HP9836C colour alignment (grey scale tracking)

[Roy J. Tellason]

On Thursday 02 October 2008 13:52, Tony Duell wrote:
> > On Wednesday 01 October 2008 16:59, Tony Duell wrote:
> > > > > Very few HP classic desktop computer service manuals contain
> > > > > scheamtics. You might, if you are lucky, get PSU schemaitcs, but
> > > > > not for the processor, etc, sections. I am, of course, trying to
> > > > > remedy this...
> > > >
> > > > That stinks.
> > >
> > > Waht does? The fact that HP desktop computer service manuals are
> > > anything but, or the fact that I am trying to do soemthing about it?
> >
> > The former of course,  and I don't know why you'd ask me such a silly
> > question...
>
> Well, sometimes I can be a pedant, and one thing that irritates _me_ is
> the fact that English is not a precise language ...

Well,  it can be in the hands of some,  and I envy those who have that 
knack.  :-)

> [Graphics PCB RAM chip allocation]
>
> > At least it was possible to figure it out.
>
> Yes. The circuitry is a little odd -- there are 16 4416 RAM chips, and
> the data pins are not liked -- effectively there's a 64 bit databus on
> that card. Each RAM chip can be written individually from the approprate
> 4 68K data lines -- there are a couple of '138s which select the
> appropriate chip (or pair of chips for a word access -- 2 pixels at a
> time) for wriing. I forget how the data is handles for reading, I think
> there re 8 off 8 input multiplexer chips which bring the 64 data lines
> down to 8, these then go to the appropriate 68K data lines again
> ('appropriate' here means the low nybble of each byte, so likes 0-3 and
> 8-11). The decoders and muxes are controlled by the low 3 bits (A1-A3) of
> the 68K address bus.

Some engineer got very clever there,  it sounds like,  and then somebody else 
in the hierarchy decided not to let you in on how that worked,  in detail...

> The data also endes up in 4 16 bit shift registers (a total of 8 '166s I
> think), which send it a a 4 bit serial pixel stream to the colour lookup
> table and thence to the DACs.
>
> I guess this was the easiest way to get the necessary pixel rate from
> slow-ish DRAMs.
>
> Having worked all that out, it was still not obvious what diagnostic
> error was telling me. Since it occured at the first RAM location in the
> graphics RAM (there's a pattial memory map in HP's CE handbook for this
> machine), I first looked to see eich RAMs were being written to. When I
> found it was only 4 of them, all on the high half of th word. I guessed
> the diagnostic stopped as soon as it found an error, and that this was a
> simple read/write test. Then, by carefully grounding RAM data pins
> (rememebr everything was buffered, so this wouldn't crash the processor)
> and seeing how the R: value changed when I re-ran the diagnostic, I
> figured it out.

Ah.

> Then it was just a matter of desoldering the RAM chip and fitting a
> replacement.
>
> > It's a nice feeling,  isn't it?  I miss that lately.
>
> Indeed yes!. I love it when a dead machine, or one with a reported fault,
> works perfectly again.

Yes.  I spent some time getting my stuff organized today,  hopefully I'll be 
able to finish that up and get it to the point where I'll be able to start 
doing something with all that stuff...

> [..]
>
> > > I prefer to use a 'scope or logic analyser whenever possible (and when
> > > finding the memory problem I've just desicribed, I did look at the RAS/
> > > CAS/ etc signals first...)
> >
> > The scope  was my most-used tool for that sort of thing,  and I never had
> > the use of a logic analyzer,  mostly working for myself I could never
> > justify the expense.
>
> Over 10 years ago I bought an HP LogicDart. It was expensive, but I have
> never regretted it. I've got other instrumetns that do all it will do and
> more, but that's a useful handheld tool that will find 99% of
> digital faults in classic computers, if used with that most important
> piece of diagnostic equipment -- the thing inside your head.

Yes...

