On Monday, June 4, 2018 at 6:30:48 PM UTC-4, 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.

Then I removed the dropping resistor (returned jumper) and tried
removing resistor R516 - 180K from the B+ input side of the HV output.
No difference.

Tried changing the value of C519 0.047ufd/200V, however increasing the
value made no real difference in the HV output.

Does anyone have any ideas on how to reduce the HV, from near 30KV to
around 19KV, that is safe and reliable? The original circuits have an
X-Ray protection that kicks in when the B+ reached about 10% over the
nominal value and I need to retain that safety feature. I would prefer
something that is pretty idiot proof.

Schematics of Electrohome GO7-19-CBO & Electrohome GO7-13-FBO:

https://www.flippers.com/pdfs/Temp/E...-schematic.pdf

Thanks!

John :-#)#

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I wouldn't worry so much about the HV as I would about the filament. If the filament voltage stays at or below 6.3, I wouldn't give it another thought. Most of those tubes have no trouble with a few extra KV. If the filament is high, drop that or the tube will have a life span emissions-wise in months.

Another way of lowering the global output of the fly is add some capacitance to the retrace cap. This will of course affect all the secondaries.

I'm not crazy about adding resistance to drop voltage on any load that's dynamic. If you scope the collector of the horiz output with and without a dropping resistor, you'll see the waveform will be different. You may have short horiz output transistor life. If the monitor uses some sort of series/pass regulator (most 70s regulators are) you can drop the voltage by playing with the regulator and keeping the supply tightly regulated to the fly.