The MW6-2

[Refugee]

I will have a look in the next couple of days and come back with a better idea. I am pretty sure that there are plenty of 1170s in one drawer and there are a couple of other types in a couple of other drawers.
My component store came from a closed down computer screen repair shop so some may be faster types and may have to be built with different timing capacitors.

[ppppenguin]

The field coil will be largely resistive but somewhat inductive. That means a fair size voltage spike during flyback. I've driven field coils with current feedback using a sense resistor. This ensures a very linear scan.

The line coils are another matter. Here the inductance will nearly always be dominant so the voltage waveform will be almost constant during the scan and a sharp spike during flyback. If you want very linear scan you can use current feedback but the amplifier will have to be able to deliver a high voltage. It may be easier to use classic line output methods, probably with a FET and energy reovery diode.

[Murphyv310]

Hi Jeffrey.
Yes I was considering a FET for line scan and a recovery diode. I'll concentrate on the frame timebase first. I'm half way through the build of it using Fairchild's circuit, not sure as yet if I will need to do any mods or not. Once the time bases are done the next question is to find out if grid or cathode modulation of the CRT is best, the MW6-2 is a triode.

[Murphyv310]

Hi.
Well I've built up a frame osc/output with the TDA1170S on a small piece of matrix board, it was a bit tight but it worked first time........ yipee!!. With just 9v from a PP3 I was happy to see that I can overscan the tube. So next is to devise a line stage that is efficient.

[ppppenguin]

You can measure the current required for full deflection, also the line coil inductance. This will give you the basic parameters for the timebase. Let's take a simple example. Say the coils are 0.5mH and 0.5A is needed to deflect the beam from centre to edge. If you need to do this in 32us then you need 0.5mH * 0.5A/32us = 7.8V. This is simply from the relationship:

V = L * (dI/dt)

Since I is meant to be a linear ramp, we don't need to worry about the calculus.

At the end of the stroke we have 0.5 * L * I^2 energy stored in the coil. 62.5uJ in this example. For flyback this will be transferred to a capacitor in (for this example) 12us. The peak flyback voltage can be calculated either by working out the value of C and using 0.5 * C * V^2 or more simply by multiplying the forward voltage by 32/6 (half the flyback period, to correspond with half the forward stroke). Hence 7.8*32/6 = 42V.

Not sure what EHT you're running at or if you plan to increase it, but remember that the deflection current is proportional to the square root of the EHT voltage. This is one important reason why TV tubes have almost all used magnetic deflection. With electrostatic deflection the scan voltage is proportional to EHT. For 'scope tubes with narrow deflection angles the deflection sensitivity problem was solved by post defeletion acceleration.

[Murphyv310]

Thanks Jeffrey for the info.
At the moment I'm running just under 5kv final anode volts, the frame scan is pin sharp but a little duller near the centre of the tube. I'm aiming between 6.5-7kv which hopefully will cure the unequal illumination. The little Chinese EHT DC to DC converter can supply 50uA without sagging and I'm pretty sure it will do the job, so line scan can be independent of EHT. I'm also hoping that the eventual outcome will mean I can do at least 405 & 625 lines.

[Refugee]

It looks like you have got the frame scan licked.

[Murphyv310]

Hi Ref.
Yes it looks promising,  I'll post up a few pictures once I've woken up properly.

[Murphyv310]

Hi.
Here are some quick shots,  all self explanatory.
I'll also post up the frame circuit later.
   
   
   

[Murphyv310]

With just 3v P-P video,  4.5kV EHT and a low level sine wave at 10,125hz fed to the scan coils I get this,  line is not locked but the 10,125 hz is from my phones tone generator,  very accurate.
   
   
So will this be a minimalistic TV once all done??