> > I don't think I've seen many diodes in TO-3 packages.  Plastic ones, 
> > yeah.
>
> This is the modern equivalent of an EZ80 or whatever you call it :-). 2
> diodes in one (To3) package with a common cathode connection (broung out
> to tthe case) and separate anodes (to the 2 pins). It's used for the
> classic biphase rectifier circuit with a centre-tapped transformer. I
> think I've seen them in DEC machines too.

As long as I've been working with this stuff there are still surprises for me 
now and then.

> [...]
>
> > > > > > I wonder if it would be possible to get from some aftermarket
> > > > > > suppliers? I
> > > > >
> > > > > Als I doubt it. IT was not a common machine, so I doubt anyone
> > > > > bothered to make a nrw flyback for it.
> > > >
> > > > How certain can you be that they went and made one that was unique to
> > > > that machine?
> > >
> > > I cna't be. All it has on it is an HP part number (which is not a lot
> > > of help). I have no idea who actually made it, whether it was used in
> > > any other HP product, or whether it was actually a standard one that
> > > was re-labelled (although I think that is unlikely).
> >
> > That's what I was wondering,  if there were any possible way to research
> > that sort of thing.
>
> Did any companies produce catalogues of flyback transformers giving the
> winding connections to the pins and the inductance/number of turns of the
> windings? If so, it would be possible to see if any of them matched up at
> all.

I know that years ago you could by some "universal" type replacements for 
flybacks and yokes,   but that was a really long time ago,  probably mostly 
for TV uses,  and likely to predate most monitors.  Whether that's something 
that's been continued since then I can't say,  though my feelings are that 
since it was done once that shows that such a thing is possible and that it 
might've been done again in the intervening years.  Although I used to run a 
service shop I've not done so for a nontrivial time -- 16 years this past 
spring was when I closed it -- so I'm seriously out of touch with what goes 
on in that business these days.

> The 3rd-party suppliers over here will sell you a flyback for TV model
> <foo> or monitor <bar> but they don't give any more details than that.

A generic replacement type flyback?  Or an OEM part?  If it's the former then 
maybe there's some hope.

> > > > Different strokes,  etc.  I suspect that with a little digging it
> > > > might be possible to find out who actually made that.  They likely
> > > > didn't make it themselves.  And an off-the-shelf product is much
> > > > cheaper than one that's custom-made for one specific product.
> > >
> > > THis is HP we're talking about...
> >
> > Which is not the same as saying it were say,  IBM.   :-)
>
> I am not so sure. HP used a lot of custom parts in their machines. The
> earlier desktop machines mostly had custom-designed processors, for
> example :
>
> 9100 : Custom, and very strange discrete-transistor thing. No ALU, even...
>
> 98x0 : Bit-serial 16 bit processor built from TTL chips.
>
> 9805 : Arithmetic/Registers and Control/Timing HP custom chips (as in
> handhelds), I think
>
> 9815 : Motoroal 6800 (OK, standard)
>
> 9825/9845/9835 : HP BPC processor custom hybrid module
>
> 8x series : HP custom 8 bit procesor (Capricorn?)
>
> 9000/200 series (the ones wr're talking about) : 68000, 68010
>
> I would not be at all suprised if HP made/had made custom flyback
> transformers.

I will defer to your experience with them,  but such stuff makes less and less 
economic sense as time goes on.  I have no particular feel for the chronology 
of these models either,  unfortunately.

> > > > Did you figure most of it out or come across it somewhere else?
> > >
> > > Now, given that I've had this machine in many bits over the last few
> > > months what do you think?
> >
> > If I had that much of an idea I wouldn't be asking now,  would I?
>
> Ok, OK... Well, if I get information from other sources I do try to give
> the reference (to a manual, a website, the person who told me, etc). So
> in this case I claim I figured it out myself, given some help from the HP
> service manual (connector pinouts), the CE manual (memory map) and plenty
> of databooks.

I think that giving credit is a good thing,  but too often I can't remember 
where I read someting these days...

-- 
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space,  a critter that can
be killed but can't be tamed.  --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James 
M Dakin