[NJARC] [NJARC Selenium Rectifier replacement mea culpa

Mon Apr 18 10:21:16 EDT 2016

  Bill -
           It’s OK….. I already forgot what I had for breakfast….. lol
  Bob B.
On Apr 18, 2016, at 9:17 AM, Bill Zukowski <n2yeg at optonline.net> wrote:

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> _________________________________________________________
> OK, egg on my face.  It wasn't a TO, it was a Hallicrafters S-72, kinda sorta like a TO.  
> 
> I had restored the S-72 last month, then worked on a H500 that I had purchased for myself.  Old age being what it is, I said TO when I meant the Hallicrafters S-72.  BUT THE REST OF THE STORY IS TRUE!
> 
> Realizing that the 1 volt tubes were very fragile to higher filament voltages, I used that as a gauge to determine the correct resistor value.  I used 1.3 volts, as that's listed as the normal operating voltage for these tubes.  Surprisingly the voltages were not all equal, but off by a few hundredths of a volt, I guess to to slight variations in filament resistances (never would have noticed it with an analog meter).  The B+ did drop a few volts, but operation wasn't affected (I didn't document it, but it was still well within 10% of specifications).
> 
> Sorry for any mental anguish that I may have caused.  
> 
> Gee, it's 9:15 AM already, time for my nap.
> 
> Bill
> N2YEG
> 
> On 4/17/2016 3:30 PM, antqradio at sbcglobal.net wrote:
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>> Reply = Poster
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>> _________________________________________________________
>> 
>> 
>> Bill
>> Which Transoceanic?  I checked the G500 and H500 schematics, which appear to be identical with respect to power supply, but didn't see a 20 ohm resistor associated with the Selenium rectifier.
>> 
>> I see your point though, the TO is a much different radio then the AA5 AC/DC radios which I was initially referencing.  Looking at the "H500 Restoration Guide" ( http://www.renovatedradios.com/articlePages/H500%20Restoration%20Guide.pdf ), John Kopp suggests, among other ideas, adding a 9.1 volt Zener from the junction of R23 and S3 to B- so that the filament string never sees anything above 9.1 volts when operated from the AC line.  
>> 
>> The Zener diode was added after John had already replaced R21, R22 and R23 in, what seems to me, to be an unfocused attempt to set the filament voltage across each tube to 1.3 volts.  He seemed to have replace those resistors because they were out of tolerance but nonetheless changed the resistor values from those specified by Zenith.  
>> 
>> Interesting to note that the 9.1 volt Zener will allow about 1.5 volts across each filament section at a filament current of 50 mA.  But that said, the Zener diode is much saver option then adding a few ohms to the 2k or so ohms already in the power rectifier circuit.  This in an attempt to drop the B+ supply a bit when changing from Selenium to Silicon to power the filament string when operating from the AC line.
>> 
>> Again, the filament voltage is much more important to tube life than a somewhat higher B+, (see http://www.bext.com/eimac2.gif )  note that this graph is for power tubes but it is still applicable.  It would be prudent to also add a 24 ohm, 1/4 watt resistor between S3 pin 5 and the junction of R19 and pin 7 of the 3V4.  This would drop about 1.2 volts and bring the filament string voltage to about 1.3 volts across each tube, assuming equal voltage drop across each tube filament section.  This will be helpful whether operating from AC line or 9 volt battery.  A lot less work and a better outcome then changing all of those power resistors willy-nilly.
>> Jim
>> 
>> 
>> From: David Sica <dave.sica at njarc.org>
>> To: William Zukowski <n2yeg at optonline.net>; Jim Whartenby <antqradio at sbcglobal.net> 
>> Sent: Saturday, April 16, 2016 4:04 PM
>> Subject: Re: [NJARC] Selenium Rectifier replacement
>> 
>> Thanks Jim, and Bill.  I wish this email reflector had a "like" button as I always appreciate hearing all the knowledgeable opinions. 
>> 
>> -- Dave
>> 
>> 
>> New Jersey Antique Radio Club
>> 
>> On Sat, Apr 16, 2016 at 2:43 PM, William Zukowski <n2yeg at optonline.net> wrote:
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>> _________________________________________________________
>> 
>> I replaced a selenium rectifier in a TO with a 1N4007 and had to increase the series resistor from 20 ohms to 47.  The series filaments are fed from the rectified DC, and the 1 volt tubes needed the additional resistance get them to the normal 1.3 volts.  So yes the resistor is necessary.
>> 
>> Sent from my iPhone
>> 
>> On Apr 15, 2016, at 17:38, Sal Brisindi via NJARC <njarc at mailman.qth.net> wrote:
>> 
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>>> _________________________________________________________
>>> You should put a resistor in series with the silicon diode as the B+ will be higher when replacing the selenium diode.
>>> 
>>> Sal
>>> From: Matt Reynolds
>>> Sent: ‎4/‎15/‎2016 5:10 PM
>>> To: NJARC Reflector
>>> Subject: Re: [NJARC] Selenium Rectifier replacement
>>> 
>>> Just remember
>>> Reply = Poster
>>> Reply All = Everyone
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>>> _________________________________________________________
>>> 
>>>    I'm probably the least knowledgeable\qualified person on this thread thus far when it comes to radio repair but I was always under the impression the resistor was added to the diode to help account for the differences in how the selenium rectifier works\"acts" in relation to the silicon diode replacement.  It was my understanding the resistor was not added to protect the diode, but the resistor was to protect "the radio" from being "harmed" by the different characteristics of the silicon replacement.  I don't ever think I've heard of fear of the modern part being damaged/burnt out.
>>> 
>>> Matt
>>> 
>>> ________________________________________
>>> From: NJARC <njarc-bounces at mailman.qth.net> on behalf of Raymond F Chase <raydio862 at verizon.net>
>>> Sent: Friday, April 15, 2016 4:35 PM
>>> To: 'David Sica'; 'Jim Whartenby'; njarc at mailman.qth.net
>>> Subject: Re: [NJARC] Selenium Rectifier replacement
>>> 
>>> Just remember
>>> Reply = Poster
>>> Reply All = Everyone
>>> 
>>> _________________________________________________________
>>> The predominant failure modes for silicon diodes are: 1. Overheating the
>>> junction by gross current overloads and 2: Exceeding the peak inverse
>>> voltage (PIV) rating.  Silicon diodes with 10 times the expected current
>>> rating and several times the PIV cost pennies so they will be the last to
>>> fail.
>>> Ray
>>> 
>>> -----Original Message-----
>>> From: NJARC [mailto:njarc-bounces at mailman.qth.net] On Behalf Of David Sica
>>> Sent: Friday, April 15, 2016 4:03 PM
>>> To: Jim Whartenby <antqradio at sbcglobal.net>; njarc at mailman.qth.net
>>> Subject: Re: [NJARC] Selenium Rectifier replacement
>>> 
>>> Just remember
>>> Reply = Poster
>>> Reply All = Everyone
>>> 
>>> _________________________________________________________
>>> 
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> <S-72 pwr sply a.jpg><n2yeg.vcf>______________________________________________________________
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