Lazy Technics SL-PG490 ?

enryfox

Member
2019-02-06 8:08 pm
Recently I have been trying to get a working Technics SL-PG490 but failed so far. I actually got two of them and more or less they ended up with the same problem.
The first one I got could not read any disc and I bought a new optical pick-up: It worked fine for 30 minutes then it started developing problems.
The second one I got was sold as working and indeed it was working when I tested it. But after a week it started having problems too.

The problems they both have is the following: when started from cold they even fail to detect the disk; after some minutes they detect the disk but might fail to lock to the track (tracking issues). After some time they start working more or less fine even if tracking is pretty weak and they might skip easily.

The SL-PG490 has a CD mechanics which is common to all Technics CD players of those years (SL-PG380/480/580, SL-PG390/490/580 and even higher end model) based on the Philips CDM12.1 with a single chip basically controlling all servo's and handling the audio signal from the pickup to the analog out (with 1 bit MASH DAC).

I checked for cracked solder joints but could not find any; I checked for voltage levels drifts over time but they are all pretty stable. I checked voltage levels on IC pins and the only discrepancy I found is on pin 16 of IC 701 (it amplifies all optical pick-up photodiodes output - see attached snapshot). Pin 16 should be H (5V) when in stop mode but it is indeed at 0V (L); OFTR is the off track signal which is passed to the main IC. Is the IC faulty or is the manual wrong ? When trying to read a CD, pin 16 levels varies from H to L so it is not stuck to 0V.

I really do not know what could be wrong, both players I have exhibits the same problem with different degree of severity, but it appears mainly linked to tracking problems. They both have a faulty CDM12.1 ? Is it something in the IC's ? How can the same optical pick-up fails to read the disc and then, after some time, read it with no major problems.
Is it a faulty design ?

Thanks for any info.
 

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Take the spindle motor out, wash it in methylated spirits; place it in a little jar full of methylated spirits and shake it well... and see if that works...

The rotor's commutators get shorted over time...the starting torque drops substantially because more than 2 poles are "magnetised" at stat-up...
 

enryfox

Member
2019-02-06 8:08 pm
A motor issue could be an explanation, but I was mostly thinking to the sledge motor not able to accurately re-position the sledge. The spindle motor would mostly cause issues with the CLV servo locking, while in my case it mostly fails locking the tracking servo.

Anyway even a motor failure would not explain all issues: the failed cold start (it won't even focus !!), the wrong logical level on IC 701 pin 16, the fact that some CD's are read flawlessly while other fail to even read the TOC ...
There are many CD players of this family (390/490/590) on sale and several have the "no disc" problem; I'm starting to think there is a latent design fault or weakness in the components.


I have a similar Philips CD 723 with the very same CDM12.1 traverse unit and it works perfectly even reading CR-RW.
 

Extreme_Boky

Member
2003-12-07 11:57 am
A motor issue could be an explanation, but I was mostly thinking to the sledge motor not able to accurately re-position the sledge. The spindle motor would mostly cause issues with the CLV servo locking, while in my case it mostly fails locking the tracking servo.

Anyway even a motor failure would not explain all issues: the failed cold start (it won't even focus !!), the wrong logical level on IC 701 pin 16, the fact that some CD's are read flawlessly while other fail to even read the TOC ...
There are many CD players of this family (390/490/590) on sale and several have the "no disc" problem; I'm starting to think there is a latent design fault or weakness in the components.


I have a similar Philips CD 723 with the very same CDM12.1 traverse unit and it works perfectly even reading CR-RW.

You may need to adjust the laser power..? Search the threads for either my name, or for an eye pattern.
PS the motor did not fail.... the commutator copper segment can get shorted with carbon deposits from the brushes, so I'd give a clean a try as per my 1st reply
 

enryfox

Member
2019-02-06 8:08 pm
I should definitely check the eye pattern, but at the moment I do not have an oscilloscope ...

Regarding the wrong level of pin 16 of IC 701, reading the troubleshooting procedure I find out it is actually correct at 0V in STOP mode. Service manuals are not always 100% reliable.


I never had any luck adjusting laser power level, it is really a desperate move: the laser has a back face photodiode which adjust the drive current to keep the output power constant. Increasing the laser power can be a sensible operation to compensate for cloudy optical elements, but if the laser is EndOfLife, increasing the drive current just kill it faster.
 

enryfox

Member
2019-02-06 8:08 pm
After tweaking, cleaning and lubing the traverse, I finally broke the CD player.
I took apart the CDM 12.1 to full clean the sledge rails, but the disassembly and re-assembly, killed it completely (I was wearing ant-static wristband though).
Up until yesterday it would read most of CD's (albeit with occasional skip), but now it fails to read the TOC and keep moving the sledge back and forth forever. Service mode reports the pickup is failing to lock on the track, so it is again a tracking issue. I checked all PD's with the diode tester and they all measure fine (~ 1V). I now have two players with the same problem, they both detect the disc, but fails to lock on track. Is in both cases a faulty CDM 12.1 ? I also tried to adjust the laser with the pot on the pick-up, but, as always, it made the situation worse, maybe that is the final nail in the coffin for the traverse.
 
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