HP9836C colour alignment (grey scale tracking)

Tony Duell ard at p850ug1.demon.co.uk
Thu Oct 2 12:52:33 CDT 2008

> 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 ...

[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.

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. 

Then it was just a matter of desoldering the RAM chip and fitting a 

> 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.


> > 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 

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.

> 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.

> > > > > 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 

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.

> > > 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 

> > > 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.


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