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portable tv composite mod

I’ve got a few of these old portable CRT TVs laying around and I thought I might use one of them for a display for a homebrew computer I am working on. The computer outputs a composite video signal, but these TVs are RF only, so I figured I’d try to mod one to add composite in.

The TV I went with is a Bentley 100C. Mainly because I like the way it looks.

I opened it up and took a look at the board. Most of the examples of composite mods on the internet deal with TVs that have a dedicated IC for the RF demodulation. This TV is all discrete components.

The demodulation circuit is under the metal shield in the image below.

I removed the shield and probed around until I found an area that I can inject a composite signal. I cut the trace leading to the base of Q201 and soldered a temporary wire to clip on to. The video came through great, so I moved on to sound.

This TV did have a sound IC, the KA2101. The audio out was on pin 8 going into a capacitor. I cut the trace from pin 8 and soldered the sound wire onto the capacitor.

The traces that I cut are marked in red in the image below.

 

I ran the wires to the rear panel and soldered them to some panel mount RCA jacks. I soldered a 75 ohm resistor across the video input for impedance matching.

I mounted the jacks in the battery compartment.

With the battery cover on, you can’t tell any modifications have been made.

The picture and sound quality is actually quite good. It came out better than I was expecting.