Servo and data zapped by damaged preamp

[pepe]

Hi Guys,

Just thought this topic might be interesting for everyone on the board...
Just an experience, and a bit of warning.

A damaged preamp with good PCB can do wonders to the data... and servo as well.
This drive came to me after some DR firms took some measures. The servo could have been killed before they got it, or during pcb swap trials, we will probably never know. But it is wise to check preamp before starting a drive after pcb swapping. Otherwise we can easily get similar results.
Some comment to the images:
0. Red signal is a longer sample of the head signal, the purple one is a zoomed portion of the red one.
1. it is normal for the data signal to have modulation over the sampled period, this is a result of the read head crossing tracks when cannot quite lock to servo.
2. The servo signal should not disappear even if the head is somewhat offtrack, so what we see in the first 2 images is absolutely talking about servo signal problems. They should be quite constant in amplitude, as in the third image.

Servo signal vanishes in the ether...

bad deeds of a damaged preamp with uncontrolled writing current

H1 has pretty normal signal, just a bit weak, as that head was weak. that's why it cannot really lock servo.

Signal from H1... Looks a bit better.

pepe

[BlackST]

Talking to the walls, Pepe. Seen soooooo many times and it was almost always happened in consequence of uncontrolled pcb swap after hw problems or attempts to repair.

[Alexii]

Thx Pepe =) Looks like fun.
Edit LeCroy oscilloscope =) nice. What model ?