Figure 4 - Brute force testing to power up the viewfinder
Figure 4 - Brute force testing to power up the viewfinder

Another way to decode the wiring of your viewfinder of you cannot probe the camera board is to use a "brute force" hack attack. This operation is a bit more dangerous, as you may apply power in reverse to the viewfinder circuit, but by using a current limiting resistor or power supply, chances of damaging the board are greatly reduced. Figure 4 shows another CRT viewfinder connected to a breadboard, which contains a 9 volt battery and a 100 ohm resistor.

By running the battery in series with the resistor, you reduce the current draw in case of a short circuit or supply reversal, so damage is not likely to occur. Often the internal circuitry in the viewfinder is very robust and may include a power regulator or clamping diode, so this operation is normally safe. If using a battery, choose dry cell over an alkaline as it will deliver much less current if shorted or reversed. Also, a power supply with an amperage control is great, as you can limit the current to a hundred milliamps and then look for spikes as you randomly test the wiring combinations. A power reversal will pin your current reading or meter instantly, alerting you of the situation.

To test your viewfinder using the brute force method, bare all of the wires and plug them into a breadboard as shown in Figure 4. Using a current limiting power supply, or a dry cell (non alkaline) battery and a 100 ohm (or close) resistor, start trying combinations until you see your viewfinder begin to glow. Sometimes you can listen for the high voltage transformer buzzing, but seeing the screen light up is your best bet. Also, don't touch the circuit board while the viewfinder is running, as there are a few kilovolts coming from the tiny high voltage section. Due to the extremely low current, you won't hurt yourself, but I guarantee you won't enjoy a shock from 10,000 volts either! If you can identify the high voltage wire, then drop a small neon bulb from the output to the metal cage on the HV section and it will glow the instant you have power connected properly.

After a bit of brute force hacking, I decoded my viewfinder's wiring map as: yellow = power, red = ground, and orange = video input. This is practically the opposite of what you might expect for wiring colors, but hackers like us are not usually a concern during the manufacturing process!

Figure 5 - The CRT will glow blue once powered up
Figure 5 - The CRT will glow blue once powered up

The dull blue glow shown in Figure 5 is what you will see when your CRT viewfinder finally powers up for the first time on battery power. There won't be any image yet, but the glowing blue tube is a great sign that you are very close to having the viewfinder wiring fully decoded. After a few seconds of being powered on, remove the battery, and then feel around the tiny circuit board for any hot spots. If your board seems hot, try a lower voltage until the screen no longer glows or until it shrinks below the screen borders. Usually 9 to 12 volts will run these viewfinders without any hassle, but the odd one may be designed for only 5 volts.

If you have failed to decode your viewfinder using the last two methods, then you will need to explore the actual circuit board for clues. Looking for large capacitors is a good start, as the negative side of the can will always be going to ground. Often, there is just one common ground, and it may be connected to several wires on your connector. On mine, there were four or five redundant ground connections. Another easy way to decipher the wiring is to identify the small IC on the board and then dig up the datasheet to figure out which pins are VCC (positive) and VSS (ground). Almost all viewfinders will have a single large IC, which will be called a "television on a chip" or "complete NTSC video processor IC". I have not found a viewfinder I could not decode, but sometimes a little hacking and experimentation will be needed, especially with multiple wires of the same color. Feel free to scan or post an image of your viewfinder board in our forum and you may find someone else with the same unit already decoded.

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