Help on HP 82901 drive

Tony Duell ard at
Sun Jun 10 15:15:12 CDT 2007

> Hello.
> I have a partially defective 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 found the problem on the controller card.
> I found a service manual on HpMuseum site, named 
> "82901-82902_CEServiceHandbook-Partial_17pages_1982.pdf". Following it, I 
> made all self tests and the problem is on the read circuit. I'm sure about 
> that because the two Tandon drives work fine with another controller card.
> In this case, the service manual suggest to verify read oscillator 
> frequency. Unfortunately the section VI of the manual describing the 
> procedure is completely missing from the pdf file.

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.


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