Help on HP 82901 drive

Roberto Bazzano rbazzano at
Sun Jun 17 06:02:49 CDT 2007

>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).

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.


More information about the cctech mailing list