HP9836C colour alignment (grey scale tracking)

Roy J. Tellason rtellason at verizon.net
Tue Sep 30 23:06:20 CDT 2008


On Tuesday 30 September 2008 18:24, Tony Duell wrote:
> > > But what worries me is a comment in the HP service manual. It basically
> > > warns you not to twiddle these adjustments (the same sort of message is
> > > printed on the metal cover over the monitor chassis).
> >
> > I lost a great deal of trust in their stuff some time ago when I ordered
> > a "service manual" for some H-P product and received a very small bundle
> > of paper that was shrink-wrapped,  and on opening it I found "The monitor
> > is normally not repairable,  but is replaced as a unit..."  I sent it
> > back and
>
> Alas, yes....
>
> 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.

> As an aside, I feel it should be illegal to sell a 'service manual' for an
> electronic device that doesn't contain full scheamtics. Those are essential
> for servicing/repair. Of courwe I'd have no objection to companies
> selling 'boardswapper guides' provided they don't misrepresent them by
> calling them 'service manuals'. 

While I agree that that sort of thing should not be sold as "service manuals",  
I don't think that the solution is to beat somebody up with government,  
which furthers their inclinations toward taking more power upon themselves.  
On the other hand,  what we have here in this medium (and the web,  etc.)  I 
have a few complaints of my own here:

http://mysite.verizon.net/rtellason/clueless.html

A continuation of further efforts along these lines will eventually have some 
effect,  I'm sure.

> Anyway, the HP9836 'service manual' is a boardswapper guide, and contains
> very little information that's not obvious from 10 minutes of looking at
> the machine. There's a large section on running the diagnostics, with
> what to do for each error, and alas the latter is 'repalce the <foo>
> board'. And very often those error messages give a lot more information
> once you've learnt to interpret them

I've seen some diagnostics that were like that,  and some that were much more 
specific,  telling you what chip you might want to look at changing out.  But 
not since the early days of the Zenith Data systems stuff.  The trouble with 
that sort of diagnostics is that most of the machine needs to be working in 
the first place,  or they don't load and run.  Printed versions of the same 
sort of thing are uncommon,  and I guess they'd take a lot of time and effort 
to compile,  and would still be incomplete when you were done.

> The only really useful section is the pinouyts of all the motherboard
> connectors (this series of machines has amotherboard in the bottom of the
> case which contains the keyboard interface and HPIB circuitry. The
> keyboard connects to an edge connector on the side of the motherboard,
> the CPU, floppy controller and expansion backplane plug into edge
> connector sockets on the motherboard. The video system consists of 2 PCBs
> -- in the 9826 the text board plugs into the motherboard, the graphics
> board plugs onto the top edge of the text board (and extends forwards
> over the internal monitor), in 9836s the 2 video boards connect to ribbon
> cables on the front of the motherboard). Anyway, know the location of
> things like the address bus was a great help!

Yes,  I can see where that sort of information would definitely come in handy.

> Be warned there are some errors in those pinouts, though. The main one
> that I've found relates to the 'video coax' in the 9836C. In all 9836s
> (monochome and colour) there's a special ribbon cable consisting of 4 (75
> ohm?) coax cables bonded together. It's terminated by 8 pin sockets at
> each end. One end goes onto a header plug at the rear of the motberboard
> (which is connected to the adjacent DA15 connector for the monitor), the
> other end plugs into the text video board under the disk drives. On the
> colour model, one of the cables isn't used, the other 3 are R, G, B.
> Anyway, it's clear that one row of 4 pins is the screens, the other row
> is the centre conductors, and it's equally clear that the latter carry
> the video signals. But the pinout in the manual shows the colour signals
> on 3 pins at one end of the connector, which would put one of them on a
> screen. It's obvious how this has happeend, it's the difference between
> numbering the pins
>
> 1 2
> 3 4
> 5 6
> 7 8
>
> and
>
> 1 5
> 2 6
> 3 7
> 4 8

I've often wondered why there wasn't some standard way of numbering those.   
This sounds like the serial connectors that were out there (and I have a pile 
of them) for different I/O adapters,  if you didn't happen on the right ones 
the serial ports just wouldn't work,  and it wasn't obvious why.

> The dismantling procedures in that manual are a little odd too. Anyone
> who can follow the procedure for removing and replacing the internal CRT
> of the 9826 and not end up dropping it or using choice language is a lot
> cleverer than I am.

I've thought for a long time that "using choice language" was a part of being 
a technician.  :-)

> You're supposed to hold the CRT against the front bezel (from the inside)
> And fit 4 screws (2 of which are almost impossible to get to as components
> on the monitor PCB get in the way, as does some chassis metalwork) with a
> founding spring between 2 of them. Not thanks!. My method may take a little
> longer, but it's not stressful. 

I remember getting (and using) some *really long* tools for stuff like that.

