Cd Player Startup Problems 电路图.pdf

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1、BY S A M G O L D W A S S E R S E R V I C E E D I T O R Troubleshooting CD-Player Slartup Problems A STARTUP PROBLEM IS A CATCH-ALL TERM FOR ANY SITU- ATION WHERE A CD PLAYER OR CD-ROM DRIVE REFUSES TO RECOGNIZE OR PLAY/ACCESS A DISC. IN THIS MONTHS INSTALLMENT WELL DESCRIBE IN DETAIL WHAT HAPPENS WH

2、EN you insert a CD and push PLAY, and well look at what to do if the unit doesnt want to cooperate. Whats a Startup Problem? With an old-style record player (you know, vinyl LPs, technology that was state-of-the-art sometime before dino- saurs roamed the earth), the tone arm could be plunked down an

3、ywhere to start play. Not so with a CD player. Aside from the fact that you cannot even get to the pickup on a CD player to move it by hand and plunk it down, a lot must take place successfully before the first note is sent to your loudspeakers. Startup problems cover all situations where the player

4、 does not successfully read the disc directory. Nearly every- thing in the optical deck and much of the main-board electronics needs to be func- tional to read the directory. Therefore, any failure in any of a rather large num- ber of places can prevent successful start- up (and subsequent play). On

5、 a changer, a startup-sequence failure will probably result in similar symptoms, but then the unit will move on to the next position in the carousel or cartridge. It is likely that the player will remember that it was unsuccessful at loading a disc for each position and eventually give up after all

6、possible discs have been tried. Possible causes of a startup-sequence failure include a defective disc, dirty lens, defective laser or photodiode array, bad focus or tracking actuator or driver, dirty track, insufficient or dried-up lubrica- tion, dirty or bad limit switches or sen- sors, defective

7、spindle motor, faulty elec- tronics or control logic, damaged parts, faulty optical alignment, and/or need for servo adjustments. The bad news is that this is a large number of possibilities. The good news is that with such a large number of possibilities, there is a good chance that the problem wil

8、l be minor and inexpensive to fix; actual failure of the (expensive) optical pickup itself is rel- atively far down the list of likely causes. First, be sure that you do not overlook the trivial: Are you loading the disc cor- rectly? Most CD players want the disc label-side up. However, there are so

9、me, like Pioneer magazine-type changers, that want the label-side down. If you have just acquired the CD player, dont over- look that possibility, Startup Sequence The exact sequence of startup events and the symptoms they cause when they fail will vary depending on the type of player and its design

10、. For example, the display might flash, be blank, display “-:- ” or the word “disc” or “error,” etc. In any event, the unit wont play the disc. By understanding the following summary as it applies to your player, you should be able to determine what is going wrong. When a CD is inserted, the player

11、should go through the following routine: 1. Drawer closes (or with portables, lid is closed manually) and CD is clamped to the spindle. 2. Interlock engages, if present (always in portables). If there is no inter- lock, there may be an optical sensor or the optical pickup may act as its own disc sen

12、sor. 3. Pickup resets to starting (index) location towards center of disc, which is usually located using a limit switch or optical sensor. Before describing the remaining steps, it would be helpful to explain a bit more about the organization of the photodetec- tor. The illustration in Fig. 1 is ty

13、pical of units that have a three-beam pickup. Blocks E and F are not present (or at least not used-they will be disabled and grounded) in units that have a single- beam pickup. The four-quadrant (ABCD) photodetector is found in all systems. The main return beam is detected by the ABCD array. The tra

14、cking beams return to E and F. Detector E is offset slightly off track to one side and F is offset to the other. Average signals from E and F will be equal when the array is centered on the track. Now, lets return to our discussion. 4. Laser is turned on and focus search routine is started to positi

15、on the lens at the correct vertical position. Once correct focus is achieved, the focus servo is activated to maintain it. Focus, which must be accurate to 1 pm, oper- ates as described in the next paragraph. 5. The optical path in the pickup includes a cylindrical lens or astigmatism that causes th

16、e laser-beam spot to be cir- cular when it is correctly focused, but elliptical when it is not. When it is not correctly focused, the major axis of the ellipse is offset 90 degrees, depending upon whether the lens is too close or too far (e.g., major axis of -45 degrees for too close and +45 degrees

