So long story shot i got an fm towns in not so great condition a while back , and have been slowly getting it working , I've already replaced the PSU and managed to get to run columns and also had to repair a trace to make the volume panel work but the cd drive wont read discs , it spins up then stops , replacing the laser had no effect , any ideas?
A problem I had with my CD drive was due to the lid close switch being dirty so it wouldn't register that the lid was closed and so the CD drive would not start. To test whether that is the problem just bypass the switch and see if the CD drive works. Also you can try to measure resistance in the switch when it is closed, it should be near zero.
Clean it using contact cleaner if it has internal resistance.
Thanks for the suggestion , I checked it , I'm getting about 0.70 ohms of resistance , is that about right?
Well i bypassed the switch but it didn't help , its still spinning up the cd , making noises then going ka chunk and stopping. The CD player seems to be getting the 12V it needs. Would adjusting the potentiometers help?
Okay so I managed to prop the CD PCB up and it looks like the worm gear and the belt isn't moving when the cd is spinning.
Does the laser diode light up? If you replaced the laser it might need adjustment before it can detect that there even is a disc to read.
Is there a way of checking if the laser diode is sending out a signal using an oscilloscope and measuring the drives pcb?
That is above my pay grade. I would suggest you mark where the focus pot is located then try adjusting it to see whether the reading process will make it further. Just turn it a bit, turn on the machine and observe if it makes it further, then do so again. Start from one extreme end to the other.
I forget how many pots the FM towns CD mechanism has, and how easy they are to reach. I know the one on the Marty was a complete pain in the ass to get to.
It has 6 , I tried adjusting them but no dice , also the drive makes a creaking and clicking sound but adjusting the pots can make it go away.
I also powered the turntable motor and stepper motors with batteries and they spin , checked the back stop switch with a multimeter which , does work , and made sure the worm gear and shaft would move the laser assembly via battery , and I've lubricated them both.
So my assumption is the noise might be coming from the laser unit itself.
Also just to check , I'm using a regular music CD as a test , that should work right?
I could adjust the lasers power too i guess but I'm worried about killing it.
6 pots, good god. I remember trying to adjust a 3 pot CD mechanism once and it was a nightmare, they all affected one another. The noise is usually an indication of the CD laser struggling to focus, so it is a good way of assessing whether one is on the wrong track.
In the past I heard that some lasers are shipped with two solder pads bridged to avoid static electricity damage, but upon installing one is supposed to remove the bridge, not sure if that is the case with yours. I suppose if the laser diode produces light it is already ok on that front.
Without an oscilloscope, test points and references to judge performance adjusting a laser with 6 pots is quite a Sisyphean task. It might be doable that you can get lucky and adjust everything just right, but the odds are quite minuscule.
I would return all pots to their original position and just try adjusting one at a time to see whether just one pot can bring life back to the CD reading.
I forget whether FM Towns has a built in CD player, so not sure if an audio CD is the correct test. At the very least it should try to read one a little.
Yeah this is from another NFG post and it seems to be the same setup for mine.
VR1 PD.BAL -- photodiode balance (?)
VR2 TR.BAL -- tracking balance
VR3 F.GAIN -- focus gain
VR4 F.OFF -- focus gain
VR5 T.GAIN -- tracking gain
VR6 T.OFF -- tracking offset
In terms of the static protection I compared both lasers and they were both the same so either the solder pad wasn't bridged or it had been removed at some point , so that shouldn't be an issue.
But yeah I do have an oscilloscope , one of those portable little zeewii ones.
I'm just not sure where to measure and what values to apply , I do wish there was a CX 20 service manual.
I guess I probably should set them to the OG positions , which i did mark with a marker , and just do tiny adjustments till I get some kind of motor movement.
Thanks for all the suggestions so far.
I've seen some videos on YouTube of people adjusting CD lasers using an oscilloscope. There might be videos explaining what you are supposed to look out for. I imagine you would have to have some detailed technical understanding of how CD data is read and processed to be able to interpret readings form an oscilloscope.
Before ruining your drive, tell us the condition of the bipolar capacitors in the drive.
I assumed capacitors had already been replaced, as that is always the first point of suspicion for malfunctioning vintage Japanese PC hardware. If you have not done so, then for sure, rule that out. Hell, you may as well replace the caps in the CD drive whether they are out of spec or not, I replaced all the caps in my 20F as the first thing.
Quote from: kamiboy on October 17, 2025, 04:49:57 PMI assumed capacitors had already been replaced, as that is always the first point of suspicion for malfunctioning vintage Japanese PC hardware. If you have not done so, then for sure, rule that out. Hell, you may as well replace the caps in the CD drive whether they are out of spec or not, I replaced all the caps in my 20F as the first thing.
I think change caps won't help.
FM-TOWNS are famous for their problem in laser pick-ups. I have twice with laser not working (one was fine for only a week, then died). I also try to take the laser from an old cd-player, but with no luck. I'm using the bluescsi drive as cd now, waiting for an ardscsino plus with CD-DA support.
As cool as booting from an actual CD is, I have to admit, an ODE is just so much more convenient and functionable.
Erm , are the CD Drive's bipolar caps particularly problematic or something? , I did replace one or two polarized ones and i checked the ESR of the originals and they measured the correct values so.
I also adjusted the laser power of the original pickup but it didn't help ,I have the spare laser installed at the moment , I upped the potentiometer far less on that one , just to be sure not to overdo it.
But that laser also just sorta spins the disc for a few seconds and stops like the original laser , I tried messing with the 6 adjustment pots but no dice , I guess ill try actually hooking up the oscilloscope and measuring the signals since nothing else has worked.
If you have tried all the software-based troubleshooting steps and the drive still won't read discs, it may be a hardware problem.
Okay so I ended up getting a second CX20 unit , which i needed as my first was not in the greatest cosmetic condition , this one does read commercial CDs (i got Battle Chess to run) but not burnt CD-Rs.
If I tweak the laser a smidge would it run burnt discs? , I tried to get it to boot Populous but it gets stuck on reading it.
I know dialing up the laser is a bit of a risk , but I'm willing to give it gamble if there's a chance.
Don´t do it. Every Laser from FM Towns i tried to tune was dead after that. Try a little bit with Gain and Offset.
Right , well so how do Gain and Offset help , do they focus the beam more?
Hookay so bit of an update ,
I managed to actually get it to read burnt discs , so what I ended up doing was to use my Windows 98 retro PC to burn some CD's at 4x speed with an old pioneer burner from circa 2000-2003ish and so it looks like I wont need to adjust anything. I'm pretty chuffed right now.