Help on HP 82901 drive

I don't know this drive by model number, but IIRC it's a daul 5.25" drive with an HPIB interface. I have an HP drive that fits that description, unfortunately I can't get to it quickly, so I can't verify the model number, but I do remember some things about it. I would strongly recoemdn that you don't _adjust_ anything at this point. It's fine to check the freqeucny, but don't tweak anything until you are sure the fault is just misadjustment. Anyway, from what I remeber, the controller card is based round a 6802 processor [1]. The actual disk contorller chip, anohter 40 pin device, is made by Fujitsu, and has the same pinout as the 1793 _apart from being +5V supply only_. [1] If you've got chips marked with the HP house codes (1820-xxxx numbers, maybe soem 1818-xxxx ones), let me know what they are and I can see if I can find the standard equivalents. That mioght be enough to get you started. Grab a 1793 data sheet and check the clocks at the pins of the disk controller. There is probably a proper procedure for adjusting the free-running requency of the data separate VCO, but at this point, what you want to do is see (a) is there a read clock and (b) is it possibly a sane frequency. I can't remember much abotu the PSU in this unit. IIRC there is a seaprate PSU PCB (and presumably you can check the outputs of that quite easily). It's quite possible the VCO circuit needs a -ve supply line. This might come from the main PSU, but on at least one HP controller, it was producaed by a charge pump circuit clocked from a divided-down version of a master clock. That might be worth looking for and checking. If you're really stuck, I'll excavate my unit, pull the covers, and see what I can quickly deduce. -tony