Hamurabi Focal source

Tony Duell ard at p850ug1.demon.co.uk
Fri Apr 2 14:05:51 CDT 2010


> > I restored one of thoise a couple of years back -- or actually the -A 
> > version which has simpler electronics (but much the same functionality). 
> > I got the HP9800 interface for it (HP11202 Opt A01 IIRC) on E-bay. As you 
> > probably know the outputs of the 2748 are pilled up to 12V, but through 
> > 10k resisotrs, whcih means they will feed the 180R/390R-terminated TTL 
> > inputs of the 11202 with no prolems. Of course the -B model that you have 
> > has clamp diodes anyway so you can force the outputs to TTL levels by 
> > jumpering 2 pins on the connector.
> 
> Indeed.  I was a little worried about depending on that jumper (and the
> associated connector pins) to ensure that the zener clamp would stay in

THe -A model doesn't have the clamp diodes or jumper...

HP connected the outputs of the 2748 stragiht into the TTL level inputs of
a 11202 interface. The reson nothing got fried is that the 11202 has a 180
Ohm pull-up (to +5V) and a 390 Ohm pull-down (to ground) on each input
while the output colelctor resistors in the 2748 (to +12V) are 10k.  If
you work it out, the votlage at the IC input is well within TTL specs. 


> place else fry my microcontroller.   So, I put a front-end on the micro
                    ~~~~~~~~~~~~~~~
I might have guessed....

Mind you, when I wanted RS232 output from my 2748, I used a set-up with a 
few _hundred_ ICs... The fact that there was an HP9830 calculator 
involved has something to do with that...

> using 26LS32 line receivers and accept the 12V levels directly or any
> level above about a volt I guess.
> 
> > What was wrong with yous? Mine had a bad mbearing in the magnetic clutch. 
> > T oreplace that I had to make special tools to press the clutch unit 
> > aprt and put it togetehr again. The ball race itself was a standard size 
> > and I had no problems getting one. Of course I also had to make the 
> > alignment mandrel to ge the clutch parts centred up wehn I put the motor 
> > unit back on. 
> 
> Wow.  I was much more lucky.  Mine had numerous burned out blubs in the

Just after I got mine working, somebody listed on E-bay a 'DEC 2748 
head'. It turend out to be the read head module for this reader. I bid on 
it, and fortunately nobody else was interested.... 

> illumination head and a bad transistor on one of the detector channels.
> And it was pretty dirty inside.  But, mechanically, I think I lucked out.
> 
> > There are a few photos in my flickr account (tony_duell) of the internals 
> > of the read head and of taking the motor/clutch apart.
> 
> Cool. I'll check it out.

If you d oever take the read head apart, you will need a couple of 
alignment pis to get it back together. The reason is that the metal block 
that holds the photrasistors is not ground, it's connected to a supply 
rail (-5V IIRC). It therefoe sits on insulators. To gt everything alighed 
you put the alignment pins into a coule of blind holes inside the read 
head. slot on the insulator sheets, then the phototranssitor block, and 
then fit and tighten the 3 mouting screws (with insulating bushes). Then 
with everythinc clamped up you remvoe the pins. 

These pins are just 1" (or so) lengths of 1/8" diameter metal rod. A 
slight bevel on the end (trivial if you have access to a lathe) makes it 
slightly easier to assemble, but is not essential.

There are pictures of this on flickr.

And note this is not in any HP manual that I've seen...

> Mine works real nice for reading these old tapes.  I posted a thing to

I still prefer the Trend USR and HSR readers with their self-setting read 
thresholds.

-tony




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