Table of contents
I bought this unit for 6€ as the store was emptying its stock before the impending shutdown of the analog TV transmitters. It was marketed as a multi-function set for camping. It really has it all :
Checking out the features. I think you'll enjoy the part where I test the radio without recording any sound. |
Pointless video of me fruitlessly trying to find an active channel one year after the transmitters have been officially shut down. |
 |
 |
 |
 |
 |
 |
 |
 |
 |
I didn't particularly care for any function besides the TV to I didn't check them in detail but the TV and radio seemed to work fine. I probably wouldn't have trusted my life in the compass and thermometer which both seem to be around 10 degrees off. I doubt anyone in their right mind would be interested, but I still scanned the manual (in French) for posterity.
Naturally next step was to take it apart and see how the TV portion could be separated from the rest. Here it lies eviscerated :
 Board top |
 Board bottom |
 Lamp |
The left side of this board handles the radio function. From the top is the AM antenna, next the tuning capacitor, and then the CD2003GP integrated radio chip. On the right side are some switches and the inverters to generate the high voltage needed by the fluorescent lamp.
 Board top |
 Board bottom |
The black DIL package IC at the bottom is a CD7313GS audio power amplifier. Both the audio from the radio and TV are routed through this board.
 Board top |
 Board bottom |
 CRT |
 Close-up of the TV decoder chip |
The whole board is built around the CD5151CP. Excluding passives this chip seems to have everything needed to build a TV receiver : video and audio intermediary frequency, tuner frequency control, automatic gain control, FM detector for audio, detection of synchronization pulses and horizontal and vertical deflection oscillators and drivers. There is also a TA8164P, which is a radio tuner IC. I'm not sure what its purpose is, since the CD5151CP already has integrated FM decoding circuitry. It may have something to do with the SECAM-L standard. In any case the CD5151GP is where I'll be looking to try to access the video signal.
The datasheet of the CD5151GP is only available in Chinese, which I unfortunately cannot read, so I tried poking around a bit on the PCB and pins of the CD5151GP. I didn't learn much from this, only that the image seemed to stabilize when I applied a composite signal from a game console on pin 6 of the IC, so I figured there must be some synchronization circuitry here. However that was not enough, and since there are no more TV transmitters operating in my area, if not the whole country, I couldn't tune the receiver and find the familiar composite signal anywhere. Fortunately I found out that the CD5151GP is equivalent to the AN5151N (datasheet still partly in Chinese), which is itself equivalent to several ICs including the KA2915 for which I could find a datasheet in English.
Let's have a look at the block diagram :
Pin 6 is synchronize input, but more importantly pin 5 is video output. So if I cut off this pin and replace it by my composite signal, I should see something.
I'm clearly on the right path and there are some bits of the image I'm expecting appearing on screen. However the text is in black and there is a bright stripe going down across the screen and some horizontal lines that are not part of the image. Looking at the standard composite signal timings, the issue is clear : the signal is simply inverted, white text appears black and the synchrnoization pulses appear as bright stripes. Let's invert the signal using a BC337 transistor and see what goes :
I'm quite happy to see this :-) Almost there. The image is still wobbly and not as crisp as it should be though. I'm thinking the wobbly edges may be due to some 50Hz superimposing on the signal or the beat frequency of the oscillators in my game console and the TV chip. In any case the oscillogram below shows that the inverter I built has a way too low bandwidth, all the high frequency components disappear and this is what causes the washed out picture. There is also a large DC component to the signal, but I'm not sure it matters since it still synchronizes well.
Well that's where I'm at now, I'll post more when I figure out how to improve the inverter.
Comments
http://modelrail.otenko.com/c64amiga/commodore-64-using-an-ancient-bw-tv
I've just hooked up composite input to a Trakton TR-735 portable BW TV.
Note that I also had a dodgy picture. It wasn't until I also passed in the audio channel that the picture cleaned itself up!
i love old school hacks