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MC1377P video circuit drops to almost 0 when terminated with 75 ohm. Any ideas?

baldengineer

A project I've been working on for several months, or years depending on how you count, is build a Mini Apple IIe. (Originally it was a portable, now its just a mini.) The key to the project is the MEGA-II ASIC from the Apple IIgs. It contains all of the support logic found in old Apple II computers. (Approximately 80 7400-series chips!).

 

Since no one has ever used the MEGA-II in this way, I decided to build a couple of prototype boards to test three key sections: 6502, MEGA-II, and VGC. Turns out, we have the 65c02 and MEGA-II to be seemingly working. It even attempts to boot the Apple IIe ROM. (And then it appears to panic.)

 

So, I moved our attention to the video board. There are two basic sections on the board: the digital side and the analog side. The digital side consists of the VGC (video graphics controller) ASIC and three 4-bit DACs for RGB. Long story short, the MEGA-II generates the classic Apple II video bitstream, the VGC+DAC section converts it to analog R,G,B.

 

The analog part of the video circuit is based on an MC1377 RGB-to-NTSC encoder. That is the part of the circuit giving me trouble. In this iteration of the project, I just copied the circuit from the Apple IIgs. (My final project won't have composite.) The problem is that when I attach a video receiver or terminate the MC1377's output with 75-ohms, its output drops from 2.6 Vpp to 300 mVpp. A bit too small.

 

Here's the circuit:

image

The Full KiCad file is available here.

 

Here's the comparison of output:

image

In both cases I am, effectively, probing at J18's signal. On the right, I terminate that node with a 75 ohm resistor. (Same behavior with an actual composite receiver.)

 

A bunch more waveforms and link to MC1377P datasheet here: https://www.baldengineer.com/mc1377-measurements-on-mini-apple-iie-prototype-video-board.html .

 

Any ideas why the output is clamping so hard?

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  • in reply to wolfgangfriedrich

    Golden Delicious was my original "code name" for the project. I thought of it because I was taking chips out of the IIgs. so: Golden deliciouS. I thought Apple IIgd would be clever.

     

    Back then, I also planned to make a portable/handheld computer. But, I recently decided to ditch the portable aspect. Instead I want to make something similar to the "The C64 Mini." The final PCB will go into a scaled-down version of a IIe case.

     

    Also, thanks to the most recent KansasFest, I've heard the original name for the IIgs was "Cortland." It's a type of apple that is cross bred between the McIntosh and the Ben Davis. McIntosh should be an obvious connection. However the Ben Davis, back in the day, was known as a cash-crop for apple growers. So the name "Cortland" is absolutely perfect for the IIgs.

     

    Cortland was a cross between the new desirable and well-established, just like the IIgs.

  • in reply to baldengineer

    Oh, I love good code names. Thanks for sharing the details and the history.