RF Only CRT Composite Mod (Philips/Magnavox PR1305C1)

[gonjona]

So I had an old CRT TV with an RF only input that I got from my Grandma. At some point I might RGB mod it, but I'm really interested in a composite mod and then S-video.

Luckily, I was able to find a good URL service manual for this model. When I opened the TV I then found the model numbers of the microcontroller and jungle chip.

Jungle Chip TDA8846 URL service manual

Microcontroller SAA5542PS URL datasheet


So to preface off the bat, at this point, I don't have any mods working. So this will be a log of everything I have tried, to no avail. I'd really appreciate any insight to be given as there seems to be less documentation here and on the internet overall regarding composite mods rather than RGB. Don't ask why I want composite/s-video over RGB, its essential to my setup.

I started by just soldering to the first composite input I saw, which was on pin 23 of the MCU. I assumed that this would somehow be sent out of the MCU into the jungle chip.

Spoiler

After no success with that basic approach, I postulated that the reason it didn't work was because I needed to tie the blanking pin on the jungle chip high in order for the composite signal I inputted into the MCU to be picked up by the jungle chip. I soldered in a blanking switch following the model done in The 8-Bit Guy's video. I lifted pin 35 out of its hole and soldered it to a toggle switch, with the common being the point it was originally soldered to, and the other throw being the supply voltage pin of the MCU, which according to the datasheet should be 3.3V. The switch seems to be working, as the RF static goes away when I flip the switch (black screen), but I can still see the channel number, volume display, and other OSD stuff. But still no signal from my composite input (a Gamecube).

Spoiler

Now I should mention something I didn't try, you can see in the image above the two wires for composite and ground (yes, I tested continuity with some other ground points around the board and with chassis grounds and they had continuity). I didn't lift the composite leg out of the PCB. Is it possible that this would have worked had I done that since some of the components the leg was connected to could have been messing with the signal?

At this point, I decided to move the composite input somewhere else on the board. I focused in on pin 13 of the jungle chip. It seems like this is where the demodulated RF (now composite) enters the jungle chip. It seems like it's also the default selected input as well. So I lifted the leg of pin 13 out of the hole and soldered the composite input in its place. As soon as I turned on the TV the background static was completely different than the standard RF static, even with nothing connected. Unfortunately though, this remained unchanged when I turned on the Gamecube. Just to be sure, I flipped the blanking switch (the background pattern went to black) and went through every channel, and also went through every channel with the blanking switch off. Nothing.

Spoiler

So this is where I stand currently. I could try soldering the composite to every possible pin on the schematic that mentions composite, but it's getting late and I don't have time for that so I decided to make this post and see what I'm missing here. According to the datasheet of the jungle chip, the Y input pin 11 serves as a composite pin as well, which would be the next one I'd think to try. Or, potentially lifting pin 38 of the jungle chip and soldering the composite there in its place. Any ideas of how I should proceed are much appreciated.

[matt]

Adding composite to an RF only set is not a good idea. They typically have a hot chassis, and adding an A/V input can be a shock hazard both to you and the equipment you're using it with. You'd be better off searching for another TV.

[Osirus]

Skimming the service manual it appears to be an isolated hot/cold chassis.

https://www.manualslib.com/manual/69936 ... 103#manual

Also has a microcontroller with typical OSD RGB out/blanking.

[gonjona]

So I got the composite working, kinda. There is a weird distortion issue that hopefully one of you knows about a fix for.

Here's a picture of the composite video working:

Spoiler

Now the distortion issue seems to be related to the brightness of the signal, and is most noticeable in the bottom half of the picture, but is present pretty much everywhere. When the color is brighter, it seems to shift right. When the color is dimmer, it seems to shift left. Here are some images to explain what I mean. The right border of the picture seemed to be the best place to show the effect on camera.(look towards the bottom) I have the composite input connected to pin 13 of the jungle chip (lifted out of the board to disconnect it) through a 1uF ceramic capacitor with a 75R 1% parallel resistor to ground.

When the image is dimmer, a concave effect:

Spoiler

When the image is brighter, a convex effect:

Spoiler

Here is a video of the effect as well: (look at the bottom of the FIGHT text and at Roy's eyes)
video


Anybody know of a fix for this?

Next, lets talk about the audio. I traced the schematic and found the audio amplifier. Component 7954, TDA7052. Datasheet can be found here. I soldered an RCA jack to pins 2 and 3 (signal and signal ground). Got some really bad background static, which you can hear in the video posted earlier. Not sure what the cause of this is, but no matter, I can just use a custom amplifier. I would like to use the onboard audio controls from the TV. According to the schematic, this is at pin 4 of the amp. It seems to be between about 0-1V for the volume control. If anyone has any suggestions for an amplifier IC that can take an analog input for volume control, let me know.

So as I said before, I would really like to have an S-video input. Unfortunately all of the other composite inputs on the jungle chip are disabled on this model. They are controlled via the I2C bus, so theoretically I could change them by editing the EEPROM, but screw that. But there is one possibility left for getting S-video. Take a look at this section of the schematic. The problem is that the chroma has already been split by the time it goes into the matrix outside the chip. A possible workaround would be using an aftermarket chroma demodulator. I looked it up and there are a few chips I can buy but I don't know much about it. Are they all the same, as in is anyone good to buy for this purpose? Or is there something specific I should look for? Do you guys even think this would be possible? I wish I could just get another TV but that doesn't seem possible lol.

Spoiler

Thanks for the help everyone!

[KPackratt2k]

So as I said before, I would really like to have an S-video input. Unfortunately all of the other composite inputs on the jungle chip are disabled on this model. They are controlled via the I2C bus, so theoretically I could change them by editing the EEPROM, but screw that.

I know this is a long shot, but have you tried looking for an EEPROM dump from another Philips/Magnavox TV that has a similar chassis (or at the very least, the same Micon + Jungle Chip combination)? In my case when I was modding an RF-only RCA TV for Composite and RGB, I was able to find an EEPROM dump from a nearly-identical model that had an AV input to enable that feature on mine.

As for what you've described as visual distortion, it looks like typical CRT blooming to me. Does it do the same thing on the RF input?

[Brandon9677]

They have a hot chassis, and adding an A/V input can be a shock hazard both to you and the equipment you're using it with. You'd be better off searching for another TV.