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Re: Problem Using a Commodore 1084s Monitor on a IIgs
To: ferdimh
ferdimh wrote:
<snip>
Originally I thought it was probably the main 150uF 400V capacitor
making the noise (by ear it sounded in the vicinity), so I replaced it
with a known good one that I had. It made no difference.
Where is it? I didn't find it on the schematic...
The capacitor I was referring to is at C109 which is listed as a 200uF
200V capacitor on the schematic, however, because I am in Australia we
are using 240V power, so I guess the need for a higher voltage capacitor?
<snip>
This is nearly always a problem in the horizontal deflection or driver
circuit.
The noise and horizontal jitter problem is caused by the horizontal
deflections failure to produced equally long lines. This causes the
current through all parts in the horizontal output stage to vary
making them vibrate and generate some noise. They are not at fault!
If you increase the load to the horizontal output stage by increasing
the picture's brightness (remember that the HOS also generates the
high voltage for the picture tube; in this set all voltages for the
video section are generated this way - thats what T202(flyback
transformer/FBT) is for.), you make it run more stable.
Really fixing this is difficult; it can be caused by
- bad(usually low) sync input (but not here, because it also happens w/
o sync)
- the oscillator circuit (I201)
- the horizontal driver, HOT(horizontal output transistor, Q202)
- the circuitry around the yoke
- low or high line voltage or power supply fault (suspect here: most
likely C222, but also C117, C116)
- poor design
- bad luck (i.e. combination of poor design, low/high line voltage,
and tolerances of exactly this model)
OK, I hope its not bad luck, because in its current state its borderline
usable. If I choose to have a black background and black border via the
IIgs control panel, the image is unstable and I get the noise no matter
how far I turn up the brightness/contrast. I haven't tried replacing the
capacitors you suggested as yet, but I will endeavour give that a try
over the weekend.
Because bad luck is the most likely cause there is usually nothing to
replace, but there are workarounds:
The problem usually disappears if the parameters of the HOS are varied
slightly. this includes:
- Horizontal phase R203 (moves the picture horizontally)
I had already tried adjusting R203 without success.
- Horizontal hold (but you've already tried it)
- Horizontal width L201 (you should be able to CAREFULLY screw in/out
its core - try it with power off before!)
I tried adjusting L201, but it didn't help.
- Horizontal drive level (modify R210, R211 or R212 slightly -
dangerous w/o oscilloscope)
I haven't tried the above - I don't own or have access to an oscilloscope.
- Horizontal feedback level (modify R255 or R247, e.g. use 33K or
39K) - you won't blow anything with this mod.
I haven't tried the above as yet.
- Power supply voltage (my favorite - but there is no pot for it in
this set)
- live with it (many modern VGA monitors show this problem in out of
the box in text mode (mostly black screen - got it?) - especially
Samsungs.)
As I mentioned above, it would be hard to live with as it is now. The
combination of the noise plus the unstable image is too much to live
with IMHO.
I swear by the end of all this I'll be able to open my own monitor
repair shop. :-)
You won't be able to make a living with it. there are not enough CRT
monitors out there any more...
I was only joking. Once I get this thing sorted out, I don't want to see
the insides of another monitor for a long time.
Oh yeah, and why can't I get a monitor that just works?!?
I asked that myself a couple of times, but now I know: It's just
boring.
At this point, I'd kill for boring. ;-)
Thanks for your help,
Mike
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