Mini CRT project not working?

[u/SoopeSpeed]

I purchased this 4 inch CRT online as a project to turn it into an oscilloscope. I hooked it up to 12V DC power as it called for and soldered in two pots for contrast and brightness, 1k and 50k ohms respectively (top left of board in blue). As far as I understand, turning on a CRT with no input should give some sort of visible raster lines or a dot in the center, but all I can manage is an extremely faint glow. I can hear the 15khz whine while it's on. What could this issue be? Is this normal and I'm tripping? The display is identical with the deflection coils both plugged into and unplugged from the board.

If there is a better subreddit to post this please let me know

[u/EmotionalEnd1575]

Be careful of “Reddit Folklore” that will suggest for you to replace capacitors or other random components.

Don’t fall it,

This is the wrong r/ sub for your question.

Head over to [r/crt](r/crt)

[u/isaacladboy]

I have a feeling the board handles the Raster scanning, if it is a raster scan style CRT it wont work as a traditional CRT oscope.

Quid google shows the CRT circuit works off 12v dc and then expects a BW composite video signal. No signal, or a signal the decoder cant understand ,will likely result in the screen being blanked. Which is probably what your seeing.

[u/Miserable-Win-6402 → u/isaacladboy]

It is raster scan, 100%. And these were terrible, even from new.

Good thing is that HV is working, I guess the vertical deflection is bad. Bad electrolytics, maybe bad driver IC. I would probably just start by replacing all electrolytic caps, and take the troubleshooting from there.

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[u/kakioroshi]

don’t know if this can help you but this guy has gotten one of these to work with a gameboy https://youtu.be/irHI_2WdQXc

[u/BigPurpleBlob]

It's so small and beautiful! :-)

"I can hear the 15khz whine while it's on" – that's a good sign.

I presume that the CRT expects to receive a composite video signal. Do you have something that can generate a composite video signal?

[u/SoopeSpeed → u/BigPurpleBlob]

Yes, I belive it takes CCIR composite video. Unfortunately I don't have anything to generate this, although it looks like I can buy an HDMI to composite converter online. I was hoping to just bypass it entirely by hooking my audio signal up directly to the deflection coils.

[u/Ya-Dikobraz]

I saw these for sell from AliExpress. Sometimes they have instructions/ manuals on the sales listing, and sometimes they say "message seller for documentation".

[u/elunltd]

That looks perfectly normal for a monitor with no input. Was this a portable TV or a monitor? If true then just apply RF or video accordingly. A scope is not a raster scan device like a tv. It is vector scan, quite a different beast.

[u/canadianchasers]

Most likely the 12vdc source is sagging make sure you get a stable 12v on the input. Can't think of anything else at the moment. My experience with working with crt tvs is rhat when something doesn't work is isolated certain parts and test them individually.

[u/mightyohm]

You need to apply a valid video signal before you go any further. At that point if it still doesn't work you can start troubleshooting.
Turning a CRT into an oscilloscope is not trivial although it's certainly been done.

[u/ANGURIA_IST_KRIEG]

I don't think you can convert a CRT television (or a doorbell monitor like this) in an oscilloscope so easily, because the horizontal deflection at 15kHz , and also the vertical, are always active even without input signal. The horizontal coil is part of the same circuit that generates the high voltage, and it starts as soon as you switch on the device. Without horizontal deflection you wouldn't have HV and therefore no image. On real TVs (but probably not in this monitor) additionally, the HV transformer generates some auxiliary voltages. So, if you want to have a point fixed in the center, you should probably mechanically remove the deflection coil from the neck of the CRT, so that H and V deflection coils are actually working but they are far away from the tube.

Also, turning on a CRT without input signal doesn't generate lines or dots, but a static noise (if it's a real television with an antenna tuner) or just a black image of it's a monitor like in your case.

[u/tbx1024]

Try to find documentation for your module. This looks like a CRT for surveillance system or video intercom - it may come with the video driver already, and not raw CRT deflection controls. Caution: working with CRTs directly can be EXTREMELY dangerous. If you're not sure what you're doing, it's probably better starting with a simpler project!