Laser RF Amp Circuit (PC Engine Duo)

this article takes a deeper dive specifically into the laser diode and photo diode part of the laser assembly, and the rf amplifier controller ic (cxa1081) that drives the laser and amplifies the returned photo diode signal before sending the signals (rf, focus, tracking) out to the servo signal processor (cxa1082bq) that interprets that data the laser head contains the laser diode (the laser that shines onto the disc), the photo diode (receiver diode that reads the reflected laser beam back) and all the motors to move the laser head and spin the disc the main chip we are interested in is the cxa1081 rf amplifier and all its signals let's work through the pins in order and what we should see rf input (pin 1 in) this is the same as the rf output only centered around 2 5v instead of 4v rf output (pin 2 out) this is the eye pattern test point where you measure with an oscilloscope the eye pattern this is the data being read out of the laser, amplified ready to be sent to the servo signal processor a well tuned laser should have an output of around 1 to 1 6vpp it is centered around 4v dc but using ac mode to capture is easier to see if you add persistence of previous signals and trigger on a rising or falling edge, you should see a clear diamond pattern this is a sign the data is being read smoothly and in a periodic fashion rf feedback (pin 3 in) this is a 2 5v dc signal on a well tuned system laser diode p/n (pin 4 in) the laser diode p/n switching you cannot attach flying leads or probe it with an oscilloscope directly without instantly changing its signal and preventing the game loading or seeing anything useful on the oscilloscope you would need an active probe that does not interfere with the signal at all to try and read it, including directly probing with a super short ground lead even just touching this pin with your finger is enough to make the disc load fail interestingly you can lift pin 4 and leave it floating and the discs read fine it is used as part of a feedback into the laser diode amp that sends power to the laser diode, so with this floating i guess it just keeps the laser diode always powered laser diode power (pin 5 out) this pin is the output power driver that controls the power being delivered to the laser diode on a working well tuned system this this should output a flat dc when idle it is 4 5v when a disc is spinning its around 3 6v when the laser diode is under powered, it outputs a pulse tune the laser head potentiometer until you see a smooth dc while spinning photo diode power (pin 6 in) this mimics the laser diode power signal but is offset at 0v not floating around 3 6v photo diode a + c (pin 7 in) connected to the photo diode pins a and c too sensitive to see with an oscilloscope probe but sits around the dc bias voltage so appears as a flat dc photo diode b + d (pin 8 in) connected to the photo diode pins b and d too sensitive to see with an oscilloscope probe but sits around the dc bias voltage so appears as a flat dc vc (pin 9) half of the vcc so 2 5v focus (pin 10 in) focus input into the focus amplifier, from the f terminal of the photo diode too sensitive to see with an oscilloscope probe but sits around the dc bias voltage so appears as a flat dc use the focus error pin 19 to observe errors error (pin 11 in) error input into the error amplifier, from the e terminal of the photo diode too sensitive to see with an oscilloscope probe but sits around the dc bias voltage so appears as a flat dc use the error output pin 12 to observe errors error output (pin 12 out) the output of the error amplifier errors can be caused by the focus (pin 10) and error (pin 11) not summing up (float the focus pin for example will cause 50% more errors than with focus connected) they can be caused by tracking errors, focus errors, and defects sits at 2 5v while idle, and pulls down to about 1 9v offset, with errors shown as ac waves around this bias point when there are errors reading data error input (pin 13 in) the feedback input of the error amplifier used to adjust the error gain shows waves on top of the dc bias when there are errors reading data should be totally flat dc when reading well here is how it looks when it's reading poorly with errors vr (pin 14 out) half of the vcc so 2 5v cc2 (pin 15 in) the defect bottom hold outputs are capacity coupled to this input will pulse when it detects errors cc1 (pin 16 out) the defect bottom hold output will pulse when it detects errors vee (pin 17) ground fe bias (pin 18 in) the bias pin for the focus error amplifier this is set via vr102 fe (pin 19 out) focus error amplifier output this pulses around the dc bias every time the focus has an error locking into the disc data note the focus error output won't register anything until it detects a disc is present with no disc on the output will be the dc bias point here is a perfectly tuned successful game load notice you still see focus errors, but they are acceptable and allow the game to load still here is a poorly tuned laser or a disc being held in place to force errors, where the focus errors show a little more aggressively te (pin 20 out) tracking error amplifier output this pulses around the dc bias every time the track has an error staying inside the track line here is a perfectly tuned successful game load notice you still see tracking errors as all discs are not perfectly helical in their shape for data so there will always be corrections here is a poorly tuned laser or a disc being held in place to force errors, where the tracking errors show much more aggressively defect (pin 21 out) the defect comparator output when it fails to read data, it pulses high if you hold the disc still to prevent it spinning for example this would show multiple pulses to 5v as its constantly failing to read data mirr (pin 22 out) the mirr comparator output this is a super useful pin it shows the actual usable game data as the game loads you see pulses this is a long capture of the boot up of the japanese game secret garden close up you can clearly see the data being loaded when the game is idling and not needing data it pulses once every 133ms in this particular game cp (pin 23 in) has a hold capacitor connected to the mirr this is the non inverting input of the mirr comparator sits at 1 4v jumps to 3 3v when reading disc cb (pin 24 in) has a hold capacitor connected to the defect comparator sits at 2 5v when loading game data it drops to 0 9v dgnd (pin 25) digital ground asy (pin 26 in) auto asymmerty input sits at 2 5v efm (pin 27 out) efm comparator output shows 5v digital data at idle its short random bursts when a game is loading, the data is wider fok (pin 28 out) focus ok comparator output 5v when the laser has focus, and ground when it doens't have focus !ld (pin 29 in) inverting laser diode active pin low when laser diode is turned on 5v when the diode is off vcc (pin 30) 5v supply power rail