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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?

Parents

  • Hi James,

     

    Sounds like a great project! Looking forward to seeing it in a video or writeup.

    I've not seen the Apple schematic, but it seems more complicated than the datasheet one. The datasheet suggests that the load should be 150 ohm, is the R54_2 connection going to ground or something?

    I couldn't see where that net went.

    What is TF1, is it to reduce noise? It may be worth shorting it out as an experiment, just in case the inductance or capacitance is too high.

    Also, I guess the MC1377 is from a reputable source, otherwise it may be already faulty perhaps : (

    image

  • in reply to shabaz

    Thanks for the taking the time to give it a look over and suggesting a few ideas . Look below for a reply, we found the problem. R9 was not connected.

     

    The 150 vs 220 doesn't seem to matter. Not sure why the Apple design went with the 220. I'm pretty sure there is a 150 somewhere else in the design.
    TF1 does appear to be for noise/EMI. On a functioning two IIgs, I removed and shorted the path. It had no effect. Though, I was thinking for a while it did some kind of isolation.
Reply
  • in reply to shabaz

    Thanks for the taking the time to give it a look over and suggesting a few ideas . Look below for a reply, we found the problem. R9 was not connected.

     

    The 150 vs 220 doesn't seem to matter. Not sure why the Apple design went with the 220. I'm pretty sure there is a 150 somewhere else in the design.
    TF1 does appear to be for noise/EMI. On a functioning two IIgs, I removed and shorted the path. It had no effect. Though, I was thinking for a while it did some kind of isolation.
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