The PC-Engine, also marketed under the name TurboGrafx-16  is an 8-bit games console made by NEC/Hudson Soft which appeared in Japan in 1987. In terms of units sold, for some time it exceeded Nintendo and its famous Famicom (NES in Europe). Despite this success, it was never officially distributed in Europe. Sodipeng was the only company to market it, but it remained a pretty well kept secret. Nowadays, people who want to play again with this excellent machine are faced with a problem of incompatibility of the video signals, as the PC-Engine’s NTSC video output may not be compatible with some PAL/ SECAM television sets. The only way to be able to use this console and obtain a color picture is to connect directly to the HUC6260 video processor which provides the red, green, and blue primary signals plus sync.
As luck would have it, these signals are directly available on the machine’s rear expansion port. This port also provides the left and right audio signals, along with a 5 Vdc power rail. Even though the RGB signals are at the standard level of 0.7 V p-p, they still can’t be fed to the TV set directly, as the HUC6260 is not capable of driving into a 75 Ω load. This is where you get out our soldering iron, oscilloscope, and calculator! The principle of this circuit is very simple and is based around a single IC, the LT6551 from Linear Technology. The package contains four independent video amplifiers with a fixed gain of 6 dB. This IC is available in MSOP format, which means the overall size of the circuit can be kept down. The RGB + sync signals are picked up directly from the expansion port. The input impedance of the circuit is set at 10 kΩ so as not to overload the HUC6260. R9 for the sync circuit, R10, 11, and 12 for the RGB. Next, we need to eliminate the 3.6 V DC component and set the RGB signals at a more suitsuitable level. If the signal were to be amplified as is, the amplifier would be bound to saturate. So the choice of a proper level is vital in order not to distort the reproduction of the image being amplified. Capacitors C12– C16 provide coupling, and only the wanted AC component of the signal passes on to the next stage.
This AC signal needs to be fixed or ‘clamped’ to an optimum level. The specifications of the LT6551 offer an input range from 0 to 2.5 V maximum with a 5 V supply (see data sheet). R5/R13 and the three other identical pairs of resistors create voltage dividers. By choosing the values of 8.2 kΩ and 39 kΩ, you obtain an operating point around 0.86 V. A little calculation just to check: 0.7 V plus 0.86 V gives a maximum input signal of 1.56 V. It’s important to choose the coupling capacitor value correctly, according to the value of these resistors. Together, they form a highpass filter that attenuates the lower frequencies of the wanted signal. As a rule-of-thumb, you need to calculate this filter in such a way as to set the cut-off frequency at one tenth of the lowest frequency to be passed, which in this case is 30 Hz, the NTSC frame rate (25 Hz for PAL/SECAM). So let’s take 30 Hz as the cut-off frequency. The formula for the cut-off frequency of a first-order filter fc = 1/(2πRC) gives C = 3.9 μF (with R = R5//R13 = 6,775 Ω and fc = 3 Hz) and so you’ll choose the slightly higher value close to this: 4.7 μF for example.
The LT6551 amplifies the video signal by a factor of two (+6 dB) and so we find at its output terminals a signal of 1.4 V, together with a DC component. A capacitor (C1, C3, C4, C5) removes this unwanted DC component and the output impedance is set to the standard value of 75 Ω by a resistor (R1–R4). This 75 Ω output impedance is effectively in series with the 75 Ω impedance of the TV set’s input stage, which divides the voltage by two, bringing the video signal back down to its standard value of 0.7 V. And that’s why we need to use an amplifier with a gain of 6 dB. An 8-pin DIN socket carries the RGB + sync signals. The sound signals are filtered of any DC component and the RGB switching signal needed by the SCART input is also provided. All that remains is to make up the cable with the correct pin-outs. This little project helps us remember that video games can generate very serious activities, and that in electronics nothing is ever chosen by chance. Enjoy your gaming!