lvbi maintenance log

June 6, 2002

Write test tape and read back (a new "Dan's tape"). (written with stack 1).

Displays say signals from odd heads have high PE rates. Even heads look ok.

To see if we have a write problem or a read problem:

Use lvdt=,2.547 to displace read stack one head pitch.

For Pass 9 (the fwd pass), the lvdt voltage is -1.666. To this we add 698.5/165.79, where

698.5 is the head pitch in microns and 165.79 is the read head lvdt constant in microns per volt. This gives 2.547 volts.

Result:the displays still say the odd heads have high PE rates while the evens are still ok.

Conclusion: We don't have a write problem, since a good read channel will read good data from any track and a bad read channel will read back bad data from any track.

To see if the read problem is in the preamps at the head or down stream in the read module,

we interchange the even head cable and the odd head cable going from the preamps to the read module.

Result: The displays now say the odd heads are good and the even heads are bad.

Conclusion: The fault lies in the preamp for the odd heads.

Jun 7,2002

Analyzed Read Preamp electronics. Spice analysis shows amp has voltage gain of 36 (28.67dB) yet gain stage has Rc/Rc = 3300/47 = 70. The factor of two is caused by the emitter signal voltage on this stage being half as large as the base signal, rather than the emitter signal 'following" the base signal. A closer analysis shows that Vb/Ve= 1 + Rb/(Re[ +1]), where

Rb, the "base resistance" is Vt/Ib which here is 26mV/[0.563ma/100] = 4600. Thus

Vb/Ve = 1 + 4600/(47[101]) = 2. The gain of the stage is therefore 1/2 3300/47 = 35, which agrees with the Spice analysis.

Running an ac analysis on the circuit shows it to have a surprisingly narrow bandwidth: the upper 3dB cutoff frequency is 4.5 MHz. At 16 MHz, it seems to be down about 12 dB (fcator of 4 in voltage).

June 11, 2002

Look at read output (scope monitor) with tape stopped. Odd channels are noisier, as expected, and show a periodicity that is likely oscillation (which Dan Smythe warned could be present).

Use FFT in Tek scope to find odd channels do have a strong component at about 12 kHz, with second harmonic ,about 6 dB down, near 25 kHz.

Nov. 5, 2002

Begin looking for Head 18 problem. Head 18 seems bad with head cables normal or reversed,

i.e. using either headstack, suggesting problem is ahead of the head. But interchanging the write drivers had made no difference. This suggests problem is ahead of the drivers. These tests done some months ago. Today we wrote a new test tape. Using repro=bypass,18,18,bypass. we could see as we wrote that the recovered clock was very jittery for head 18, again suggesting that the problem ahead of the heads. Could be problem inside the foratter, in the cables from the formatter to the VME write module, the cables from the write module to the write drivers.