> Here it is (basically) 
>
> Remov top cover, board hold-down strip, PSU PCB, CPU PCB, floppy
> controller and expansion backplane. Remove graphics board (on top of
> monitor cover -- 3 screws) and text video board. Remove the internal
> floppy drive (6 screws + LED and power cable). Remove metal plate under
> floppy drive. Remove keyboard (5 screws on underside of machine, unplug
> ribbon cable and unscrew earth wire at keyboard end. Unplug ribbon cable
> and motherboard end and remove from machine. Unscrew power switch
> assembly from case, unclip and open cover, then release actuator from
> switch and remove the actuator/cover parts. Remove monitor cover (4
> quarter-turn fasteners), remove monitor to motehrboard ribbon cable
> jumper, discharge final anode to CRT ground (faston tab on monitor PCB),
> then disconnect anode cap. Unplug yoke, CRT base wiring and earth wire
> from monitor PCB. Undo  2 screw holding bezel to the bottom of the case,
> carefully tilt it backwards (mind that CRT!) and pull it fowards to free
> the 3 tabs.  Place the bezel on the bench fase down, remove the CRT
> socket, undo the 4 screws (note how the earthing spring is fitted) and
> take off the CRT moutning frame. Lift out the CRT -- NOT by the neck.

Noted.

> There's one curious thing about that servic manual. It contains no
> schematics at all. Not even of the mains side of the PSU, which is
> trivially field-repairable.

Then they really ought not to call it a service manual,  per se.  Or at least 
not in my view of things.

> Juat about the only electronic tests you are asked to make are the PSU
> outputs at labelled testpoints on the PSU board. It doesn't, for example,
> describe the use of the testpoints [1] and LEDs on the disk controller
> board. And yet it contains this complex procedure for setting up the grey
> scale tracking involving an instrument -- the photometer -- that few people
> are likely to own (many fewer than would on the equipment to do proper
> electrical tests. 

One wonders who the author of this thought they were talking to when they 
wrote it...

> [1] I think some of those are mentioned in the CE manual, which is
> similar to the service mnaul but contains a couple more useful bits of
> info.
>
> > told 'em I wasn't going to pay that particular invoice as that was of
> > absolutely no use to me whatsoever.  :-)
> >
> > > And that misadjustment can cause various problems :
> > > Visible flyback lines
> >
> > Yup,  if you crank it way up.
>
> I _think_ the only misadjustment that will cause this is the A1 ('screen
> grid') adjustment on the flyback transformer. Maybe setting the clamps
> way too low will do it.

Screen controls are singular these days,  but the early stuff used to have one 
for each color.  You'd flip a "service switch" which would give you just 
horizontal lines on the screen,  and either adjust for all white or if they 
weren't exactly overlapping for equal apparent brightness.  Or in some stuff 
till they just went out.

(Snip...)
> > 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?

> Who would buy one? If you follow the HP manual, all you'll do is replace the
> complete deflection PCB, there are no tests to do to check votlages from the
> flyback, etc. So unless you're clueful you won;'t even know you need a new
> flyback transformer. 

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.

(Snip...)
> > regulation ties into this too.
>
> This monitor is a little unconventional in that there are 2 output stages.
>
> The horizontal sync output from the computer goes through a couple of
> monostables (one of which is set by the horizotnal centring control), one
> of these monostables is held reset if the monitor's internal SMPSU
> outputs are incorrect, thus shutting down the EHT side.
>
> The output of that circuit drives the first power transistor. This has
> the horizontal yoke as its load (but not the flyback transformer). The
> flyback pulse from the yoke is detected and used to indicate that the
> yoke is doing something. This signal, along with a similar feedback
> signal from the vertical defleciton circuit are used to (a) turn on a
> green LED indicating that the deflection circuit is working and (b)
> enable the drive from the output of the monostable section to the second
> power transistor, which drives the flyback transformer.

Interesting.

> Now the power to that stage comes from a TIP122 transistor which is
> contolled by an op-amp circuit. One of the inputs to that comes from a
> potential divider connected to the EHT (25kV) supply, another comes from
> a voltqge reference circuit. This ,of course, is the EHT regulator
> circuit. A divider block on the side of the flyback transformer provides
> the A1 and focus supplies, other windings (brought out on the PCB)
> provide a -ve bias for the CRT control grid and a supply for the CRT
> heater.
>
> Of course this information is not in the HP manual...

Of course...

Did you figure most of it out or come across it somewhere else?

> > I can't imagine H-P deciding that they were going to make their own
> > flyback for this one monitor as opposed to using something else that
> > already existed. Not impossible,  but it just doesn't seem too likely to
> > me.
>
> I think it's very liekly they did this (or rather got some company who
> made flyback transoformers to make one to their specification). My
> experience with working on monitors, terminals, etc, is that the flyback
> transformer is almost always specifci to that model. I say 'almost
> always' because Philips sold a few more general-purpose ones and those do
> turn up, e.g. in KME monitors (as used on PERQs, Whitechapel MG1s and so
> on).

Still,  I suspect that there's some finite number of different ones out there.  
But it's not something that I've researched in general.  Yet?

-- 
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



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