Re: Problem Using a Commodore 1084s Monitor on a IIgs

[ferdimh@gmx.de]

On 14 Feb., 10:13, "Polymorph" <polymo...@a2central.com.remove-3n-
this> wrote:
>   To: Polymorph
> Good and bad news...
>
> OK, I have wired up the 200 Ohm pot and it does successfully fix the
> sync problem. The monitor produces a nice crisp image.  :-)  I thought
> it was a matter of case closed. My daughter played with the computer for
> quite a while before I noticed different troubles.
>
> Unfortunately, after the monitor is turned on for about 30-40 minutes,
> it starts making a noise and the screen shimmies a bit (horizontally).
> It seems to be related to the brightness/contrast somehow. Lowering the
> brightness and/or contrast makes the problem appear worse, setting high
> contrast/brightness make the problem go away.
>
> All testing from this point was done with the monitor standalone - i.e.
> there was no input source. I simply left the monitor on until the noise
> recurred and attempted the following.
>
> I opened the case again and tried the horizontal hold adjustment, and it
> changed the pitch of the noise being produced, but that is all.
>
> 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...
>
> By moving a microphone *carefully* near the components, I was able to
> determine where the noise was coming from (without the use of the
> microphone it was impossible to tell where the sound was coming from). I
> narrowed it down to two components - L301 or L202. I copied the
> schematic for the 1084S-D onto my website:
>
> http://apple2.sytes.net/1084S-D.gif
>
> I took a couple of photo's of the suspected noise makers:
>
> http://apple2.sytes.net/1084a.jpghttp://apple2.sytes.net/1084b.jpg
>
> Now, for my questions:
>
> 1) is it likely that one of the components making the noise is faulty,
> or one of the surrounding components? (e.g. a capacitor)
> 2) if L301 or L202 faulty, are those parts easy to find replacements for?
> 3) is there something else that is likely causing the noise? Bear in
> mind that I'm fairly confident that the location I have indicated is
> where the noise is coming from (the record levels peaked out around here).
>
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)

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)
 - 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!)
 - Horizontal drive level (modify R210, R211 or R212 slightly -
dangerous w/o oscilloscope)
 - Horizontal feedback level (modify R255 or R247, e.g. use 33K or
39K) - you won't blow anything with this mod.
 - 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.)

> 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...
>
> 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.
-----
Ferdinand Meyer-Hermann