17、 for too far). Focus RadioFans.CN 收音机爱 好者资料库 Error is equal to (A + D) - (B + C), which will be 0 when focus is correct since, with the circular spot, the outputs of all four quadrants will be equal. 6. The disc starts spinning up to 500 rpm and the constant linear velocity (CLV) servo is activated

18、to maintain cor- rect speed. The CLV servo uses a PLL (phase-locked loop) to lock to clock tran- sitions derived from data read off the disc. Data is derived from A + B + C + D (look at Fig. 1). 7. Tracking servo is activated to keep the laser beam centered on the track. With a three-beam pickup, tw

19、o addition- al laser spots are projected onto the disc, one in front of and one behind the main beam. Those are offset on each side of the track just enough so that the Tracking Error = E - F = 0 when the beam is cen- tered. With a single-beam pickup, similar information is derived using only the ma

20、in beam; Tracking Error = (A + B) - (C + D) = 0 when tracking is correct. 8. Disc directory is read and dis- played. 9. Unit shuts down awaiting com- mand or goes into play mode, depending upon how it was activated. Some of the steps listed above may be performed concurrently. If any of the first ni

21、ne steps fail, the laser is turned off and the machine will display some kind of error or no-disc message and then return to idle mode, or in the case of a changer, load the next disc and try again. Validating the Startup Sequence WARNING: The procedures below may require access to the optical picku

22、p while the laser is powered. The laser diode is infrared (IR), 780 nm, and for all intents and purposes invisible. The only indication of laser output will be a tiny red-appearing dot of light when the lens is viewed from a safe position of at least 6 inches and an oblique angle of at least 45 degr

23、ees. Dont be fooled into thinking the laser is weak-the actual beam inten- sity may be 10,000 times greater than it appears! However, the red dot is a good indication that the laser is at least pow- ered (though not necessarily that it is operating correctly-an IR detector and/or a laser-power meter

24、 are really needed for that; see below). To narrow down possibilities, try fol- lowing this procedure when the disc is not recognized, but the drawer closes completely: Start by double checking the drawer closing/opening mechanism. Without FIG. l-HERES HOW THE PHOTODIODE ARRAY in a unit with a three

25、-beam pickup is arranged. Also shown here is the geometry between the array and the information pits on a disc. exception, Sony CD players that have belts need them cleaned and eventually replaced. If the drawer does not really close completely, then the disc may not be clamped properly, or other er

26、ratic problems could occur. Once you have verified that the draw- er operation is fine, you need to deter- mine that the lens is clean. In general, the lens should look shiny with a blue tinge. Any gunk or crud can and will degrade performance. You may have to remove part of the clamping mechanism t

27、o get at the lens. If the lens is not perfectly shiny, clean it carefully as explained previously in this series (see “Service Clinic,” July, 1998). A dirty lens-perhaps one not even visibly dirty to your naked eye-can result in any number of startup (or other) problems. Therefore, cleaning the lens

28、 should be done before looking for more obscure mechanical or electronic faults. If lens cleaning does not improve the situation, the next step is to verify that the pickup has reset itself to the inner (cen- ter) track of the disc. If necessary, manu- ally move the pickup away from the cen- ter by

29、turning the appropriate pulley or gear. If there is a linear actuator or rotary positioner (no gears or belts), just push the pickup gently and see what happens when a disc is loaded. If you are not able to move the pickup from one stop to the other, make sure any shipping lock is disengaged! The pi

30、ckup should move smoothly toward the center, usually trip- ping a limit switch and stopping. If there is no movement, if the movement is jerky, or if the pickup gets stuck at some point, then either lubrication is needed or the motor or drive circuitry may be faulty Also check for broken or damaged

31、gear teeth, a slipping belt, and misaligned or damaged tracks. Measure the voltage across the motor that moves the pickup. If there is none, or if it is very low (under a volt or so), then there is a problem with the motor, its driver, or the system con- troller. (well discuss more about motors shor

32、tly). Determine if the machine attempts to focus. On portables, you can simply by- pass the door interlock to get the opera- tions associated with reading the disc directory to begin (you may also need to press PLAY-that need is model-depen- dent). In some component CD players, a disc actually has t

33、o be present to block an optical sensor. You should see the lens moving about 2 mm up and down (at least one of these directions will have a smooth movement) one or two times. If a disc is in place, the lens should quickly stop at the appropriate focus position. Admittedly, however, observing the le

