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Technics SL-P8 - help diagnosing strange eye pattern
Technics SL-P8 - help diagnosing strange eye pattern
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Old 20th January 2019, 07:32 AM   #1
TechnicsAg is offline TechnicsAg  United States
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Default Technics SL-P8 - help diagnosing strange eye pattern

First post here! I have a Technics SL-P8 cd player (Japanese 100V version, using a step down converter) that I've been trying everything to fix. It plays some discs fine but others not at all. I have another SL-P8 that I fixed previously by cleaning/lubricating the worm gear and laser rails and replacing the drive belt and some failed capacitors on the head amplifier board, so I tried all that to no avail. I then suspected the optical pickup was going bad; I tried taking it apart and cleaning the optics (almost killed it doing this), but neither that nor swapping it with a pickup from a similar unit did the trick. I also replaced all the electrolytic capacitors in the player (power supply, servo board, digital board, etc.) hoping to get lucky, but no dice.


Eventually I was inspired to pick up an old analog oscilloscope to try and hunt down the problem, and I noticed that the RF eye pattern looked strange - it had jagged peaks instead of smooth curves. Adjusting the focus/tracking servos didn't seem to affect the peaks, and neither did tweaking the laser power up just slightly, although it did increase the amplitude of the eye (I turned it back down to be safe).


I've attached pictures of the eye pattern on a 'good' CD playing with no issues and the pattern on a CD that reads the TOC but won't play. The scope was set at 200 mV/div, 0.5 msec/div. Any insight you guys might have would be a huge help!
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Old 20th January 2019, 07:48 AM   #2
Mooly is offline Mooly  United Kingdom
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Technics SL-P8 - help diagnosing strange eye pattern
Hard to say.

First question has to be whether you are using the correct ground point for the scope. You need to use the ground point on the pcb related to the RF amplifier.

That's pretty high frequency noise you are seeing there. Is your voltage convertor just a passive transformer or is it some other box of tricks

If you connect the scope tip probe to the scope probe ground lead you should see a 100% clean trace. Any radiated noise external to the lead would show as a noisy trace as it induced current into the lead.

Assuming you see a clean trace, try connecting the still shorted probe tip and ground lead to the ground point in the player you are using. The trace should still be clean. Is it?
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Old 20th January 2019, 08:14 AM   #3
percival007 is offline percival007  England
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Looks like noise on the Signal as Mooly says above. Try another 0V point.
Nothing to worry about really.

The right hand Trace, the one I presume doesn't play, is lower in pk-pk value. I suspect that this is why it will not play.
The disc itself will not be as 'reflective' as the one that does and therefore does not produce as much info from the Laser block.

I would suspect the Laser is on its way out.

P.
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Old 21st January 2019, 04:45 AM   #4
TechnicsAg is offline TechnicsAg  United States
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Thanks to you both for the advice...still working on isolating the issue (and also developing basic oscilloscope measurement skills ). Mainly hoping its not the laser, but I might have to face facts.
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Old 24th January 2019, 08:22 PM   #5
Extreme_Boky is offline Extreme_Boky
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The good looking pattern can be seen while the disc is playing. The bad looking pattern may be okay -> while the TOC is being read.

The RF signal can easily be tracked from the laser, via an RF amplifier, towards decoding and DAC. Try that... the good working player should help in this regard; compare the signals (oscilloscope), while tracing the RF signal from where it enters the PCB, between the good player and the bad player.

The problem might be related to the uPC chip...

Last edited by Extreme_Boky; 24th January 2019 at 08:27 PM.
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Old 4th February 2019, 07:34 PM   #6
TechnicsAg is offline TechnicsAg  United States
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With the benefit of the scope, I tried replacing the optical pickup again (I had a spare from an SL-P1 parts unit), and it turns out that was indeed the problem - the RF eye pattern looks much better and the amplitude is around 2V p-p.

However, there's a second issue. On certain discs the turntable won't spin up and the TOC won't load when the CD is loaded using the tray. But, if I turn the unit on with the 'problem' disc already in it, 9 times out of 10 it will work fine, especially if I give the disc a little manual spin before turning it on. I don't think it's a disc reflectivity issue because the RF signal is normal when I can get these discs to play, and it plays other known problem discs fine. Could this be related to turntable height, or maybe the spindle motor drive circuit?
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Old 5th February 2019, 07:41 AM   #7
Mooly is offline Mooly  United Kingdom
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Technics SL-P8 - help diagnosing strange eye pattern
Is 2v pk/pk a normal amplitude for this player? It sounds high compared to the more usual 1.2 to 1.5v pk/pk generally seen in most players.

Having first noted accurately the level on a given disc, it might be worth turning it down a little and seeing if that helps.

Other possibilities could be a sluggish platter motor... not sure what this model uses offhand, whether its a traditional DC motor or a Hall effect type. If the later then its unlikely to be a problem.
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Old 5th February 2019, 01:46 PM   #8
TechnicsAg is offline TechnicsAg  United States
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The service manual says that 1.5-3V p-p is normal, and that less than 1.5 indicates a bad laser. It does have a hall effect spindle motor
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Old 5th February 2019, 05:44 PM   #9
Mooly is offline Mooly  United Kingdom
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Technics SL-P8 - help diagnosing strange eye pattern
1.5 to 3 volts is a huge range... I would still look at investigating what effect altering the power has. Just be sure to get a valid reference first so that you can always set it back.
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Old 12th February 2019, 05:28 AM   #10
TechnicsAg is offline TechnicsAg  United States
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After tearing my hair out over this for a while longer, I think I finally got it solved! The few discs that wouldn't spin up were all from the same record label, so probably the same manufacturer. I noticed that the 'hub' area in the center of these discs was slightly larger than others that were working and guessed that the rest position of the optical pickup might be too far in towards the center of the disc. All it took was a very slight tweak of the limit switch and the discs all loaded normally. I'll definitely do some confidence testing for a few days, but I'm crossing my fingers that I finally have this player back to fully operational condition. Thanks again for all the help!
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