23 Apr
2009
23 Apr
'09
11:09 a.m.
Hi again!
I've spent a some more time with the monitor, following the CRT driver
board schematic, with some good results (I hope). Thanks to Tony, I had
a better idea of what to look for.
Tony Duell wrote:
Right. My question was really 'There isn't a PAL or NTSC decoder in the
monitor, is there?'
I don't have it, but I am not sure it'd
be any more use to you than the
schematics. Very few monitor service manuals (particularly ones for
small, relatively simple montiros like this) have any form of
fault-finding charts. Most of the time you're expected to work from the
schematic [1] .
[1] On the few occasions that I have had fault-finding charts for
something I've been repairing, I've found them to be misleading and
next-to-useless. I find it a lot easier to work from the schematics and
figure out what should be going on.
Fair enough - I've given it a go.
Right.
This is an RGB input monitor, yes?
Yes - analog RGB, as far as I can tell. OK, what is between the IC and the CRT cathodes
(I've yet to see a colour
monitor where hte video signal is applied to anything other than the
cathodes)? Most likely some kind of transistor amplifier.
Start there. Since all 3 gelectron uns are affected, look for a common
cause. Either a missing power rail, or a blanking signal. Look at the
votlage on the transistor pins, do they make sense? Or are the
transistors always saturated or cut off?
Done. The power supply to the gun drivers seemed OK - very close to
200V. Sounds about right I hope? Of that, nearly the whole lot is applied to the gun cathodes constantly; only a tiny AC
component.
Right. I assume the cathodes are connected to this supply through load
resistors. And that they also connect to the collectors of NPN
transsitors.
Now, the higher the votlage on the cathode, the greate the effective -ve
voltage on the control grid (g1), and thus the more the CRT is cut off.
With the cathodes at 200V, the CRT is going to be pretty much cut right
off, hence the total lack of illumination on the screen. So that makes sense.
Between each output of the AN5356 and the CRT itself
are two
transistors. The first looks like an amplifier (the base is driven from
the AN5356's colour output). For the second transistor on all three
colours, the base is driven by the same line, which is derived from an
output on the AN5356. I'm guessing this is the blanking part of the CRT
Does the emitter of the second transistor connect to the collector of the
first? That's quite a normal circuit, and yes the second transistor is
bart of the blanking circuit (when it's turned off, the cathodes go up to
the +200V rail, and the CRT is cut off).
driver. Looking at the output of that pin (#1) on the
AN5356, I get a
very regular waveform, which makes sense in the context of blanking.
OK.
This drives something on the main board (I'm
guessing the scan
electronics...), plus a small two-stage amplifier driven from a 12V
supply, which then drives the bases of all three of the final CRT drive
I asusme that 12V supply is present and correct (I've learnt to check
just about everything when looking for a fault...)
transistors. Things look pretty good after the first
stage of the small
amp (a nearly square waveform), but the second transistor appears to be
doing nothing, the output of the amp sitting lowish despite the good
signal into the base. So, my suspect is that transistor.
Right. Could well be the case.
The way this normally works is that there is a voltage divider (5.6k /
330 ohms) that keeps the output low. The output transistor is placed
across the 5.6k resistor; when it is driven, I expect its resistance is
a lot lower than 5.6k, pulling the voltage high.What I'm actually seeing
is the output voltage sitting at about 4V, with a very little AC
component. So perhaps that transistor has failed, sitting at a constant
resistance, and holding blanking on.
So, three simple questions for anyone who knows more than me!
1 - Does my understanding of the way this works sound correct? I'm
guessing at a lot of this stuff...
What you say sounds logical, and it roughly matches what I'd expect to find..
2 - Could I be leaping to the wrong conclusion in
blaming that transistor?
You might be right, you might be wrong.
3 - How can I check? Could I pull the 5.6K resistor
and the suspect
transistor, replace the resistor with a 5K pot set to 5k initially, and
wind the pot down a little, see if the picture comes back in some form?
Good idea, or misguided?
Why not test the transistor. Desolder it, and at least make sure the e-b
and c- junctiuons test as diodes. This doesn't prove it has any gain,
but it'll find open or shorted transsitors. I can't beleive that
transistor is very critical, you could probably replace it with just
about any small-signal transistor of the right polarity (NPN .vs. PNP)
-tony