Technics SL-PJ1 constantly spinning discs without reading them

I just got a Technics SL-PJ1 off of ebay for $30 that claimed it was "tested and working." However that was not the case. I temporarily replaced the main drive belt with an O-ring just to see if I could get it to work, cleaned the wormgear, and reseated the laser guide rails, however it never seems to move the laser all the way back in order to read the disc. Whenever the tray is closed with or without a disc, it moves back maybe a centimeter or so before moving back forward and trying to adjust focus.

Just to make sure the reason for this wasn't because of my temporary belt solution, I moved the laser all the way back when I went to clean the wormgear. When I powered it back on it moved all the way to the front, so there wouldn't seem to be an issue with torque. I took a closer look with a flashlight during operation and it does appear that the motor only moves the laser back a bit before reversing direction, so the "belt" isn't slipping.

I have also checked to make sure the back rest switch isn't making contact with itself and it isn't, so that rules out that possibility; and I have cleaned the laser lens, but since when has that ever worked?

Any help would be much appreciated. My biggest fear is that the laser is shot and this is now a hunk of junk.
 
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Hi CAPTAINKURS,
No, not a reasonable conclusion yet.

The feed motor may have a dead spot in the commutator. It isn't supposed to do a focus search until it hits the laser in switch, then it moves away slightly.

Silly thing first. Make sure the worm gear isn't slipping on the motor shaft if the motor checks okay. Make sure no gears have even the slightest crack in them. Disengage the drive and make certain the laser head moves freely and easily if you haven't done that. Monitor the laser in switch signal to see if it isn't being help low by something else.

These things run on firmware, so unless the MCU has gone a little stupid, that won't change. I have seen Technics MCU's lose programming on turntables. Then you're toast.

I'll assume you got the service manual and am following it.

-Chris
 
Hi Anatch,

The motor itself is fine since it was slipping on the original belt. Before I went in the fix it the wormgear was actually loose as it was out of the clip that holds it in place. It was working fine until I put it back in place and started slipping which prompted me to see if an O ring would do the job. While it seems to work in one direction it's possible it doesn't work as well in the other as I noticed the original worn belt had more torque going forward than it did backward when I went to replace it.

The little plastic piece that is attached to toe wormgear is fine as I was able to rotate the wormgear with my finger and move the laser sled forward and backward.

As far as I am aware, there are no gears as it is just the motor attached to the wormgear with a belt and then the sled is attached to the wormgear.

The sled also moves freely along the rails when not attached to the wormgear.

As for monitoring the switch signal, I don't have anything that can do that.

As for the service manual, I have been using one online from an SL-P1 since I couldn't find an SL-PJ1 online. Since they seem to be basically the same, I figured it would mostly transfer over.

I ordered a proper set of belts, so I'll have to see if it helps when they come in.
 
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`You'll need an oscilloscope to check if the motor is good. Also to check the power supply voltage in case the motor drags it down in a short spot and maybe resets the MPU.

Normally we would run the motor through a resistor and monitor the voltage across the motor or across the resistor. That's with no load and through a variable power supply (disconnected from the CD player). The motor runs at slow speed.

Never make assumptions. Especially if you are inexperienced, good experienced techs don't make assumptions either. Cracks in nylon can be hard to see and open up under load.

Hopefully the belts will help. I wouldn't assume the two manuals are close enough to use. They might be - or not just as easily.

Do you have a half decent DVM? Oscilloscope? CD players do take some specialized equipment and knowledge to repair. Some easy checks can give you clues. But those easy checks do not mean CD players are easy to fix. That and once you getting running, it isn't fixed until it is aligned. Anyway, I'll leave those questions to you.

No, the internet will not teach you how to properly service anything. People who atually do know what they are doing don't have time to post articles and videos.
 
Yeah, unfortunately I don't have any specialized equipment. I mostly work on cassette decks where all they need is some new belts and a good cleaning and they're working again. There's been a reason why I've never touched broken CD players and this is why. I appreciate your assistance though, you've been very helpful.
 
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Well, I'll try.

You know, for cassette decks there is over $1,000 in test tapes and gages you need. My set for Nakamichi was $10,000 at the time. I did a lot of tape products and for recording studios. You don't want to know how much the 1/4", 1/2" and 1" tape cal sets cost me. Then there are the other bits of test gear.

For any audio service, you need at the minimum, good THD meter, 100 MHz dual trace scope, Good bench meter good to 50 KHz minimum, 250 watt non-inductive dummy loads, low distortion audio oscillator. This was the minimum you needed to be authorized for basic service. Never mind tension meters, force scales, various component testers (real LCR meters) and soldering stations (irons are not allowed). An AC voltmeter was much easier to use than a DVM, often a dual channel AC voltmeter was demanded. I have everything and more.

Always check the capstan bearing first on any tape machine, R-R or cassette. If that is worn, forget it. The machine is done.

For CD players, a laser power meter is very helpful, plus filter jigs and test CDs. Then you need the previous items I mentioned. Your scope needs a 0.5 uS/div horizontal scale to see an eye patten well enough to assess it.

-Chris