Atari Tetris Sync issues

bakerhillpins

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I picked up an Atari Tetris the other day that's part of a little side project I have going (at some point I'll make a thread on that) and I drove over to a friends house to install it. Unfortunately after getting the JAMMA harness fixed up and installed I couldn't get the video working. Like nothing, zip, nada. The board was tested (and test results forwarded to me) before I bought it and it was shipped well (in anti-static bag too) so I wasn't expecting issues. Fortunately I threw a scope in the truck before braving the snowstorm to get there. I poked around the board and it all looked good. I was even getting video signals that looked OK, at least from a cursory point of view anyhow. Since I had limited time and I didn't have a 2nd board or monitor on site I packed everything (including his monitor) up and dragged it all back home to work on it here. I took the game downstairs and hooked it up to my test bed and a known good monitor. I got a scrambled image but it was recognizable so I it would seem the board works and it's a monitor issue. However, I spent probably 30 minutes trying to get the dam thing to sync to no avail. At this point it was time to break out the Oscope again and take a look at the video signals. They are attached.

One thing to note is that Atari Tetris is JAMMA (negative composite) but also has a secondary video header for Positive sync monitors. So I started pulling waveforms from there as it was easier than the edge connector.

The 1st is Positive H sync, 2nd is Positive V sync, 3rd is Red (blue/green are similar). These signals all look good. Note all are at 1v per division, but I changed the period for each.
 

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Now I compare that to the JAMMA output which should be negative composite sync. Scope attached, still 1v per division. Note that this signal is .5v p-p offset to ~1v. It also looks to be a positive sync format signal too.

Unfortunately there don't seem to be any schematics for Atari Tetris so I will need to spend some time tracing the circuit before I go any further.

I'll try to post up some sort of wiring diagram for what ever I trace. This is sooooo much less fun now that I am older and can't see for s**t anymore. :(

As the Positive sync signals are valid I could just run the board on them but I want to connect to a JAMMA harness so I am going to fix it. It also leads me to believe the error is in the logic that combines the syncs so that should limit the search and hopefully make this a quick fix.
 

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Ok, tracing the neg comp sync it hits a 7406 (F1) at pin 2. The 06 is a inverting buffer/line driver. The input that pairs with pin 2 is pin 1. When I scope Pin 1 I get the attached signal, which is good and clean.

The output is of course bad. (see previous post for neg sync) It's not inverted (most importantly) and it's not full scale so it's most likely bad. If the signal was inverted but weak then there would be the possibility that maybe a resistor went bad and was loading the output.

Now I need to locate a 06 replacement. When I do I'll report back.
 

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Good sleuthing. Note this is a fairly common issue. When there are sync issues, and it's not the monitor or wiring, the output buffer is very often the culprit.

It's a good idea to socket the 06 when you replace it, in case it happens again.
 
So after a rather lengthy delay before I ordered and received my parts I was able to replace the 7406 and return to using the JAMMA negative composite sync signal. Of course when I went to put other parts into my "stock" I stumbled upon a tube of 7406s that I had. Isn't that typical. :p
 

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