CD player focus/tracking gain adjustment

[justaguy]

Hi everybody, long time reader here, but first post.
I'm trying to fix a Onkyo C711 CD player which I picked up off Ebay for cheap. I replaced the belt of the eject mechanism and it has worked fine for years.
Recently it has started to have problems: during play every 3-5 min it skips a bit, with no audio for maybe 0.5/1s and then playing again ok for a while. This happens on different CDs, no matter of which track.
I have opened it and seeing no obvious mechanical problems I decided to have a look at the waveforms in the optical unit, as shown in the service manual on page 15.
The eye pattern looks quite ok, maybe on the weak side (around 1Vpp) but apparently still within the specs. But the Focus Error and Tracking Error look completely out, I see the waveforms centered around 2.1V where they should be around 100mV / 0V resp. according to the manual. Can it be that those signals are so way off and the player still somehow works?
As I am new to optical pickups, before starting turning pots and messing up things I decided to hear your suggestions.
The CD player is built around the common KSS-240A optical unit.

Service manual is about 6Mb so I cannot attach it here, I uploaded on dropbox

https://www.dropbox.com/s/o4fvlnoltgj4ypt/C711_SM_ONKYO_EN.pdf?dl=0

[coromonadalix]

Many KSS-xxx laser pickup(s) from Sony had a problem over time, the lens where going down in the KSS unit, the lens suspension where going bad

In some cases even doing an adjustment will nerver repair the physical problem, i changed tons of them in the past ...

On the page 16  you have the adjustments procedures : Focus Gain and Tracking adjustments.

BUT    you have a service bulletin  check it

[Haenk]

With the KSS-240A being dirt cheap, don't fiddle around, just replace it. To my experience, there usually is no need to adjust anything after replacement (unless it's a mid 80s player). Just don't forget to remove the ESD solder bridge 
(Funny sidenote: Just acquired a Denon DVD-A1. Uses a cheap Hitachi pickup, which is also only around 10-15 EUR. The player was 4000+ EUR back then.)

[coromonadalix]

You had Pioneer high class cd-rom players with a full copper casing and screws who where very sensitive and was skipping if you made a bump on the table near it,  was around 4k$, and a dirt cheap 200$ cd player was not skipping  loll

And surprise, it had a very cheap laser pickup in it.

[mzacharias]

Clean the laser lens, clean and lubricate the moving parts, sliding surfaces and motor spindles. Fixes most skipping problems.

Don't just jump to alignments.

[justaguy]

Thanks everybody for the suggestions. In the meantime I found my error: the waveforms shown in the service manual are referred to Vc which is around half of the supply voltage 5V, and not to GND as I assumed. They look fairly ok when measured correctly.
I'll start with a general cleaning and lubrication and if that does not help will replace the optical pickup.