CRT General Discussion.

[dominicbeesley]

Thanks Graham for the compliment!

Thanks Klaas for the suggestion I've just got back and had a quick look into this. However I don't think it is EHT regulation as this happens with or without flyback suppression and doesn't vary with picture content. (I tried a pure grey raster with all lines the same and it stayed like this.)

Next I tried disabling the V & H outputs and looking at the line drive waveform (to rule out feedback / decoupling issues) and there is still a problem. Watching the line sawtooth from the buffer after the 555 it wobbles at about 12.5Hz with all other circuits out (i.e. just clamp/sync seperator and H sawtooth and flywheel sync in place)

I can actually see a very small variation of the flywheel sync control voltage (just above the noise floor of my scope). This I think may be the "missing" control update during the missing sync line - I've used a double diode push/pull phase detector but this must be leaking during the flyback and causing a ripple? However this could be a symptom rather than a cause!

I'll try and have a look later tonight but I've just got back after a long and frustrating car journey (long in time not distance!) and need a cup of tea and a lie down!

Thanks

Dom

[dominicbeesley]

Hello all,

I finally tracked down the reason for the wobbly edge - it was the high-gain "error amplifier" part of the line-driver was picking up noise from the power supply. Decoupling this more effectively gave better results.

I'm now trying to design a better version using split-rails as per Steve's advice. I've now built a +/-12V and 5V power supply.

For the next attempt I'd like to swap the coils round - so as to get a lower inuctance frame coil so as to help me to use that for spot-wobble. [i.e use the original 15kHz line coil as the 12.5Hz frame coil (with a 1Mhz wobble)]

However this means using the original frame coil as the 400Hz NBTV line coil. Here I'm running into difficulties getting the flyback action strong enough with only 12V to play with. I'm guessing to get this to work I'll need to rig some kind of flyback transformer but have no idea where to start in terms of calculations for this....does anybody have any pointers? I was thinking of hand-winding something on a ferrite core but no idea what inductance / turns ratio etc I should aim for, inductors still make my brain hurt!...

Cheers

Dom

Picture attached shows my attempt so far. Green trace is line coil current (in a 4mH 3.8R coil) blue is drive voltage measured at junction of R16, R17