CD614 servo problem

I have been working on a Philips CD614 to improve its servo circuit/laser supply.
Originally this circuit was not regulated or decoupled. So i added regulation and decoupling and sound improved dramatically.
Now what happens when i turn off the player the mechanism shortly turns backwards and the laser arm swings to the end making a 'click' noise because it hits the 'wall'.
I think this is because there is still some dc in the capacitors and specially the -10 discharges slowly.
How can i prevent this? Or did i do something wrong?
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
If it didn't do this before your mods then you have to accept that you have changed something...

The swing arm reacts almost instantly to voltage across its radial drive coil (like a moving coil meter) and so any change in power supply rise and fall times (time constants) could easily cause such behaviour.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Better perhaps in some respects, worse in others...

You need to to look at why this happens and that may simply be a case of rails not collapsing at the correct rate, and the timing of such rails with regard to others.

It is often important that rails are sequenced correctly (to avoid this and so called 'race hazard' situations) and that is down to the designer/manufacturer of the equipment.

I've no easy answer for a fix... the complex route would be separate muting for the arm motor coil and platter, a bit like a speaker relay that drops the instant power is removed.

The best you can do without going down that route is to experiment with rail timings by tweaking cap values and a first step might be to look at the rails on a dual beam scope and see how they collapse relative to each other.
 
In the end studying the original design and I came to the conclusion that the minus rail was having way to big smoothing capacitors for what it actually uses. Originally none, and now equal values as the plus rail was just not 'even' in a way to the actual usage.
So it works fine now, no more laser banging to the casing ;-)
And thanks Mooly for pointing me to the right direction although it took some time and experimenting.
 
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