the grinding was impossible to produce.

Solution: He has a replacement mech where the laser and PCB will happily attach, and the Sega CD doesn't even know a different mech is installed. Only downside: Tray is now white, and not black. ehh.. As long as it works..

It's probably too late and Arfa won't see this, but I brought my whole Sega CD to a repair shop, and he was able to reproduce the problem when COLD.

With a scope, he discovered that the CD-ROM drive is in "seek" mode when playing video games, hence the disc spins faster. Possibly to boost transfer rate for the games "Full Motion Video".

Also, he discovered the grinding will ONLY occur when the drive is in it's "1.5x" mode, as when playing music CD's, the grinding was impossible to produce.

Solution: He has a replacement mech where the laser and PCB will happily attach, and the Sega CD doesn't even know a different mech is installed. Only downside: Tray is now white, and not black. ehh.. As long as it works..

Good result then !

Arfa