[rmzalbar]

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