Reading Schematics - Need Help

clanggedin

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I need help. I can't read schematics. I just picked up a Chicago Coin Demolition Derby for $16 and it appears to have some issues. The player 1 car rolls on the screen like the horizontal hold is off and only half of the Player 2 car is showing. Also the vehicles you have crashed do not stay on the screen

Is there are good site that can tech me how to read schematics? Luckily these schematic have some notes on them on which ones are for the cars, and I can tell that the rectangles are promms, but the other shapes I don't recognize.

Please help.
 
AAARRRRGH! I hate typing nice detailed messages, only to lose them when the forum hangs while submitting!!! Sigh. Ok, short recap.

Congrats on an amazing find! You are a lucky @#%@. I really want that game to complete a set - Death Race, Demolition Derby, and Destruction Derby. And maybe Alley Rally? I own DR, I have Des.D on loan, hopefully to buy soonish, and I'm looking for a Dem.D.

I second "The Book". It's a must. Also highly recommended is "Video Game Operators Handbook" (Atari, TM-043). I didn't find a PDF, but I know I've seen one before. Originals can be had from ebay.

Send me a copy of your schematics and I'll give specific pointers. If you don't have an electronic copy, compare it to this and let me know: http://www.cityofberwyn.com/manuals/DemolitionDerby.pdf
 
Send me a copy of your schematics and I'll give specific pointers. If you don't have an electronic copy, compare it to this and let me know: http://www.cityofberwyn.com/manuals/DemolitionDerby.pdf

The schematics I have are almost identical to those except I am missing the cap list and someone was kind enough to write some notes telling me what some of the I.Cs represent. I reflowed solder on some of the ICs but I think I did the wrong ones, since I was missing the page telling me which side was IC numbering started at.

I am hoping it's a cold solder joint, since the issue didn't present itself until after the move. I need to remember to remove pcbs from cabs being moved so they don't get bounced around in the back of a pickup.

I read some of the Big Book, but It doesn't explain what the triangles represent on schematics. Maybe it does and I missed that part.
 
The other thing that's crucial from Bronze Age games is a TTL databook. It will tell you how each logic chip works. You can find most of the datasheets online, one at a time (e.g. search for "74161 datasheet") but having a real book in your hand is so much nicer for this. Check your library. You'll want the book from Texas Instruments.

Other than reading schematics, what's your level of understanding for digital circuits? I don't want to bore/confuse you with the wrong amount of info.

Oh wow. WOW! I think I just found the whole thing. (but it's huge - ~35 megs) Find it here:
http://www.tech-systems-labs.com/booksdata/TI-DATA-1976.pdf
The paper version is easier to use than a PDF. You need to be able to flip back and forth. But still....this is cool.
 
Thanks for the additional data to reference. I have almost no knowledge of circuits etc.. I can do cap kits, swap tubes, replace flybacks, but when it comes to the pcbs, I have no experience. I did replace a resistor on an ARII board once. but that's the extent.
 
Ok...that makes it harder in some ways and easier in others (blank slate and all that).

Here's a crash course. I'll jump around a bit by design, and I'm sure I'm leaving things out accidentally....I'm dead tired. But it should be enough so that you can ask questions as you need to. I'm looking at PAGE 1, CLOCK AND VIDEO OUTPUT.

In the upper left corner, the thing labeled 14.391MHz is a crystal (14.391 MegaHertz). It's a metal can with 2 pins. The squigglies labeled 470 are resistors (470 Ohms). The triangles with a circle are inverters, also called "NOT" gates. That's a logic gate that reverses the input signal. If 0V goes into it (the input is on the left) then 5V comes out (on the right). And vice-versa. Any time you see a circle ("bubble") like that it means "inverse". Inside the triangles, "7404" is the type of chip. Below the triangles "B16" tells you where to find the chip on the board...row B, column 16. The numbers by the input and output tell you which pin it is. For most on the chips on this board, there will be a notch at one end of the plastic body. If that end is the "top" of the chip, then pin 1 is at the upper left corner. The pins increment counterclockwise, so that pin 14 would be in the upper right corner. Note that a 7404 "hex-inverter" contains 6 inverters on the same chip. Usuallly, power and ground connections will not be explicitly shown in a schematic like this. You'll have to look at the datasheet. You can verify the pin numbers there too until you get the hang of it. The The thing labeled ".01 100" is a capacitor (0.01 uF, 100V). Generally speaking lines between components are wires and a dot where two lines meet is a connection (a solder ball). If two lines cross without a dot, there is no electrical connection. Inside the triangles, DTQ14 is the name of that particular gate. I'm too tired to work it out now, but there's a code there that is used for when a signal has to go from one page to another. That whole section (2 inverters, 2 resistors, and ITQ14) forms the master time base for the game...the "clock". If you were to somehow measure the signal on pin 6 of B16 you'd see that it's a 14.391MHz squarewave....it alternates between 0V and 5V (really about 3.5V) more than 14 million times a second. That's the last circuit function I'll go into for now.

The 3 lines that are connected to the input of T999 (pin 9 of B16) is the symbol for "ground", often abbreviated "GND". It is the 0V reference for the system and all other voltages are relative to this.

The other basic logic symbols are AND, OR, NAND(Not-AND), NOR(Not-OR). I'll give lookup tables or "truth tables" for how they work below. And an example of each gate from page 1. I'll represent 0V with a "F" (false) and +5v with a "T" (true) in the tables. (I don't know how to line columns up in the forums...use your imagination)

AND (A13) - if in1 AND in2 are both True, then out is True. Otherwise, out is False.
in1 in2 out
----------
F F F
F T F
T F F
T T T

NAND (F14) - notice the bubble...NAND is the opposite of AND!
in1 in2 out
----------
F F T
F T T
T F T
T T F


OR (F15) - if in1 OR in2 OR both are True, then out is True. Otherwise out is False.
in1 in2 out
----------
F F F
F T T
T F T
T T T

NOR (D15) - again, the bubble!
in1 in2 out
----------
F F T
F T F
T F F
T T F

On "PAGE 2, GAME CONTROL"....
The thing labeled "coin drop" is one of the edge connector "fingers" on the board. In this case it's pins 6 and 7. The thing labeled 1N4004 is a diode. In this case, a black resistor looking thing on the board. The circle labeled +5V means the diode is connected to the +5V supply at one end.

On "PAGE 4, CAR SHAPE"....
G6 is an XOR gate...exclusive or. If either one of the inputs is true but not both, then the output is true. If both inputs are the same, the output is false.

Most other things will show up as rectangles with labeled pins and a part number that you look up in the databook to see what it does.

This should be enough to get you started reading the schematic. And to help you ask questions in the future for more detailed help. And hopefully it's not overwhelming!! :D
 
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