KiwifruitDev - The Fast-Scan TV Chronicles (4/4)

The downconverters (again)

After experimenting and a lot of thought processing, my dad finally decided to take a crack at modifying the downconverters.

We determined that the green-labelled pot from the board was the local ocillator (LO) adjustment.

Problem is, we didn't have a signal generator.

The signal generator

I figured out how to conjure up a 1253.25 MHz signal generator using a Raspberry Pi 3 (B+) running rpitx.

I hooked up two leads from the Pi, one from GPIO 4 to the antenna and the other from a related GPIO pin to ground (I forget which one).

Using SSH, I ran the following command to install rpitx:


            git clone https://github.com/F5OEO/rpitx
            

            cd rpitx
            

            ./install.sh
            

            sudo reboot
            

            ...
            

            cd rpitx
            

            ./easytest.sh

From there, I set the output frequency to 1253.25 MHz and selected the Tune option which outputs a carrier signal.

The frequency counter

Reading more into the TVC-12G, I found out that a math equation is used to adjust the local ocillator (LO) frequency.

When we first hooked up the downconverter to a frequency counter (well, actually an ICOM receiver), we found that the LO frequency was 1240 MHz.

We then used the math equation to determine the LO frequency:

LO = F - 181.25 MHz (Channel 8)

The F is the target input frequency, which for us was 1253.25 MHz.

Therefore, the LO frequency should be 1072 MHz and not 1240 MHz.

Having hooked up one of the downconverters to the signal generator, we adjusted the LO frequency to 1072 MHz and found that the output frequency was around 181.25 MHz.

The suspense

Knowing that the old CRT TV could track channels without it being exactly accurate, we hooked up the downconverter to the TV and tuned to channel 8.

We didn't get a picture as nobody was using the repeater, but we did believe it was correct at least.

However, when we tuned to channel 50 (not 49 or 51), we got a completely black screen with static audio.

The audio carrier is usually 5 MHz off anyways, so it's not a big deal.

But picking up something on channel 50 sounds really weird.

The coil

Before messing with the LO, my dad contacted the Ham TV guy and he mentioned that there is a missing coil on the downconverter.

I can't share photos of the manual here, but on the board it was replaced with a straight wire for some reason.

We believe that this missing coil is related to the channel 7/8 output, and that's why we might be getting a picture on channel 50.

It'd take some trial and error to figure out if this is the case, but we'll figure it out.

The future

We're expecting the transmitter to arrive between Dec. 3rd and Dec. 5th.

Alongside the transmitter, my dad ordered an HDMI to composite video adapter.

We're going to try to use OBS Studio to project onto the transmitter as if it was another monitor.

I'm not sure if it'll work, but we'll see.