[R-390] Selenium rectifier, was: R-390 Antenna Relay Chatter

[Jim Whartenby]

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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-- 
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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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