Hi all, I am currently working on fixing a WM FX-37 I got for cheap because it was broken. I need to replace the belts, but while I was testing the other parts I see that the motor is not stable. I have made a video and you can hear what I am talking about. When I test the motor directly from a power source the motor is stable. This leads me to believe there is something wrong with the power going to the motor. Can any of you assist me in what I need to look for? Or have a fix for this issue? https://drive.google.com/file/d/17BOf22X7oFkyDXAJlf7aZ-7N6V68BNIV/view?usp=drivesdk I have put the video in my drive with a link, I hope this is allowed. Also attached is the service manual for your convenience. Thanks! Job
The radio works fine, so I don't believe that it is a power regulation issue. I think it is eiter a capacitor, or the servo amp. So I think it must be Capacitor C604, which is a radial electrolytic capacitor in the motor circuit, or the servo amp MM1038F. Any clue on which component could cause this issue? or if I'm looking in the completely wrong spot? Im not very experienced in electronic troubleshooting.
C604 is on the main B+ rail and is not critical for circuit operation as far as speed is concerned. Only sitation where it could cause this symptom is if it's failing short-circuit intermittently, but in this case you should also see the power LED dimming when motor speed is going down. Most likely scenario seems to be servo IC failure.
Hi, thank you for your reply! The power LED is indeed dimming when the motor slows down! So you mean there is a short in the motor? Or where should I be looking to fix the short? Thank you!
Not in the motor, in C604. Remove C604 from circuit to see if it fixes the problem. If it does, replace the capacitor with a new one.
Unfortunatel desoldering C604 did not change the behavior. So then it seems like the motor IC is the issue. Can I troubleshoot this to be sure?
Measure the DC voltage at motor terminals with and without load (with cassette/without). For more load you can stop the takeup reel by hand. If servo is working correctly, increasing load should increase the motor voltage trying to compensate for the increased load. Also variation in motor voltage during normal operation should be minimal, like in the order of dozens of mV.
