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Old 06 December 2019, 23:14   #1
rmzalbar
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1084S PAL/NTSC V-size switch mod

I'm not sure what the situation is for PAL territories, but for those of us living under the NTSC dominion, switching between PAL and NTSC screenmodes generally involves getting up and walking around to the rear of the monitor to manually adjust the V.SIZE potentiomoter to be able to fit the extra PAL lines on the screen.

I am having a problem where turning V.SIZE to minimum on my NTSC 1084S-D2 is not *quite* enough - a little bit too much of the PAL screen is still in the overscan area for my liking.

Looking at the schematic, the V.SIZE is just a 100k resistor to a 100K potentiometer to ground from the vertical control IC. I figured I should be able to add another resistor in series with the existing 100K pot to bring the adjustment into a more usable range (or substitute the 100K resistor with a larger value).

But then I thought - why not add a PAL/NTSC size switch while I'm at it? The idea is to add a resistor in series with the pot, that changes the screen size the right amount to compensate for PAL, and then mount a switch to the front of the monitor to bypass it.

When I launch a PAL mode game, just flip the switch and bam, good to go. No more reaching around the back of the monitor, and no wear and tear on that pot.

This evening (or tomorrow) I'll experiment with resistor sizes and post an update. Could be a nifty and easy mod.

Last edited by rmzalbar; 06 December 2019 at 23:28.
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Old 07 December 2019, 11:34   #2
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That went well.

Pin 7 of the TDA1675A is used to set the V.SIZE. It passes through a 100k resistor, then through a 75k or 100k trimpot (the V.SIZE adjustment knob) then to ground. The NTSC version of 1084-D2 has the 75K V.SIZE pot instead of 100K, to give it a bit taller screen size at NTSC resolution. It's too much though, most of the adjustment isn't used and turning counterclockwise all the way to the stop to maximum resistance isn't enough to get the full overscan in PAL mode.

Some experimentation shows that I should add about 25k to this circuit, and about 16k additional for my PAL/NTSC switch.

*** DO NOT DISASSEMBLE OR WORK ON THE MONITOR WHILE CONNECTED TO MAINS POWER. MAKE SURE YOU UNDERSTAND HOW TO SAFELY DISCHARGE THE CRT AND ANY OTHER HIGH-VOLTAGE COMPONENTS. ***

I desoldered and removed resistor R305 (100K, brown-black-yellow-gold, next to the vertical deflection IC 301, a TDA1675A, which stands vertically in the center of the board and is mounted to a heat sink.)

I soldered one leg of a 127.5K resistor (actually a 270k and a 240k in parallel) vertically to the frontmost pad (towards the front of the monitor) where R305 was. To the other end of this resistor, I soldered a stranded wire long enough to run out of the chassis. Protect the joint with heatshrink tubing.

I soldered a second wire to the rear pad where R305 was, long enough to run out of the chassis.

I protected everything with heat shrink, twisted the wires gently together, and ran the pair out the front through a vent hole in the bottom of the case.

I have a push-on, push-off switch. I soldered a wire to each pole of the switch.

I soldered a 15.6k resistor (actually I used two smaller values, 3.6k and 12k in series to make this) between the two poles of the switch. When the switch is OFF, the current passes through the resistor, adding 15.6k and making the screen size a little smaller. With the switch on, this extra resistor is bypassed, because it's shorted by the switch. Later, I ended up replacing the resistor with 50k trimmer instead, so I could tune it. You turn the switch ON and set the main V.SIZE control for NTSC. Then turn the switch OFF and use the trimmer to adjust for PAL. I found 18k to be optimal.

With my typical fear of commitment, I used hot glue to mount the switch under the left corner of the monitor.

Works great! With the larger value 127.5k resistor in place of R305, I have plenty of adjustment to size the screen down to see the entire PAL display. Pushing the switch to OFF expands the screen height about the right amount for NTSC gaming. No more reaching behind to twiddle the V.HEIGHT pot, and no more concerns about wear-and-tear on the pot. If the switch fails, all that will happen is that it will stay in the larger of the two sizes, it won't interrupt the V.SIZE circuit. If the wire gets *broken,* vertical scan is lost and the vertical control IC will automatically shut off the CRT voltage. No worries.

This mod was done to a 1084S-D2 NTSC. It should be the same for the D1.

It should work for PAL models too, but you probably won't need to replace R305 - the V.SIZE pot is already 100k for PAL monitors.

I also thought about perhaps using a little microcontroller as a frequency counter, to auto-adjust size based on vsync, but the manual control is better as a lot of some games that run best at 50hz PAL frequency only use the NTSC screen area. Also, I found a lot of WHDLoad dumps will ONLY run at PAL frequency even though they are in the NTSC playfield format.

Here's a video of the switch in action:
[ Show youtube player ]


-Rob
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Last edited by rmzalbar; 09 December 2019 at 21:56.
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Old 09 December 2019, 10:20   #3
Jope
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The PAL ones handle PAL/NTSC height differences a lot better for some reason. :-)
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Old 09 December 2019, 18:18   #4
brett71
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This is the exact reason why I love the Philipps/Magnavox-made monitors vs. the Daewoo ones. The P models automatically adjust when flipping between NTSC and PAL and back again. No manual adjustments necessary. I know I've talked about this in other monitor-related threads, but it is absolutely true.
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Old 09 December 2019, 21:09   #5
rmzalbar
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Quote:
Originally Posted by Jope View Post
The PAL ones handle PAL/NTSC height differences a lot better for some reason. :-)

Does it automatically rescale? Or do you have more adjustment at the knob? The PAL units have a 100K pot to the NTSC's 75K pot, which does give that extra range.
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Old 09 December 2019, 21:27   #6
rmzalbar
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Quote:
Originally Posted by brett71 View Post
This is the exact reason why I love the Philipps/Magnavox-made monitors vs. the Daewoo ones. The P models automatically adjust when flipping between NTSC and PAL and back again. No manual adjustments necessary. I know I've talked about this in other monitor-related threads, but it is absolutely true.

I just looked at the schematic for a 1084S-P, and it's completely different. It looks like the the vertical control IC knows if it's in 50hz or 60hz, and also the V.SIZE integrates something, possibly one of the outputs. This monitor and the old monitor I had in my original amiga years have both been D models.

Another control i'd like to have from the older models is Sharpness. Sometimes a bit of that is needed to clean up muddy output from old 8-bitters. It looks like they just use a filter/transistor to amp luma changes (intentional ringing) so I could probably add something.
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Old 09 December 2019, 21:31   #7
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It's been a long time since I had one of those Daewoos and I can't for the life of me remember, whether it had problems here.. The Philips ones were so much more common in these parts.
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Old 09 December 2019, 21:39   #8
rmzalbar
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A 1950-style Overscan button would be easy to implement too, that shifts and scales the screen so non-overscan stuff uses more of the screen area. OK, maybe now I'm getting into microcontroller territory after all.
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