is it possible to identify a figure out the value of a blown psu zener diode?

[mj148]

hey eevblog people,

I have an old sony video-8 ev-s1 here that belongs to an elderly neighbour and it has a dead power supply, i've tried to find schematics or any data / pictures to help online but nothing useful so far.

The unit has sentimental value to the neighbour and he would like to see it up and running again if possible.

Anyhow, apart from the leaky capacitors that have ate away some of the board I can identify all the components and should have suitable substations for testing except for a zener diode that goes across the secondary of the transformer, it was burned in half so identifying it is out of the question, I assume it is a zener as the silkscreen on the pcb is a diode symbol with diagonal arms. (It runs from just below pin 7 of the transformer in the attached image)

I'd like to try and get the powersupply up and running so I can see if the actual VCR part of the deck is still functional, usually for something important i'd try and track down another identical unit to compare / swap the part or take measurements but even that is proving difficult.

So what i'd like to know is if there is some kind of power supply theory that will allow me to make an educated guess at what value diode was in there?

thanks in advance for any pointers

[fzabkar]

What is the voltage rating of C1? That should be the upper limit for the zener voltage. Allow for a 50% margin.

[mj148]

Hey thank you!

Ahh.. I think I understand.. those two were the leaky capacitors 16v 2220uf..

So 50% over or under margin? ie.. 20v-ish?

Many thanks.

[fzabkar]

those two were the leaky capacitors 16v 2220uf.

Since the capacitor is 16V, it must never see more than 16V in normal operation. Typically a designer will overrate a capacitor by 50% - 100%.
Therefore a likely operating voltage would be 9V or 12V. That would be the zener voltage.

If this is a regulated DC supply rail, try to determine which ICs are connected to this supply by performing a continuity test with your DMM. Then look up the datasheets for these ICs.

[abdulbadii]

If manual or schematic on  hand in such it might give T Volt info , or could meter, acquire V of sec. pin 7 - 6  less ~ 1 V (D dropped V)