[R-390] Selenium rectifier, was: R-390 Antenna Relay Chatter
Barry
n4buq at knology.net
Sun Oct 9 21:01:08 EDT 2022
For CR801, I'm definitely going with silicon. The clock has already been ticking on any original replacements and it just isn't worth it for this application.
I fabricated a small bracket that uses the original mounting hole for the selenium stack and had intended to mount four standoffs with four 1N400x diodes in a bridge but then I realized that the 1N400x is only good for 1A which is a bit closer to the required current than I'm comfortable with. I've ordered a packaged bridge that will still mount on that bracket so it wasn't an entire waste of time to make it.
I hesitate to replace some of the other rectifiers (e.g. the 26Z5Ws) with silicon as voltage drop then does become a bit of an issue. Besides, I kind of like tubes.
Thanks,
Barry - N4BUQ
----- Original Message -----
> From: "Bob kb8tq" <kb8tq at n1k.org>
> To: "Jim Whartenby" <old_radio at aol.com>
> Cc: "R-390 Mailing List" <r-390 at mailman.qth.net>
> Sent: Sunday, October 9, 2022 7:22:50 PM
> Subject: Re: [R-390] Selenium rectifier, was: R-390 Antenna Relay Chatter
> Hi
>
> Back in the day the state of the art was a selenium rectifier. They beat the
> copper oxide stuff out by a wide margin. We went through Germanium and
> have now forgotten about them. Silicon is not the “best of the best” but it sure
> wins the “lowest cost” race by a wide margin.
>
> If you want to keep the radio in “as issued” shape the indeed dig around for
> the original part. The same thing applies to the tube / silicon diodes you use
> for the HV supply. It also applies to that ballast tube ( which Fair Radio seems
> to have in stock …). It is *very* much up to you. There is no right and wrong
> here. You need to decide on what is right for your radio.
>
> Performance wise, go with the silicon stuff. Cost wise, go with the silicon
> stuff
> (by a very wide margin). Reliability / MTBF wise, silicon wins yet again. Yes,
> it
> might be silicon plus resistor or this or that, but the net result is the same.
>
> If you have ever had a selenium die and stink up the shack, I’d say you likely
> would vote silicon as well. Yes that’s just a personal opinion ( based on a
> really
> stinky shack ….).
>
> Bob
>
>> On Oct 9, 2022, at 5:53 PM, Jim Whartenby via R-390 <r-390 at mailman.qth.net>
>> wrote:
>>
>> GlennI've seen this link before but frankly it doesn't make any sense to me. If
>> you increase the foreword resistance of the Selenium rectifier, then the
>> foreword current must decrease. Less current, then less I^2R loss, so not a
>> great increase in heat. Now if the crappy Selenium rectifier reverse voltage
>> drop decreases, then more reverse current will flow, so a greater I^2R loss, so
>> much more heat.
>> This second scenario does make sense to me so I'm in the Emilio Ciardiello camp
>> but I go much further then he does. I don't bother trying to repair them, I
>> replace them! Read his paper (link is below), if you haven't already; it is
>> interesting.
>> The foreword to reverse current ratio in a typical Selenium rectifier is about
>> 500 mA to 8 mA or 62.5 times the reverse current. The silicon 1N4007 is 1 amp
>> to 5 microamps or typically 200,000 times the reverse current. The higher
>> reverse current in Selenium will, at least, increase the power supply ripple
>> voltage so this may be an overlooked reason for the higher B+ voltage when
>> Selenium is replaced by silicon.Regards,Jim
>>
>> Logic: Method used to arrive at the wrong conclusion, with confidence. Murphy
>>
>> -----Original Message-----
>> From: Glenn Little WB4UIV <glennmaillist at bellsouth.net>
>> To: r-390 at mailman.qth.net
>> Sent: Sat, Oct 8, 2022 2:07 pm
>> Subject: Re: [R-390] Selenium rectifier, was: R-390 Antenna Relay Chatter
>>
>> https://en.wikipedia.org/wiki/Selenium_rectifier
>>
>> 73
>> Glenn
>> WB4UIV
>>
>> On 10/8/2022 1:37 PM, Jim Whartenby via R-390 wrote:
>>> I'm not sure of how the Selenium rectifier actually fails. Lots of speculation
>>> abounds but there seems to be no hard evidence. One take on the failure mode
>>> can be found here:
>>>
>>> http://www.ase-museoedelpro.org/Museo_Edelpro/Catalogo/Overview/Articles/selenium%20rectifiers.pdf
>>>
>>> One curious observation is that there is little to no data, from any
>>> manufacturer, on the reverse current of the Selenium rectifier. Has anyone
>>> disconnected the load from the rectifier circuit and then measured the voltage
>>> across the low ohm current limiting resistor with an o'scope to calculate
>>> reverse current? An isolation transformer would be helpful!
>>> As everyone knows, the reverse current in silicon rectifiers is in the microamp
>>> region. I suspect that the normal leakage current in Selenium is in the single
>>> digit milliamp region as Emilio describes or perhaps even higher. If this
>>> reverse current raises as the reverse voltage knee approaches zero then this
>>> can certainly produce the heat needed to cause rectifier failure.
>>>
>>> Emilio Ciardiello has also written an article on the Collins development of the
>>> PTO which can be found here:
>>>
>>> http://www.collinsradio.org/pto-overview-specifications-history/
>>>
>>> The Collins PTO has it start with the ARC-2 but the ARR-15 is another story
>>> which is often overlooked. IMHO, this is the seed that later developed into
>>> the R-390 and all of the variations of the 51J and 75A series of receivers. I
>>> would like to get a copy of the RFP written by the Signal Corps that resulted
>>> in the R-390. It would answer a lot of questions.
>>> Jim
>>> Logic: Method used to arrive at the wrong conclusion, with confidence. Murphy
>>>
>>> -----Original Message-----
>>> From: Glenn Little WB4UIV<glennmaillist at bellsouth.net>
>>> To:r-390 at mailman.qth.net
>>> Sent: Fri, Oct 7, 2022 6:28 pm
>>> Subject: Re: [R-390] R-390 Antenna Relay Chatter
>>>
>>> I would suspect that the selenium rectifier has gone the way of all
>>> aging selenium rectifiers and has an increased voltage drop across it.
>>> Change the rectifiers to silicon and add a dropping resistor to get the
>>> relay voltage in spec and all will be good.
>>> Dropping resistor my not be needed.
>>>
>>> 73
>>> Glenn
>>> WB4UIV
>>>
>>>
>>> On 10/7/2022 12:59 PM, Barry wrote:
>>>> When in STANDBY, the antenna relay in my R-390 produces a bit of chatter. Is
>>>> this normal? I see where that relay is fed, unfiltered, from CR801 which, from
>>>> the pictures in the manual, appear to be a stack of selenium diodes and I'm
>>>> wondering if that rectifier stack might be a problem. Anyone know what to
>>>> expect from that relay in STANDBY and, perhaps, what I'm seeing might be
>>>> normal?
>>>>
>>>> Thanks,
>>>> Barry - N4BUQ
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>>
>> --
>> -----------------------------------------------------------------------
>> Glenn Little ARRL Technical Specialist QCWA LM 28417
>> Amateur Callsign: WB4UIVwb4uiv at arrl.net AMSAT LM 2178
>> QTH: Goose Creek, SC USA (EM92xx) USSVI, FRA, NRA-LM ARRL TAPR
>> "It is not the class of license that the Amateur holds but the class
>> of the Amateur that holds the license"
>> ______________________________________________________________
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