27 Apr
2007
27 Apr
'07
6:11 p.m.
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 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. > 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 have an
_old_ -- over 40 eyars old -- HP frequncy counter.
Sounds like you are discussing an HP 5245L... 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.