HP-150 MS-DOS Computer

Warren Wolfe wizard at voyager.net
Thu Apr 26 21:21:20 CDT 2007

On Wed, 2007-04-25 at 23:20 +0100, Tony Duell wrote:

> > > 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?

    Okay...  Yeah, I muffed the explanation somewhat.  I don't, upon
re-thinking it, have a much better explanation.  However, if you would
imagine a scenario in which you are TRYING to make a "normal" HP-150
look like mine, I CAN tell you how to do it, quite simply.  First,
replace all characters on the screen with spaces in the same scheme
(normal or inverse video) as the characters were, and then invert the
whole screen, on for off and off for on.  Then, all the "on" bits are
made bright.  Other than that, it seems to work just fine.

> 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

    Yeah.  <Grumble, grumble.>  Most problems I've seen with computers,
probably more than three quarters, do NOT need schematics to solve.

> 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.

    Just from the symptoms, I'd bet a reasonable meal on the S153 mux.
If it's not, it's almost GOT to be multiple problems, and that seems
more than a little unlikely.

> 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 
> correctly.
> 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've never seen them.  Must be that "pond" thing, what?

> > > 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

    Now THAT was some fine engineering.  Old enough that they had to use
Nixie tubes, but, whatever.  Watching them was more than a little
hypnotic.  I repaired many of them.  Which is not to say they were not
reliable...  The pool of instruments was quite large.

> >     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.

    The calibration of the crystal oscillator takes about three days to
do correctly, and stays within tolerance for about 15 months, if I
remember the data correctly.  You'd be disappointed by the rubidium beam
frequency reference, however.  It drifts, too, albeit not quite as fast
as the crystal oven oscillator.  You'd have to calibrate *IT* every few
times you did the counter.

> > 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.

    The GOOD thing is that those equipments ALWAYS drift UP in
frequency, very regularly, and very slowly.  The calibration process
involves measuring the drift over a couple days, and comparing that with
previous data if available to check for anomalies.  Then, the oscillator
is set slow, JUST within tolerance, and the drift figure obtained in the
first part is used to predict the length of time it will be within


            Warren E. Wolfe
            wizard at voyager.net

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