How do you test a Selenium Rectifier?

[t***@myshop.com]

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.

[Fox's Mercantile]

Post by t***@myshop.com
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.

A DMM won't read correctly. No enough voltage compliance on the current
source to read ohms correctly.
Just replace it with a 1N4007 diode and about 100 ohms series resistor.

[Cursitor Doom]

Post by Fox's Mercantile
A DMM won't read correctly. No enough voltage compliance on the current
source to read ohms correctly.

True, but it *might* give some rough indication, like if the thing has
somehow shorted out. I have a few of those lying around somewhere; I'll
measure 'em with a DVM just out of curiosity and see if I get readings
like those reported by tabbs.

Post by Fox's Mercantile
Just replace it with a 1N4007 diode and about 100 ohms series resistor.

Not kosher if you want to keep the internals original! Many collectors
are extremely fussy about that.

[t***@myshop.com]

On Sun, 2 Dec 2018 10:57:07 -0000 (UTC), Cursitor Doom

Post by Cursitor Doom

Post by Fox's Mercantile
A DMM won't read correctly. No enough voltage compliance on the current
source to read ohms correctly.

True, but it *might* give some rough indication, like if the thing has
somehow shorted out. I have a few of those lying around somewhere; I'll
measure 'em with a DVM just out of curiosity and see if I get readings
like those reported by tabbs.

Post by Fox's Mercantile
Just replace it with a 1N4007 diode and about 100 ohms series resistor.

Not kosher if you want to keep the internals original! Many collectors
are extremely fussy about that.

I plan to leave it on the chassis for looks, but replace it.
My voltage is real low, so it seems it's weak or worse....

[Big Bad Bob]

Post by t***@myshop.com
On Sun, 2 Dec 2018 10:57:07 -0000 (UTC), Cursitor Doom

Post by Cursitor Doom

Post by Fox's Mercantile
A DMM won't read correctly. No enough voltage compliance on the current
source to read ohms correctly.

True, but it *might* give some rough indication, like if the thing has
somehow shorted out. I have a few of those lying around somewhere; I'll
measure 'em with a DVM just out of curiosity and see if I get readings
like those reported by tabbs.

Post by Fox's Mercantile
Just replace it with a 1N4007 diode and about 100 ohms series resistor.

Not kosher if you want to keep the internals original! Many collectors
are extremely fussy about that.

I plan to leave it on the chassis for looks, but replace it.
My voltage is real low, so it seems it's weak or worse....

right they'll probably short out when they fail and blow the filter caps.

You could try a resistive load (10k-20k, several watt resistor) and
disconnect it from the filter capacitors when you do it. Then look at
the waveform. (I suggested this in a different post already, just
summarizing)

[Phil Hobbs]

Post by Fox's Mercantile

Post by t***@myshop.com
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.

A DMM won't read correctly. No enough voltage compliance on the current
source to read ohms correctly.
Just replace it with a 1N4007 diode and about 100 ohms series resistor.

And watch out for confetti. ;)

Cheers

Phil Hobbs

[Ralph Mowery]

Post by t***@myshop.com
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.

The same way as with any other diode. However the selenium drop about a
volt for each plate that is on it. Each plate is only good for about
25 volts , so you may have 5 to 10 volts to overcome. Therefore most
meters will not have enough voltage to over come that drop.


Get a voltage source of around 10 volts or more (maybe even a 9 volt
battery will be enough) use your meter on the voltage scale and put hte
battery , diode , voltmeter in series. Then turn the voltage or diode
around and measure again. There should be a couple of volts dropped one
way and many more (almost all) the other.

[Cursitor Doom]

Post by Ralph Mowery
Get a voltage source of around 10 volts or more (maybe even a 9 volt
battery will be enough) use your meter on the voltage scale and put hte
battery , diode , voltmeter in series. Then turn the voltage or diode
around and measure again. There should be a couple of volts dropped one
way and many more (almost all) the other.

+1

[Big Bad Bob]

Post by t***@myshop.com
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.