30 Sep
2008
30 Sep
'08
3:24 p.m.
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 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.
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. 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.
[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. :-)
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.
And that misadjustment can cause various problems
:
Visible flyback lines
Yup, if you crank it way up. Missing colours
If you don't have it turned up high enough. Incorrect colours (compared to another 9836C)
Either of the above.
Significantly decreased reliability
Maybe if you crank them _all_ up?
Now, the first 2 are very ovbious, and I'll
not have problems with that.
The third doesn't bother me at all, since I only have one HP9836C. But
the last does worry me.. Particularly if the part that fails is the
flyback transformer. The CRT seems to be a standard one (I can find no
data on it, but some CRT testers/boosters list it as one of the CRTs they
can handle). I would have a chance of finding a replacement. The
transistors on the video board are all standard, so I could find
replacements. But HP never supported component-level repair on these
machines, and the flyback transformer was only avaialbe as the complete
deflection PCB. And that's long unobtainable.
I wonder if it would be possible to get from some aftermarket suppliers? I I don't if
any other HP monitor used the same transformer -- the horizontal
scan rate is unusual -- about 25kHz -- but other HPs used the same sort of
rates.
The scan rate supported (with the flyback time being the proper portion of a
scan line and such) is not only determined by the transformer but also by
what all else is hooked up to it, capacitors in particular. And HV 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.
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...
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).
-tony