HP-150 MS-DOS Computer

Tony Duell ard at p850ug1.demon.co.uk
Wed Apr 25 17:20:30 CDT 2007

> > What's the problem with your unit? Does it appear to be a digital 
> > problem, or a problem with monitor circuit?
>     Digital, I suspect.  Although, this is a rather bizarre problem, and
> COULD be a video problem, I suppose.  When I boot it up, it works
> correctly, as far as I can tell, with the single exception that the
> screen shows what it should be showing, only in reverse video, and
> without the characters being visible, whatever the brightness level.  In

I am not sure quite what you mean here. Do yuo mean that all characters 
(other than space) appear as solid blocks?

Boy do you need the schmatics! I've looked at the 150 'Video Alpha 
Display Sybsystem' and the overall design is what you'd expect, but the 
details are odd. And I susepct it's one of those odd bits that's causing 
the problems

OK, the basic dsegin is a CRT controller -- here an SMC9007  (U315) which 
addreeses the video RAM. The output of the video RAM is 16 bits wide 
(character and attrboutes), the character part -- 10 bits of it -- goes 
to the address lines of a character generator ROM U512. The output of 
that goes to a shift register (U511 and U612, 'S195) which does the 
obvious dot serialisation.

The attribute logic is compiclated, but based round a 16L6 HAL (mask 
programmed PAL) U314. I do _not_ have the PAL equatiuons. The outputs of 
that are latced (U614, '174) and feed the 'Dot Stream Mixer', a 'S153 mux 
which combines the alpha dots and the graphics system dots, and which 
then produses the Full Brightness and Half Brightness signals to the 
'Sweep' (monitor) PCB.

The complicated bit is round those shift registers I mentioned. There's 
some logic to, I think, make the line-drawing characters touch on-screen. 
This is controlled bu U46b ('S74) and is based rounf U78b ('S112) and 
U713b and a ('S09),, U712d ('LS00) and U610a ('S32)

There's even a couple of gates between the 2 shift register ICs, but I 
think if that was the problem you'd get half of each character displayed 

I think I'd start by looking at pin 3 of U713. That's the alpha dot 
stream going into the mixing logic. What you're looking for is fine 
enough pulses to be character dots, rather htan complete character cells. 
If You've got that, suspect a problem round U715. If not (and I think you 
won't have it), go back to U78b, etc.

> > [1] Although I always thought Tektronix made better 'scopes :-)
>     I think the Tek's may have been more fully-featured, but I liked the
> HP 'scopes just fine.  If you leave the two companies out of it, you've
> cut off almost all the truly great 'scopes.

Oh, I quited like Nicolet and LeCroy :-). Not that I own any of their 
instrumnets, alas..

> > I have an _old_ -- over 40 eyars old -- HP frequncy counter.
>     Sounds like you are discussing an HP 5245L...

Exactly. Well, it could also have been a 5243L, which is the 10MHz 
version (the 5245 being 50MHz). I have one of each. Actually, a fair 
number of the PCBs are common to both instruemtns. I also have some, but 
certianly not all, of the plug-ins

>     Yes!  Our lab had a cesium beam that we used as an external standard
> for the 5245Ls.  Truth be told, however, there really wasn't much

Alas I have to 'make do' with the internal crystal. I am told that HP 
sold a rubidium beam (sub)standard that was in a 19" rack module, and 
which you simply cabled up to the external oscillator input on the 5245L. 
I don't hjave it, thouhg.

> difference, especially if one "tuned up" the crystal oscillator by
> beating it against the Cs beam frequency reference.  We were ACCURATE.

That's the problem. Unless you have a Rb or CS beam reference, it's 
almost impossible to calibrate the crystal, and the crystal can and will 
drift slightly over time. Not enough to bother me (I am darn sure the 
instruemnt is more accurate than any modern thing that I could afford), 
but still.


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