How can I check zener diode?

[bshi02]

hello.
I have 29 inch LG CRT TV(CN-29H3) which won't display any screen.
and it is written "MC-7CA(MAIN) 6870VM0056C(6) 980424 L.W.G(0)" and
"MC-7CA(CPT) 6870VS0167A(1) 971014 K.J.B(0)" on the surface of its PCB.

Frankly speaking,I posted a thread about this CRT TV in the reddit forum.
But It was impossible to repair this CN-29H3 CRT TV.
https://www.reddit.com/r/ElectronicsRepair/comments/1cb7m6y/how_can_i_check_str_6707_regulator/
and I also post a thread in the badcaps forum,and I finally got a clue of this problem.

It may be related to defective zener diode or C3852A transistor.
https://www.badcaps.net/forum/general-topics/general-capacitor-questions-issues/3282862-how-can-i-test-non-polarized-capacitor#post3297183

I attached a pdf files(lg_chassis_mc-7ca_sch.pdf) because I found that Its schematic of Hot area is almost the same to that of CN-29H3.

I measured pin#9 of STR-S6707 by placing red probe of analog meter to pin#9 and placing black probe to negative termainal of C808 the biggest capacitor(400v 470uf) and It was measured 6.4-6.6Vdc.

But In the datasheet of STR-S6707,It says that its On-State Voltage is 7.6Vdc.So I think that there would be a defective component which is undoubtedly adjacent to pin#9 of STR-S6707.

And In the same way,I measured cathode of ZD808 zener diode and Q800(C3852A)base,It was measured 7.1Vdc.

I also measured positive terminal of C805 and cathode of  D807 and Q800(C3852A)collector,It was measured 10.8Vdc.

Judging from pdf files(lg_chassis_mc-7ca_sch.pdf), ZD808 has 7.5V  specification.So I desoldered this ZD808 zener diode and I checked it with zener diode tester which was made with the aid of veteran eevblog users.

It displays 7-7.5V on analog panel meter and 0.5-1V with placing opposite polarity of probe.
and It is written B.75 on its surface.

I desoldered ZD413 zener dioder which has 5.1v specification and also checked it with zener diode tester.
It displays 5V on analog panel meter and 0.5-1V with placing opposite polarity of probe.
And it is written B.51 on its surface.

I think that There is no faulty in ZD808 zener diode with zener diode tester. I don't know why Output voltage of ZD808 zener diode cannot reach 7.5Vdc..