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Ian Field Ian Field is offline
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Default How change color TV to adjustable HVPS?



"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.