"Crow Berry" wrote in message
...
O.K. Thanx for all the responses. The circuit board has a label
" Z 1" where this small glass cylinder is and under it is what looks
like a symbol for a zener diode. Now I am wondering two things - how
do you test a zener diode and how can you tell what value it is for
replacement. The board also has " 4V7" next to the diode symbol and
with a magnifier I can see " C4V7" and "CH" on the diode itself.
Also the glass cylinder sort of looks cracked, but it is hard to tell.
Thanx again, Brett

That is, indeed, a 4.7V Zener diode. Power rating is a guess, but probably in
the neighborhood of 0.5 to 1 watt.
To test a zener, you need a power supply that can supply more voltage than the
zener voltage, at about 10 to 50 ma (DC of course).
Connect a voltmeter directly across the zener.
Connect a 1K resistor between the power supply positive output lead and the
zener cathode.
Connect the power supply negative to the zener anode.
Turn the power supply on and slowly turn the voltage control up. If the
voltmeter reads zero or less than a volt, reverse the zener connections.
If the voltmeter reads in accordance with the power supply's output as it is
adjusted, continue adjusting the power supply voltage upwards until the
voltmeter reads a constant voltage. That reading is the zener voltage.
If the zener is shorted, the voltmeter will read zero or a very low voltage
regardless of the direction in which it is connected.
If it is open, the voltmeter will never stabilize on reading, meaning that the
zener isn't regulating. Of course, this will also happen if the power supply
voltage isn't high enough to make the zener conduct. In that case, get a power
supply capable of higher voltage output.

Cheers!!!
--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Life is like a roll of toilet paper; the closer to the end, the faster it goes.