[NJARC] [NJARC Selenium Rectifier replacement mea culpa

[Bill Zukowski]

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:
> Just remember
> Reply = Poster
> Reply All = Everyone
>
> _________________________________________________________
>
>
> 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 
> <mailto:n2yeg at optonline.net>> wrote:
>
>     Just remember
>     Reply = Poster
>     Reply All = Everyone
>
>     _________________________________________________________
>
>     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 <mailto:njarc at mailman.qth.net>> wrote:
>
>>     Just remember
>>     Reply = Poster
>>     Reply All = Everyone
>>
>>     _________________________________________________________
>>     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 <mailto:mattr04 at hotmail.com>
>>     Sent: ‎4/‎15/‎2016 5:10 PM
>>     To: NJARC Reflector <mailto:njarc at mailman.qth.net>
>>     Subject: Re: [NJARC] Selenium Rectifier replacement
>>
>>     Just remember
>>     Reply = Poster
>>     Reply All = Everyone
>>
>>     _________________________________________________________
>>
>>        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
>>     <mailto:njarc-bounces at mailman.qth.net>> on behalf of Raymond F
>>     Chase <raydio862 at verizon.net <mailto:raydio862 at verizon.net>>
>>     Sent: Friday, April 15, 2016 4:35 PM
>>     To: 'David Sica'; 'Jim Whartenby'; njarc at mailman.qth.net
>>     <mailto: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
>>     <mailto:antqradio at sbcglobal.net>>; njarc at mailman.qth.net
>>     <mailto:njarc at mailman.qth.net>
>>     Subject: Re: [NJARC] Selenium Rectifier replacement
>>
>>     Just remember
>>     Reply = Poster
>>     Reply All = Everyone
>>
>>     _________________________________________________________
>>
>>
>>     ---
>>     This email has been checked for viruses by Avast antivirus software.
>>     https://www.avast.com/antivirus
>>
>>     ______________________________________________________________
>>     NJARC mailing list
>>     Home: http://mailman.qth.net/mailman/listinfo/njarc
>>     Help: http://mailman.qth.net/mmfaq.htm
>>     Post: mailto:NJARC at mailman.qth.net
>>
>>     This list hosted by: http://www.qsl.net <http://www.qsl.net/>
>>     Please help support this email list: http://www.qsl.net/donate.html
>>     ______________________________________________________________
>>     NJARC mailing list
>>     Home: http://mailman.qth.net/mailman/listinfo/njarc
>>     Help: http://mailman.qth.net/mmfaq.htm
>>     Post: mailto:NJARC at mailman.qth.net
>>
>>     This list hosted by: http://www.qsl.net <http://www.qsl.net/>
>>     Please help support this email list: http://www.qsl.net/donate.html
>>     ______________________________________________________________
>>     NJARC mailing list
>>     Home: http://mailman.qth.net/mailman/listinfo/njarc
>>     Help: http://mailman.qth.net/mmfaq.htm
>>     Post: mailto:NJARC at mailman.qth.net
>>
>>     This list hosted by: http://www.qsl.net <http://www.qsl.net/>
>>     Please help support this email list: http://www.qsl.net/donate.html
>
>     ______________________________________________________________
>     NJARC mailing list
>     Home: http://mailman.qth.net/mailman/listinfo/njarc
>     Help: http://mailman.qth.net/mmfaq.htm
>     Post: mailto:NJARC at mailman.qth.net <mailto:NJARC at mailman.qth.net>
>
>     This list hosted by: http://www.qsl.net <http://www.qsl.net/>
>     Please help support this email list: http://www.qsl.net/donate.html
>
>
>
>
>
>
>
>
> ______________________________________________________________
> NJARC mailing list
> Home: http://mailman.qth.net/mailman/listinfo/njarc
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:NJARC at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
>
> No virus found in this message.
> Checked by AVG - www.avg.com <http://www.avg.com>
> Version: 2016.0.7539 / Virus Database: 4545/12052 - Release Date: 04/17/16
>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/mailman/private/njarc/attachments/20160418/f3b6d4fd/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: S-72 pwr sply a.jpg
Type: image/jpeg
Size: 71423 bytes
Desc: not available
URL: <http://mailman.qth.net/mailman/private/njarc/attachments/20160418/f3b6d4fd/attachment-0001.jpg>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: n2yeg.vcf
Type: text/x-vcard
Size: 5 bytes
Desc: not available
URL: <http://mailman.qth.net/mailman/private/njarc/attachments/20160418/f3b6d4fd/attachment-0001.vcf>