Zener diode voltage for Symphonic

[Paul]

I'm working on a small AC/DC TV/VCR ,model SC309D, that only works on
DC. Shorted power FET, open fuse, open fusable resistor, shorted
zener. I think I have ID'ed all the bad parts, but do not know the
zener voltage. It's really small... labeled 36A. D641. I don't have
the schematic,, but suspect that it could be seeing up to 1.4x line
voltage, not just 36v... I think the A stands for 5% tolerance grade,
but wonder if the 36 is a cut-off 1N- number instead of the voltage.

TIA,
Paul

 

[Arfa Daily]

I'm working on a small AC/DC TV/VCR ,model SC309D, that only works on
DC. Shorted power FET, open fuse, open fusable resistor, shorted
zener. I think I have ID'ed all the bad parts, but do not know the
zener voltage. It's really small... labeled 36A. D641. I don't have
the schematic,, but suspect that it could be seeing up to 1.4x line
voltage, not just 36v... I think the A stands for 5% tolerance grade,
but wonder if the 36 is a cut-off 1N- number instead of the voltage.

TIA,
Paul

Voltage working rating of any electrolytics across it or following it, will
sometimes give a good indication of what its likely maximum is. For example,
25v wkg cap, voltage unlikely to be more than 18v. This doesn't always hold
because if any such cap is small in value, it may well be 50v working
anyway. If we're talking a transformer supply here, you could always run the
set up very gently on a variac, with no zener fitted. When it *just* comes
to life, measure what voltage you have on the rail that the zener is
supposed to be regulating, then add about 20%. That should get you close
enough.

Arfa

 

[Mark D. Zacharias]

Voltage working rating of any electrolytics across it or following
it, will sometimes give a good indication of what its likely maximum
is. For example, 25v wkg cap, voltage unlikely to be more than 18v.
This doesn't always hold because if any such cap is small in value,
it may well be 50v working anyway. If we're talking a transformer
supply here, you could always run the set up very gently on a variac,
with no zener fitted. When it *just* comes to life, measure what
voltage you have on the rail that the zener is supposed to be
regulating, then add about 20%. That should get you close enough.

Arfa

This assumes the zener is situated across for example, the 5V or 14V lines,
and is not being used for some other purpose.
Also, if the zener was shorted, is was likely due to over-voltage (a
regulation problem). So measuring voltage across it's position in the
circuit would give erroneous info - if that voltage is running way too high
anyway. If this is the case, electrolytic caps are the usual culprit.

Mark Z.

 

[Tech Data]

This assumes the zener is situated across for example, the 5V or 14V lines,
and is not being used for some other purpose.
Also, if the zener was shorted, is was likely due to over-voltage (a
regulation problem). So measuring voltage across it's position in the
circuit would give erroneous info - if that voltage is running way too high
anyway. If this is the case, electrolytic caps are the usual culprit.

Mark Z.

It's a 36 volt 1/2 watt zener and is used for protection on the
secondaries. I've read about people getting lucky and just changing it
and all was well BUT, I think the reason it shorted was one of two
reasons (from experience). Q602 on the primary went leaky and in short,
forced the secondaries on too high causing the zener to short or, there
are cracks on the board on the primary of the switch mode where the
screws attach the board to the frame.

www.techdata-kickass.net

 

[[email protected]]

Another thing that will cause the 36v zener to short on Symphonic/Funai
chassis is the photocoupler in the feedback circuit going bad... Its
usually a good idea to replace it as well. Bout all you can do is cross
your fingers and hope. Let us know what happens

 

[Paul]

Thanks for the information.
I already replaced Q602 which was shorted.
I'll proceed with the 36v zener and fusible resistor.

Paul

 

[Paul]

So far I have not been able to locate a replacement for the fusible
resistor.
It is labeled :
3W 1R2 K
PH 00321
I assume it's a 3 watt 1.2 ohm 10% fusible resistor.
Does anyone know where I can buy one?

Thanks,
Paul

 

[Paul]

In addition to already replacing the power FET and line fuse. I replaced the
1.2 ohm series resistor and the shorted 36v zener.
Plugged it in and the line fuse popped instantly. There is a diode bridge
accross the line and a filter cap accross the DC output of the bridge, then
the FET and a transformer get involved... a switching supply, I assume.
There is still a short somewhere accross the DC output of the bridge. (I
thought I checked for that last night before installing the res & zener)
The new FET and zener seemed to survive... at least neither is shorted.
Disconnected the filter cap and that is ok. Need to keep disconnecting
things to isolate the short. (Would be easire if I had a schematic) It's
already barely worth the time/money I've invested so far. More of a favor
than originally expected.

Paul

 

[Paul]

Turns out the old FET had an isolated tab, and the new one had a live tab.
I Insulated the FET from the grounded heatsink and the short went away. Now
getting 163vdc from the linear line supply that feeds the switching FET, but
nothing happens when pressing the power switch. Would anything but the fuse
get fried with the FET drain grounded?

Paul