On 2018/06/05 3:06 PM, Dimitrij Klingbeil wrote:
On 05.06.2018 00:30, 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 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.

Hi

According to the numbers you provided, the EHT needs to be reduced by
about 7 kV (while keeping other voltages derived from the transformer at
their usual levels).

The 7 kV difference is significant, but not insurmountable for a series
dropper approach. The current draw of a CRT anode is small, the power
dissipation should still be manageable: 7 W at 1 mA (if the CRT ever
takes that much current).

There are TVS diodes with voltages up to 600 V (nominal value at 1 mA
test current) available (Littelfuse P6KE600A). A series string of 12
such TVS diodes will have around 7 kV of voltage drop across them.

The continuous power handling capability should be enough (each TVS of
this size can dissipate over 1 W, with heatsinking even multiple W).

The TVS diodes are reasonably cheap (some 40 cents a piece at Mouser for
QTY 10), so the main difficulty would probably be the insulation
(potting) of the series string. Probably best to put the whole thing
into a long insulating tube and fill the tube with epoxy.

Regards
Dimitrij



Interesting idea Dimitri, however I don't see it working out - 12 diodes
in series would make the anode wire ungainly at best. Then insulating,
etc. where I think the average tech probably won't have HV rated epoxy
or silicon glue. Appreciate the idea, and may use it elsewhere!

Another poster had suggested changing the capacitance at the Horizontal
Output transistor X01, and indeed that did work! I now have 19KV on the
HV output, screen looks normal and all is well.

Obviously I need to brush up on my Horizontal Output theory so I can
understand why this worked, but for anyone reading this I added 2 X
2200pf @ 1500V and one 1500pf @ 1500V caps. Took them off a dead GO7-CBO
- 19" chassis.

Thanks!

John :-#)#

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