On 10/19/2012 12:33 AM, Robert Baer wrote:
Ian Field wrote:


"Ian Field" wrote in message
...


"Robert Baer" wrote in message
...
Ian Field wrote:


"Robert Baer" wrote in message
...
Ian Field wrote:


"Robert Baer" wrote in message
...
Attached is a truncated version of the manual as the 2Megs would
not
transmit.
Also, are enlarged parts showing horizontal section; T703 output to
CRT is about 30KV.
Would be nice to have adjustment from around 10KV to full 30KV.

Thankx.


Do you actually need the full current that a CTV flyback stage can
generate 24kV at?

Years ago I was messing around with the transformer from the PSU
out of
an Amiga or somesuch - I'd turned the transformer around and
driving one
of the original secondaries with a low VCEsat power transistor to
get
high voltage from what had been the primary.

Apart from turning the transformer around, it was the same blocking
oscillator style self oscillating chopper as the original circuit
- the
problem encountered the first time I powered up was a continuous arc
between the pins of the high voltage winding. It was neccessary to
complete the circuitry including the regulation loop for safe
operation.

If you don't need a vast amount of current at EHT voltage, this
basic
circuit driving a diode/capacitor ladder will do just fine.
I have no idea as to how much current these nominally supply to the
CRT; suppose it is maybe 1mA or so.
Current capability could decrease as voltage decreases but it
would be
nice to have nearly the same current capability as far down as 20KV
and then decrease in current capability.

I see no sense in ripping out the xfmr, building another supply do
run
another oscillator - why re-invent the wheel when it is there and
nicely round?


So,how could one vary the output voltage?

Pretty much the same as the original self oscillating chopper, control
transistor (2N2222 etc) clamps the chopper base on overvoltage. The
control circuit can be simple as you like or complex as you need -
simple voltage divider is probably best if you sample from the end of
the ladder.
Thank you for not reading the question and avoiding what i asked for.



I chose to interpret the question as not being silly - the TV sweep
section is not designed for PWM (you might get away with burst mode
thogh).

It seemed more sensible to assume you were asking how to regulate the
voltage on the PSU which was designed for PWM in the first place.


Another thing I just remembered - a TV flyback unit is highly resonant,
that means you'd have to keep the scan yoke in circuit (and accomodate
the physical assembly on the bench) to keep the resonant system tuned.
Without resonance you don't get the full flyback EHT voltage.
Bench?? For this exercise, everything in the TV will remain except the
addition of my control and a filament disconnect (so that the CRT passes
no DC current).


If I were gonna do it, assuming efficiency is no object, I'd regulate
by changing the DC volts going into the flyback.
Might even be able to do a first order approximation of pulse width
based on desired output voltage and close the DC volts loop around that
for regulation.