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 Post subject: Replacing a selenium rectifier
PostPosted: Jul Wed 20, 2005 3:41 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 131
Location: Baden, PA 15005
I'm gathering parts for a Silvertone 8020 that I am recapping, and decided to replace the selenium rectifier that is in it. I have read on a few sites that a bridge rectifier can be used, but I'm not sure how that would be possible, since, in the schematic, the rectifier appears as a single diode.<P>If this is the case, what size diode should I use? I have a bunch of silicon diodes that I've pulled from old computer power supplies - would one of these be sufficient, or would I need something that would handle more power?<P>------------------<BR>~general0ne~


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 Post subject: Replacing a selenium rectifier
PostPosted: Jul Wed 20, 2005 5:33 pm 
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Location: Baltimore, MD
I use a 1N4005 silicon diode. Cheap, easy to find, and does a good job. As Bill said, be careful with the polarity and add at least a 5W resistor in series with it. In the few I've had to replace, the ideal resistor has been 100 ohms or less, but there may be cases where a different value is needed. <P>You might want to move this thread to the restoration forum to get more advice.<P>Tom<P>------------------<BR>


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 Post subject: Replacing a selenium rectifier
PostPosted: Jul Wed 20, 2005 7:17 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 131
Location: Baden, PA 15005
I put a post in the restoration forum. <P>Getting the polarity isn't a problem, since the anode and cathode are marked on the schematic, and the cathode is always the banded or marked end on the diode itself.<P>What is the purpose of wiring a resistor in series with the diode? Is it just to compensate since the selenium rectifier has a higher resistance than the silicon diode? Is there a way to check the rectifier (it is still operational - it hasn't blown yet) to figure out what value resistor I should use, or is it pretty much a "try one and see how it works" approach?<P>I have about 10 or 15 1N4005's from the old power supplies (plus a whole pile of un-dissected supplies), so if one or two get toasted trying to find the correct resistor value, it wouldn't be that big of a loss.<P>------------------<BR>~general0ne~


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 Post subject: Replacing a selenium rectifier
PostPosted: Jul Wed 20, 2005 7:30 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 959
Location: Buenos Aires, Argentina
mario2 wrote:
<font>quote:</font><HR>Originally posted by general0ne:<BR><B>I put a post in the restoration forum. <P>Getting the polarity isn't a problem, since the anode and cathode are marked on the schematic, and the cathode is always the banded or marked end on the diode itself.<P>What is the purpose of wiring a resistor in series with the diode? Is it just to compensate since the selenium rectifier has a higher resistance than the silicon diode? Is there a way to check the rectifier (it is still operational - it hasn't blown yet) to figure out what value resistor I should use, or is it pretty much a "try one and see how it works" approach?<P>I have about 10 or 15 1N4005's from the old power supplies (plus a whole pile of un-dissected supplies), so if one or two get toasted trying to find the correct resistor value, it wouldn't be that big of a loss.<P></B><HR>
<P>The purpose of the resistor is to drop same voltage that gets dropped by the selenium rectifier. A silicon diode will drop 0.7v while the selenium rectifier may drop anywhere between 5 and 20 Volts (or more) Measure the drop is a good starting point but keep in mind it might not be accurate as it tends to drop more as it ages (I guess that's why they overheat and burn)<P>Experiment by carefully testing voltages on the different stages (plates and filaments) this is the safest way<P>good luck<P><BR>Mario<P>------------------<BR>I know the voices aren't real, but they have some pretty good ideas!


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 Post subject: Replacing a selenium rectifier
PostPosted: Jul Wed 20, 2005 7:34 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Vieques, PR, USA
A low value resistor serves for surge protection (current limiting) of the diode. The selenium should already have one in place.<P>You generally don't need one on transformer operated sets, though.<P>A good 6-stack selenium loses about 5.4 volts 'by the book' (0.9v per section) compared to 0.7 volts for a single silicon diode. Thats usually not a problem but if you have high line voltage problems as well then it doesn't hurt to adjust the resistor accordingly to keep the B+ down.<P>-Bill<P>------------------<BR>


