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John Robertson John Robertson is offline
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Default Reducing HV output voltage from Flyback/LOPT as used in arcademonitors

On 2018/06/04 3:48 PM, Jon Elson wrote:
On Mon, 04 Jun 2018 15:30:40 -0700, John Robertson wrote:

I'm looking for a safe and effective way to reduce the voltage output of
a replacement flyback that is not original for the circuit.

The initial problem - Electrohome 13" monitors used a specific flyback
transformer that is no longer produced.

The result - People are using the 19" Electrohome flyback as it fits
exactly the same, and is still in production overseas.

The danger - However what they are not aware of is the HV for a 19"
picture tube is 22.5 to 25KV and for a 13" colour tube it is supposed to
be 18.5KV to 22.5KV. Thus the 13" tubes are running at up to 25KV which
is 10% over their original maximum rating and my concern is an increase
of soft X-Rays. My HV probes show somewhere around 25 to 30KV for the
19" flyback when used in a 13" chassis.

My original test was to use a 75R 5W dropping resistor to limit the B+
to 100VDC to the Flyback (and only the flyback/LOPT) vs. 120VDC for the
rest of the B+. The result was proper HV, however other voltages
developed by the flyback made the results somewhat less that optimum.
The image, while good, was clipped on the left side.


Ugh! If they are getting other voltages off TAPS on the flyback, then it
is going to be real hard to ONLY turn down the UHV without affecting the
other voltages. This really requires different winding ratios. Now, if
the other voltages are just taken off the point where the transistor
drives the flyback, you could fix this by just adding a few turns to the
primary winding.

Jon


Would that it was so easy! This is a sealed flyback in a plastic
housing. Late 70s design.

I have gotten a bit further - using a 75R to 100R in series with the
Flyback B+ in I can get the HV down to about 20KV, and the +12VDC supply
is fine with that. Now the problem is the image is shrunk on the left,
but that turns out to be blanking after all. It has to do with
transistor X304 and its base feed. Need to muck around with the
resistors and caps a bit I expect to try and reduce the horizontal
clipping. With X304 disconnected I do get a good, although narrower,
image. I will have to expand the width, hope adjusting the caps C513,513
or C515 will give me enough width.

John :-#)#

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