Guidance tv picture tube circuits.

[dr pepper]

Guidance on tv picture tube circuits.

I want to interface a b&w tv picture tube to a pic micro so I can produce shapes and graphics, I've seen this sort of thing done with scope tubes to make clocks, but not tv tubes.

I'm not new to electronics, but I havent messed much with tv stuff.

Having had a brief look the tube has the usual field coils to deflect the beam, that shouldnt be much of an issue, and the gun assy usually has a fly back transformer with the main HT going to the ultor on the tube, and 2 focus voltage outputs to the gun assy, the flyback tranny being driven off the horizontal coil output (so generating a 15kc waveform should do the trick for the flyback) also I'm going to need heater volts for the tube.

I'm not looking for a full raster type system, just something where software can position the beam anywhere on the screen, I dont have a problem so much with the deflection circuit, I just need to be able to produce a dot (in other words make a circuit that will produce a dot in the centre of the tube).

 

[mister_rf]

In my opinion it’s difficult to control both horizontal and vertical CRT deflection stages using variable frequencies. There are some critical timings and components with critical function/values chosen for a dedicated frequency, and if those values are modified the amplitude of the flyback pulse will increase several thousand volts up to unsafe levels.
One option to keep all related ICs for video/synchronization, H and V output stages and to concentrate only generating the composite video signal as you need. See for example some details here:
**broken link removed**

 

[betwixt]

You have discovered a fundamental difference betwen scopes and TVs. Oscilloscopes use electrostatic deflection which to the X and Y amplifiers looks like a fairly small capacitive load across the frequencies the device works over. TVs use magnetic deflection which is at fixed frequency and tuned to the scan rate. You may also have a problem with the EHT generation as the same frequency source is normally used to provide the drive waveform and the horizontal scan signal.

What you are asking for can be done but it's far more difficult than it first appears. You probably need several Amps of deflection current and to the driver circuit it would look like a high 'Q' tuned load. To simply make a dot, all you have to do is provide the correct voltages to the CRT. Be careful, a stationary dot will burn the phosphor very quickly unless you keep the current very low.

Brian.

 

[FvM]

Magnetic deflection with "random access" isn't impossible with respective current amplifiers, it will be more or less easy if you don't need high frequencies and particularly high di/dt rates.

 

[dr pepper]

Ok then.

I had thought that the deflection system miht well be tuned to a particular frequency, but maybe there are other issues with random scans as you guys just pointed out.

Also I found there are picture linearity issues with all tubes, these are sorted with the timebase generator.

I think the suggestion of keeping the deflection timebase circuit is a good idea, that would mean the micro would have to keep up with the rasta scan, and if I go to that length it wouldnt be much more comples to generate a composite video signal.

So I'd better look into if theres enough speed in a pic running at 20 mc's to generate video, generating the signal isnt so bad its all the other stuff and the updating.

Also I need to find out how to adapt my mini tv so that it has a composite video in, it doesnt allready have one.

 

[betwixt]

You are correct about the linearity issue, the waveform fed to the deflector coils is deliberately skewed to make the resulting scan linear.

Converting the TV to have composite input should be fairly easy as long as it has an isolated chassis. If you have a schematic or can give a model number I might be able to advise. I have done numerous monochrome and colour adaptions in the past.

Brian.

 

[dr pepper]

Its a little b&w roadstar 12v/240v with a 5.5" screen, the tvm5002 is very similar, the controls are the same it looks like a cosmetic upgrade of mine, I cant find diagrams for it except for a nasty one linked below scanned from the user manual (not by me), roadstar probably thought it a disposeable item and didnt print any.

Theres a single ic managing the deflection and the gun by the look of it, and a uhf/vhf tuner head at the back, if I'm lucky the tuner will output composite and all I'll need is to match the level.

https://i807.photobucket.com/albums/yy352/widgidibbie/55TvSchem.gif

The last connection on the tuner can goes to a tranny which is driving a transformer connected to the controller chip, could this be a compo video in, or is it just gonna be IF in or something no use to me.

 

[mister_rf]

I think you need a cord that has a male 3.5mm headphone jack on one side, and you can feed the composite video (and audio also) to the TV set.

 

[dr pepper]

I'm a twonk, I got that schematic from the net last nite and posted it here, its for a set similar to mine, what I didnt notice is that the schematic is for a set that has compo video in, mine doesnt have that, but mine has the same chip, the ka2915 and a ery similar schematic, if I'dve looked a bit harder I'dve seen before I posted that the schematic clearly shows a video/audio socket.

I found a cool vid on youtube for converting the same set mine to audio/video in (link below), this guy disconnects the compo video output from the chip and connects it to a video source using it as an input instead of an output, I wondered about doing that but I thought the impedance of the ic's output would be to low to inject video to it, but this guy got it to work.

The ka2915 chip is probably quite common in cheaper 12/240 sets so this might be usefull for other readers.

Gonna remove the tuner can and the audio pa to save power, and make the telly into a video monitor, I'll probably mod the circuit so's its the same as the schematic I posted as that was a manfacturers trick, probably more reliable than the youtube mod.

How to Modify analog portable Black and White TV for Composite video and Audio Inputs - YouTube

 

[dr pepper]

Its a very rough job but I sorted out the little telly.

Turns out there was a wire link on the board where video comes outfrom the if block and video discriminator and then goes back into the controller chip for sync purposes and also out to the gun driver tranny on the tube.

I just broke the link and connected a video signal to it, works a treat, even connected up the audio.

The tuner can has been removed and the tuning pot and band switch so its now purely a video/audio monitor, which was my aim.

Now to work on the pic software to generate video. heres the tv, like I said its not pretty, you dont see many of these with a scart on the back.

 

[dr pepper]

Right then, my little tv is now a monitor.

I have started on the video software, I've a pic16f88 running at 20mc's, I have an interrupt from timer 0 running at 15.625 kc (pal line timebase), upon each interrupt I call hsync and then backporch, these kinda explain themselves what they do, and after those I call videodata which takes the corresponding bytes from memory and shifts them out through portb (about 20 characters with dark zones where the new byte is fetched), bit zero is connected to the video output through a resistor (porta bit 0 is also connected to the video out through a resistor its used for hsync, video analogue is produced with a crude 3 resistor network for b&w).

Using a line counter every 312 lines (625 lines for pal) I call a sub that generates the correct vertical sync pulse train.

Every other line the software skips video out, so every other line is blank, this means that I have processor time to do other stuff as well as set up the next line ready, it doesnt look so bad and everything is done with the pic and 3 resistors, I still need to incorporate the software that reads the characters and places them in 'screen' memory, the idea is to make a basic video text terminal with rs232.