Help on HP 82901 drive

[Tony Duell]

> 
> >I think I'd first check I was getting a master clock on pin 24 (I think
> >this should be 1MHz), and then look at pin 26 (read clock) when the unit
> >was trying to read a floppy.
> 
> On pin 26 there is absolutely nothing: it's always low.
> I traced it back, and found that there is nothing because pin 11 of LM361 
> (output 1) is always low.
> I didn't go back it yet.

I've just checked my unit for you.

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.

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).

Can you check the supplies on your unit. If the -ve supply is missing it 
could explain a lot.

-tony