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Big Bad Bob Big Bad Bob is offline
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Default How do you test a Selenium Rectifier?

On 12/01/18 22:30, wrote:
How do you test a Selenium Rectifier?

Yes, I am aware that they should be replaced, but on preliminary tests
of this 1956 Zenith tube AM-FM Radio, I want to know if it's working. My
digital multimeter shows 1meg in one direction, 2.5 meg the other way.
NOT DISCONNECTED FROM THE CIRCUIT. To me, that seems bad, but I have not
worked with one of these in many years, and never had to replace one in
the old days. My meter was climbing, probably from the filter caps.
during this test.



forward/back resistance with a basic ohm meter should be around 100:1 or
better as I recall, and you might have to use a resistance bridge of
some kind (or other measuring circuit) with a voltage potential of
several volts for the higher voltage selenium rectifiers.

Selenium rectifiers might have a voltage drop of several volts during
normal operation. So maybe a test circuit would work better...

You could drive normal AC power through it, into a (safe) resistive load
(no capacitors), and then look at the downstream waveform on an o-scope.
It should be half wave AC at a reasonable peak voltage, into a
reasonable resistive load (let's say a 10k or 20k several-watt resistor).

I assume it's for a radio that runs on 110V [or maybe 220/240V for EU
and AU radios]. So you'll see a half-wave 50/60 cycle waveform that
peaks at around 1.4 times the input voltage.

If you look at it with an o-scope and see too much on the negative
cycle, you'll know it's bad. Otherwise it should be ok to use it as
long as the resulting output voltage is correct.


--
(aka 'Bombastic Bob' in case you wondered)

'Feeling with my fingers, and thinking with my brain' - me