Weird JVC 503B "RF converter" pinout?
 

I've come across a shiny box that despite calling itself an "RF converter" is actually an RF modulator for a VCR or something. What's strange is that it looks more "modular" than any other VCR modulator I've seen (wink wink), and only outputs on channel 3. Most of these have a switch to change between channel 3 and 4. Also instead of being hardwired to a VCR, the connections are exposed by contacts on the PC board like a computer card. So you can disconnect iwithout desoldering.
All in all it seems to be designed for extreme ease of replacement.

https://i.ibb.co/85MnfqN/MG-6822.jpg
https://i.ibb.co/vVrspYj/MG-6819.jpg
https://i.ibb.co/qrN6rym/MG-6812.jpg
https://i.ibb.co/zVZMGJL/MG-6814.jpg

Now, I would like to make use of this box but I don't know the pinout. All I know is that pins 1 and 6 are tied to the metal case (so ground or shield). That leaves 4 possible connections, +12v, -12v, Video in, and RF out.
Since I really have no clue about how this sh*t actually works, and the search engines don't yield thy results, I would appreciate help with identifying the pinout of this module.

This has a heck of a lot more guts to it than the usual VCR RF modulator. Seven transformers/coils and four pots? Hard to imagine what that's for unless maybe this has a lower sideband suppression filter so it can be used in a master antenna system and not interfere with another source on channel 2.

Is there a frequency marking on the crystal in the lower right?

Think this comes from an early top loading VCR. It's a lot more probable that it only has one power rail and takes both line level audio and video (if I owned the VCR it goes with I'd be rather mad if the RF didn't contain sound) input and has RF output.

Pin 4 is most likely RF out.
What you need to do now is find B+... It'll connect via resistors and chokes to multiple transistors and have capacitive filtering.
Audio and video should just have capacitors coupling them to 1 transistor each.
If I could identify B+ I'd work it from positive 5-12V (watching current draw), hook a TV to pin 4 and touch composite video to the 2 remaining input pins to figure out which is which.

It says:

30.625
42703-5
MEW

625... Is this a PAL converter???

I think pin 5 is B+. It goes from a ceramic cap to the coil pictured at top. From there it goes several places including the red wires. Red means positive. Then for instance a red wire goes through a resistor and onto a transistor's collector.

Probably for an old U-Matic.

Am I supposed to put -12v on the case? If I do this and put +12v on pin 5 the amp meter jumps a tiny bit.
What is a channel 3 signal supposed to look like on the oscilloscope?

30.625 MHz is one-half the RF carrier frequency for U.S. analog channe; 3, so that makes sense. Nothing to do with PAL.

I'd guess the other markings are a part number and maybe manufacturer's ID.

https://broadbandlibrary.com/signal-...owers-that-be/

See the right side of Figure 9.
You need a wideband scope (at least 75 MHz or so) to display this modulated carrier waveform.

Well, it's an RF modulator all right, and it works. The pinout is:

1 - Ground
2 - Audio
3 - Video
4 - RF output
5 - +12v
6 - Ground

For my use case I put -12v on ground. You must have 12v for optimal picture quality. Which also means you can run this module off a car's cigarette lighter but that's besides the point.
Too bad +12v accidentally came across my video source during experimentation and wrecked the Raspberry Pi's composite output. Luckily, HDMI can be used and this unit only costs $15. Still feels stupid though!

This is why on any test source even on service grade test pattern generators that are designed to be connected to B+ energized video circuits I tend to put a 630V cap on series with the hot lead...I rather waste the effort of adding a DC blocking cap than troubleshoot something that did work before I started using it to fix something else.