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 Post subject: Replacing a selenium rectifier
PostPosted: Jul Wed 20, 2005 8:08 pm 
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Location: Maryland 20709, USA
Leigh wrote:
<font>quote:</font><HR>Originally posted by general0ne:<BR><B>...I have about 10 or 15 1N4005's from the old power supplies</B><HR>
<P>The 1N4005 is rated 600 volts PIV, which would be marginal in this application. At a 120 volt line, the actual PIV across the diode is about 490 volts. The 600 volt rating leaves very little safety margin.<P>I use R1800 diodes (catalog number 583-R1800F) from Mouser <A HREF="http://www.mouser.com" TARGET=_blank>http://www.mouser.com</A> , rated 1800 volts PIV. They cost about $0.12 each.<P>------------------<BR>73 de Leigh W3NLB | | Leigh@AtwaterKent.Info<BR><A HREF="http://www.AtwaterKent.info" TARGET=_blank>http://www.AtwaterKent.info</A> | | <A HREF="http://www.Synchrophase.info" TARGET=_blank>http://www.Synchrophase.info</A>


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 Post subject: Replacing a selenium rectifier
PostPosted: Jul Thu 21, 2005 2:50 am 
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Location: Livermore, CA
Hi Leigh<P> Always good to use higher rated diodes but 400 PIV (1N4004) are usable on our AC lines.. 1N4004 has been used for years although spikes can blow them or almost any diode. Those 1800 PIV diodes sound great.<P> PIV of a 125 volt AC line is only 354 volts. (125 x 2.828)<P> When power silicon diodes first became available people would try to use 200 PIV. (1N2069) Some times they got away with this as diodes are usually higher. There were none rated 300 so 200 may actually be higher and hold up. Series resistance can also be added. <P>------------------<BR>Norm


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 Post subject: Replacing a selenium rectifier
PostPosted: Jul Thu 21, 2005 8:03 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Vieques, PR, USA
US voltage has never been rated at 100 volts. You may find an item labelled at 110 from time to time but the practice has always been 115-120.<BR>True, many folks do encounter high line voltage. I've experienced (twice) a spell of it >124 at all hours and it did a fair amount of damage before they corrected it.<P>In a typical selenium rectified set running with the ac voltage 10% "too high", yes, adding an additional 5 volts by changing to a silicon diode may tip the scale on B+ ratings. Hence my previous post about maybe needing to add resistance.<P>Thats not a cure-all, though...the filaments are still running high...and so is your refrigerator, VCR, etc.<P>If you have high line voltage and cannot get it corrected you might indeed have to take some extra precautions. Modifying the set is one valid option because even with the extra resistance it will still perform for the next guy (should you sell it) who is not suffering from "todays higher line voltages".<P>You shouldn't be blowing filaments in battery powered sets regardless of the line voltage <IMG SRC="http://antiqueradios.com/forums/smile.gif"><P>-Bill<P><P>------------------<BR>


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 Post subject: Replacing a selenium rectifier
PostPosted: Jul Thu 21, 2005 7:30 pm 
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Leigh wrote:
<font>quote:</font><HR>Originally posted by Bill Cahill:<BR><B>Remember, guys, while you are giving this good advice, that yesterday's line voltages were rated anywhere from 100v to 117v ac.</B><HR>
<P>117 volts was the average of service voltages around the country, so half of the service areas were higher. Many Atwater Kent voltage table readings were done at 120 volts nominal.<P>------------------<BR>73 de Leigh W3NLB | | Leigh@AtwaterKent.Info<BR><A HREF="http://www.AtwaterKent.info" TARGET=_blank>http://www.AtwaterKent.info</A> | | <A HREF="http://www.Synchrophase.info" TARGET=_blank>http://www.Synchrophase.info</A>


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 Post subject: Replacing a selenium rectifier
PostPosted: Jul Sat 30, 2005 8:53 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 265
Location: Beverly Hills, CA, USA
What's wrong with using a silicon bridge rectifier, snubbed with four 0.0047 mf ceramic caps?<P>Also, what's the differnece between a "christmas" tree bridge, with four 1N4007 and a premade bridge rectifier?<P>Vacuum Tube Valley sells bridge rectifiers for vacuum tube applications.