34、ns may be difficult or impossible with the disc in place. Dentists are probably good at this! If the focus action is identical whether a disc is in place or not (it keeps up the search pattern and then gives up), verify that the laser is being powered. In most cases and as outlined above, you will b

35、e able to see a tiny spot of red light when the lens is viewed from an oblique angle during the focus search. From a safe dis- tance of at least 6 inches and 45 degrees or more off to one side, you should be RadioFans.CN 收音机爱 好者资料库 able to see this dim red light in a dark- ened room while the unit i

36、s attempting to focus. If you do see this, you can assume that the laser is at least being powered, although this is not a sure test for an actual IR laser beam or proper optical- power output. In most cases, however, the red light indicates that the laser is working. An IR detector would confirm at

37、 least that there is an IR emission. If there is no dot of red light, then either the laser diode is bad, it is not being pow- ered, or you are not looking from the correct angle. For a more exact test, you can pur- chase an inexpensive IR-detector card from most electronics distributors or build yo

38、ur own using a photodiode, a few resistors, a general-purpose transistor, and a standard LED-all powered by a 9-volt battery. Figure 2 shows the circuit for this device. If you do build one, you will also find it useful for testing IR remote controls. (This is the same circuit we showed you when we

39、discussed remote-control repair in the June 1997 installment of this column.) If the lens is hitting the disc at the top of its excursion, there is a possibility that the spindle table has been pushed too far down, perhaps by something falling on it, for example. A bent shaft and wobbly spindle is a

40、lso a possibility caused by this kind of damage. This kind of fault is much more likely to occur with a top- loading “boom-box” or portable than to a drawer-loading machine. While the lens hitting the disc with the spindle table set at the correct height is not impossible on some players, it is unli

41、kely. On most lens- es, a ring around the outside of the lens itself prevents the critical central area from actually contacting the disc, so acci- dental contact does not usually damage the lens. However, it can scratch the disc. Similarly, if the spindle is too high, the lens might not be able to

42、reach high enough to focus properly. On a player with the height adjusted properly, there is usually about 2 mm between the laser shroud and the bottom of the disc. The spindle height is not super-critical, but if it is way off, you can- not establish proper focus. Incorrectly adjusted focus offset

43、or gain can also result in the search pattern being too high or too low. Either of those can result in inability to recognize discs, noise, or even erratic tracking during play. Once focus is established (and some- times concurrent with this operation), the spindle should start to rotate and quickly

44、 reach 500 rpm. The speed may be ramped up or controlled in some other search pattern since there is no speed feedback until the data coming off the disc becomes available. A partially short- ed drive motor will keep the disc from reaching full speed even though the spin- dle and disc are turning. C

45、heck the voltage on the spindle motor when it starts. It should reach 2 volts or more. If you read less than this, but not zero, a partially shorted motor or a weak driver is likely. A permanent-mag- net-type motor can sometimes be revived by a quick squirt of degreaser through the ventilation holes

46、 and/or by disconnect- ing it (very important, or else you could fry your driver circuit and create a new problem!) from the circuit board and applying 9 VDC or so to it with each polarity for a few seconds to spin off the “gunk.” If one of those fixes helps, you can continue to use the player until

47、 you get a replacement motor (which might never be needed depending on the sever- ity of the original problem). In some models, the design of the driver circuits is the underlying cause of motor failure, and there are circuit-board changes that would need to be performed to avoid the problem in the

48、future. FIG.2-THIS SIMPLE IR TESTER is a sure way to confirm that the laser is emitting an IR beam. You can also use it to test IR remote controls. If the voltage reads zero at all times, there may be a bad driver, or the machine may not realize that focus was established and is not issuing the spin

49、dle-motor start command. The required speed of 500 rpm, just over 8 revolutions per second, can be estimated by using a disc that has a dramatic label, or you can simply put a small piece of tape on the side of the disc that is visible and watch it spin. Keep in mind that a dirty lens can sometimes create symptoms that are similar to those of a bad spindle motor, so always clean the lens first when servic- ing a CD player. I almost learned this the hard way. Once the disc is up to speed, the speed control (constant linear velocity- CLV) and tracking servos will be activat- ed (in some equip