Color Classic 640x480 mod sense line issues

[lilmul123]

Hi all,

I just performed the VGA mod on my CC as outlined in JDW’s excellent video. At first, it didn’t seem like it was working. The image was squished horizontally and no amount of adjustments worked. It took me a bit to realize that it was actually in 67Hz mode! The sense lines appeared to be reporting the display as a “Multiple Scan Display”.

I checked my points over and over again. All the solder points checked out fine, and the pins that needed to be isolated were surely isolated.

This is where it gets weirder. Against my better judgment, I decided to try the old school HiRes mod to see if the screen would fill correctly with the refresh rate at 67Hz, and as expected, it did. So I adjusted the monitor and figured I was going to do need to do the improved version of the Hires mod.

That was until I restarted and then the screen changed to 60Hz! Now the screen I painstakingly adjusted was off the screen. So I adjusted it back again. But sure enough, when I came back later after it was off for a while, it was back to 67Hz! And then when I used it for a half hour and needed to restart, it switched back to 60Hz!

I am at a loss. FWIW, I did do a full cap kit before the mod.

Does anyone have any idea what might be happening here? I’ve attached some pictures of the VGA mod (before I moved the horizontal coil leg over to 84V), and the display showing both 67 Hz and 60 Hz options.

 

[Paralel]

Would 7 Hz change in refresh rate really change the geometry that much? I don't remember dramatic differences from back when I'd switch refresh rates on CRT monitors.

 

[JDW]

@lilmul123
To begin, your entire Analog Board needs to be recapped. You made no mention about that, so I need to make mention of it. Not likely to cause a switching between 60 and 67Hz, but still a very important thing that must be done to avoid unexplainable problems.

Next, your photos show a very crusty soldering job which makes it hard to see if connections were made perfectly, so I would suggest 99% IPA and a toothbrush to clean off the flux. Could be that will reveal a dry solder joint, and I would suggest the addition of a little flux on the joints with a touch of a soldering iron to make the solder flow perfectly.

With that said, your photos show the 68.4V standard VGA mod that gives you only 60Hz and not 67Hz. Bad connections, bad cuts and/or dry solder joints could cause you to get something odd.

67Hz (which you mention in your opening post) is attained via original Hi-Rez Mod (aka 84V Mod). You cannot get that via what is shown in your photos (which is the Standard VGA Mod). Here's a still photo taken from my 84V Mod video:

As shown, pin-8 must first be isolated and then examined with a good magnifying glass to ensure the isolation cut was perfect, then solder joints 8 and 10 above must be shorted to each other via a rather thick wire as shown.

Not only that, but for the 84V mod (67Hz mod), the sense lines must be like this...

Why the solder bridge at the top between pins 25 and 26? Because as I explain in my 84V Mod video, you must UNDO the regular VGA mod if you have done that before (and your photos show you have). That means no more jumper wire on pin 20! And... J79 must have its pads SHORTED as shown above, while J78 pads must be open like above. That completes the 84V mod that gives you 67Hz.

Here are the 2 videos that explain everything in detail:

1. 84V 67Hz (VGA) Mod
2. 68.4V Regular VGA Mod

So it's all a matter of doing the right mod correctly, soldering and cutting correctly, and having clean cuts and good solder joints, all on a recapped Analog Board.

I also recommend the Beefy Component upgrade too.

 

[lilmul123]

Thanks for the input! I did do the analog board recap (I mentioned it in the same line as “I am at a loss”). And I want to mention that your videos are incredible and I appreciate the level of detail you go into. I wouldn’t have even attempted this without them.

I did figure out the issue and I’ll post it here for posterity. I have always had trouble with the grounds on this machine since the day I received it (at one point, the machine wouldn’t power on at all and I figured it was the CUDA or the sound chip and ended up desoldering and replacing both with no change! It turned out it was a poor ground wire connection lead that goes to the CRT). In this case, I was having a poor ground connection between the tabs on the bottom of the logic board and the shield. I solved the issue by simply taking an alligator clip lead, and directly connecting one of the logic board tabs to the metal shield. I have not seen it re-enter 67Hz mode again. I then undid the 84V mod, put it back to the 68.4V mod, did the adjustments, and it runs flawlessly.