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Date Posted: 20:05:25 07/11/01 Wed
Author: Chris Lawson
Subject: CC VGA modification success

As promised, I did the VGA (not the 13"/High-Res) modification this evening. Here are the juicy details, coming soon to a web site near you.

As Mr. Watanabe and others note, pin 8 on the power transformer needs to be isolated, just as in the 13" modification. This pin contains the horizontal deflecting voltage signal that we are increasing, and thus must be isolated either way. Note that a continuity tester will, even after a successful isolation, register a contact between pin 8 and pin 9, to which it was formerly connected. You need to make sure you can see white where you cut, because the white is the base colour underneath the copper conductor. If you can't tell whether you've fully isolated pin 8, cut away more copper until you can.

Similar to the 13" modification, pin 8 must be jumpered to another pin on the power transformer, but in this case, you want to connect it to pin 12. Use wire of approximately the same thickness as the pins on the transformer, rated for a voltage of at least 100V. (This shouldn't be a problem with most wire this size.)

Once you've done that, you're ready for the "fork" in the procedure. There are three ways to fool the sense lines into thinking you have a VGA monitor attached.

Way #1, the easiest way, is simply to connect wires #20 and #24 in the IDC cable leading to the analogue board connector. Frogeye's Takky Correlation Tables have information about this (although it is directed more toward the 630 connector than the 57x one). I didn't want to mess with that IDC cable so I decided to leave this one alone.

Way #2 involves a slight modification to the logic board. The Gabezing Room has details of where the jumper should be placed, and you can choose to either cut the #20, 24, and 25 wires connecting them to their contact pins on the logic board connector or cover their contact pins with electrical tape. I recommend tape. As I wanted a plug-and-play solution (no modifications to logic boards), I also avoided this method.

Way #3 has not been detailed before on the Internet to the best of my knowledge, so I'm claiming credit for it :) (Of course, I couldn't have done any of this without the help of the CCSCC and its associated pages, Stuart, Eric Neumann, or Mr. Watanabe.) I decided, since I was modifying the analogue board anyway, that the modifications should stay on the analogue board. That way, I would be able to put any 575 motherboard into the CC without worrying about whether or not I had configured it for VGA signals. NOTE: this modification will PREVENT you from using the original CC motherboard, as it will not output a VGA signal. DO NOT PUT A CC MOTHERBOARD IN A CC THAT HAS HAD ITS ANALOGUE BOARD MODIFIED THIS WAY*.

I isolated pin 25 on the analogue board in the connector in the same manner I isolated pin 8 (i.e., with a Dremel and a cutoff disc, at low speed, with great care). Pin 25 is just below the location for J78/J79. I then removed J78 entirely, thus isolating pin 20 as well. I then connected pin 20 to pin 24 with a small wire. This leaves 25 isolated and sets the proper sense coding for a VGA monitor.

If you are missing J78/J79, you'll need to isolate pin 20 by cutting its trace to ground just as you do with pin 25. All other directions remain the same.

* I lost my blue colour circuitry after trying this; I'm not sure if it was coincidental or not. (The board was a bit questionable anyway.) I'm in the process of isolating the problem; if anyone knows the colour circuitry on the CC analogue board well, please contact me via e-mail.

cl

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Replies:

Re: CC VGA modification success -- Chris Lawson, 21:05:16 07/11/01 Wed
Re: CC VGA modification success -- Stuart Bell, 23:52:33 07/11/01 Wed
- Re: CC VGA modification success -- Chris Lawson, 04:49:45 07/12/01 Thu
  - Re: CC VGA modification success -- Stuart Bell, 05:48:54 07/12/01 Thu
    - Re: CC VGA modification success -- Chris Lawson, 08:10:23 07/12/01 Thu