Adjusting The Commodore 1702

Yesterday I was fiddling around down in the “byte cellar” for a bit and went to reconnect my Apple IIgs’s audio to the A/V switch after hijacking its cables to record a bit of Wii audio for a recent post. While I was digging around behind the machine I found a loose RCA cable tied at one end to the IIgs’ composite video jack. After wondering for a moment what that was about, I recalled that a couple of years ago someone had asked me to verify that the IIgs composite video signal was indeed color and not greyscale (Amiga 2000-style). I did so by hooking the IIgs to the neighboring Commodore 64C setup’s Commodore 1702 CRT. Yep, it’s color.

For no particular reason I reconnected the GS, which has its own 12-inch RGB monitor, to the 1702 and fired up an old 8-bit game. Looking at the display on the RGB and composite screens sitting next to each other I was reminded how much better Apple IIe-style video looks on a real composite CRT. The Woz’s “high-res” graphics mode generates color by taking advantage of NTSC artifacting using black and white pixel patterns on the 280×192 pixel screen to create four colors (other than black and white) at an effective 140×192 resolution — brilliant stuff way back when. The problem is, the IIgs RGB video output, needed to best represent the far more advanced GS-specific graphics modes, has to sort-of emulate these NTSC artifacts for IIe-mode graphics and the results just look, to my eyes (and despite the aforelinked, overexposed image of the side-by-side displays), rather wanting as compared to “the real thing” on a composite screen. So, I decided to leave the 1702 hooked to the GS, sharing it between the Apple and Commodore computers (thanks to its switched, dual inputs). But this new arrangement underscored a little issue I’d been meaning to take care of for a while now….

After posting several photos of the Commodore setup some time ago I received a few reader comments indicating that the image on the screen seemed to tall. Looking at a few, more familiar Apple games (when it comes to 8-bits, I was an Apple guy back in the day) on the screen last night, I was reminded of the adjustment I’d been wanting to make…and the reason I’d not yet made it: there is no “v. size” knob among the front controls or around back. This meant opening up the enclosure, grabbing my plastic CRT tools, and fiddling with pots — which I promptly did. But which pot? I found five right away, but they appeared to be unlabeled (I think they may have had labels…obscured with a 23-year-old layer of dust). So, I went online and spent a great deal of time searching for a guide to 1702 adjustments or some sort of schematic when finally I found a hand-drawn schematic and various other, useful 1702 images. Just what I needed. A little twiddling on the proper pot to the glare of a few test images and things are looking just about right, finally.

My purpose in rambling on about all this has simply been to share the link that helped me so that perhaps it can be more easily found by the next guy needing to make a few 1702 adjustments. Browse the complete directory full of related goodies.

And it’s definitely worth noting that mucking about in the innards of a CRT CAN KILL YOU!! Only do so if you’re sure you know what you’re doing!