17 Jun
2007
17 Jun
'07
4:02 a.m.
I get 1MHz on pin 24 of the FDC chip (as expected) and
about 250kHz on
pin 26 (read clock). There is twice this frequency (500kHz) on pin 11 of
the LM361.
Frequeny on pin 26 is missing.
More interestingly I've checked the supplies to the
LM361. I get +12V on
pin 1 (+ve supply), +5V on pin 14 (output stage +ve supply) _and_ -9.5V
on pin 6.
So there is a -ve supply (I'd half-suspected there was, knowing that
other HP drive units which use a load of transsitors + an LM361 as the
VCO/PLL also have a -ve supply). I've not yet discovered where it comes
from, it must be generated on the controller board (my guess is that
those 2 metal-can transsitors at the front left corner have somethign to
do with it).
Bingo!!!
In my unit there is not a -9.5V on pin 6: I have 0V on that pin.
So the problem should be the missing -9.5V.
A few more investigations around the LM361 chip:
- on input 1 there is always around 3 volt
- on input 2 there is a signal from 0 to 3 volt when the drive reads a disk
- both strobe 1 and strobe 2 are high (4-5 volt, I don't remember)
- there is +12V on pin 1, +5V on pin 14, and 0V on pin 6 as told before.
Roberto