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 Post subject: Replacing a selenium rectifier
PostPosted: Jul Sat 30, 2005 10:29 am 
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Location: San Jose, CA USA
Hi Omer,<P>Bridge rectifiers are "full wave" rectifiers and put out a different waveform than a single diode "half wave" rectifier.<P>Although the output from a full wave rectifier requires less filtering (and therefore smaller capacitors) than one for a half wave rectifier, in the old days, rectifiers were expensive. You either used a tube or a selenium rectifier, and using four of them to make a bridge was considered too expensive. In those days, whenever a full-wave rectifier was desired, a centertapped transformer was used. With a centertapped transformer, you can make a full-wave rectifier with just two diodes, and this was usually done with a dual diode tube like 5Y3 or 5U4 or 80.<P>In the cheapest radios (and in those used for AC-DC operation), no transformer was used, so a half-wave single diode rectifier was the solution, either using a single diode tube like 35Z5 or 35W4, or a selenium rectifier. Bigger filter caps were needed (typically 50 uF + 30 uF), but this was cheaper overall than either a centertapped transformer or a 4-diode bridge.<P>If you replace a single diode half-wave rectifier with a full-wave bridge rectifier today, you'll end up with smoother (better filtered ) DC, but also a B+ voltage that is higher than originally intended. You could add resistance to compensate.<P>In the end, you wouldn't really be restoring, but "improving" on the original. Nothing wrong with that, but I don't hear of people doing this particular modification very often.<P>There is no functional difference between a bridge made with four separate diodes versus a single-unit bridge. It's just packaging.<P>Tom<P>------------------<BR>


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 Post subject: Replacing a selenium rectifier
PostPosted: Jul Sat 30, 2005 10:57 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 265
Location: Beverly Hills, CA, USA
In the German radios I have been restoring, their radios already have the full wave selenium rectifiers. This is the reason I was posing this question.<P>As for the half wave rectifiers, I think the principles of radio restoration would be violated by changing the circuit to something else!<P>Other than,hiding mylars in wax caps, hiding new electrolytics in old cans, and replacing bad cords with polarized ones, radio restoration should do its best to keep what's going on inside the radio as much as its designers intended, lookwise and circuitry wise!<P>


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 Post subject: Replacing a selenium rectifier
PostPosted: Jul Sat 30, 2005 11:19 am 
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Location: San Jose, CA USA
Omer, you raise a good point. Yes, lots of German radios had full-wave selenium bridge rectifiers. Replacing those with full-wave silicon bridge units makes a lot of sense.<P>Siemens and other German companies seemed to have a good inexpensive process to manufacture those bridge rectifiers in the 1950s. The packaging was very compact as well (in some cases the modern Si bridge is bigger!).<P>Those radios almost always have a power transformer as well. For the Germans, it was apparently cheaper to use those little bridge units instead of a centertapped transformer.<P>That's one of the fun things about collecting antique radios from different countries. In different places, people came up with different solutions.<P>Today, with our worldwide marketplace, designs seem to be very similar everywhere.<P>Tom<P>------------------<BR>


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 Post subject: Replacing a selenium rectifier
PostPosted: Jul Sun 31, 2005 3:40 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Loserana
The only problem I can see with installing a bridge rect. is there enough room to mount in order to provide a a heat sink.<BR>As far as diodes I use ECG 125, (NTE)<BR>1000 PIV and 2.5 amp<BR>Got bags of 'em around<P>------------------<BR>There was this really bright flash and loud bang.....


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 Post subject: Replacing a selenium rectifier
PostPosted: Jul Sun 31, 2005 3:51 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 131
Location: Baden, PA 15005
If you get the square style, that has a mounting hole in the center, it may be possible to mount it to the chassis, using the chassis as a heatsink. I'm not sure if that would provide enough heat dissipation, though.<P>------------------<BR>~general0ne~


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 Post subject: Replacing a selenium rectifier
PostPosted: Jul Sun 31, 2005 5:29 am 
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Location: Livermore, CA
Hi<P> Radios don't draw a lot of current. You may not need a heat sink? 100 MA times .7 volts is only .07 watts. Even with a bridge shouldn't get above 1/4 watt. A bridge uses 4 diodes but each is handling less current.<P>------------------<BR>Norm


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