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 Post subject: Selenium Bridge replacement on Nordmende chassis 1/613?
PostPosted: May Mon 06, 2019 6:00 pm 
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Joined: Nov Tue 15, 2011 11:19 pm
Posts: 441
Location: Woolwich Twp (Swedesboro), NJ
OK. So I got the diagram (don't have the radio yet), and noticed that the bridge rectifier is selenium. I have replaced Selenium diodes before with silicon (1N4xxx). However the bridge is new for me. I know the junction voltage is much lower on the silicon diodes.

My questions are:

1. Should I replace it, if it is not bad, as a preventive?

2. If so, do I need a drop resistor following the bridge before the transformer, or is the voltage increase not significant enough to not warrant one?

3. I would supposed 1N4005 or higher should be adequate?


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 Post subject: Re: Selenium Bridge replacement on Nordmende chassis 1/613?
PostPosted: May Mon 06, 2019 7:25 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 717
Location: Merrick,NY,USA
If you're going to play the radio regularly, I would change out the Selenium Bridge. You can make your own bridge out of 4 diodes or buy a silicon bridge rectifier such a KBU810 8A @ 1000V–you can buy 10 of these for about 6 bucks on eBay.

If you go the discrete diode route use 1N4007s. Opinions differ about using a dropper resistor, but personally I do. I use 100 ohm @10 watts between the + terminal of the bridge and the first filter cap and dress it against the chassis to dissipate the heat.


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 Post subject: Re: Selenium Bridge replacement on Nordmende chassis 1/613?
PostPosted: May Mon 06, 2019 9:14 pm 
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Joined: Sep Fri 14, 2018 7:56 pm
Posts: 28
Location: Switzerland
Please consider that selenium rectifiers should have a soft startup after years of beeing idle: The junction looses its ability of blocking in reverse direction.
Applying full load after decades will destroy it for shure. Begin with half tension for 15 minutes then rise to 3/4 again for 15 minutes. If its temperature hasn't rose immoderate, apply full tension for half an hour. Then let it cool down and measure the reverse current and decide if it can be used again. The ageing/rise of the forward conductability can not be reversed.
Source (in german): https://www.radiomuseum.org/forum/ersatz_eines_selen_brueckengleichrichters.html, Post 7

Another thing to consider is the fact that the dropping resistor only drops the voltage after plate current flows. Before, the higher tension is applied to the load and filter capacitors which stresses their dielectrica.

I tried to translate it quickly, please apologize my poor english.


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 Post subject: Re: Selenium Bridge replacement on Nordmende chassis 1/613?
PostPosted: May Mon 06, 2019 10:01 pm 
Member

Joined: Nov Tue 15, 2011 11:19 pm
Posts: 441
Location: Woolwich Twp (Swedesboro), NJ
Michael Simos wrote:
If you're going to play the radio regularly, I would change out the Selenium Bridge. You can make your own bridge out of 4 diodes or buy a silicon bridge rectifier such a KBU810 8A @ 1000V–you can buy 10 of these for about 6 bucks on eBay.

If you go the discrete diode route use 1N4007s. Opinions differ about using a dropper resistor, but personally I do. I use 100 ohm @10 watts between the + terminal of the bridge and the first filter cap and dress it against the chassis to dissipate the heat.



I have 1N4007s in stock. However, the bridge would be likely much more easily to mount. So I think I may do that. Thanks.

I found wirewound 10W resistors fairly cheap at Mouser. I assume those would be OK since it is the power supply section.


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 Post subject: Re: Selenium Bridge replacement on Nordmende chassis 1/613?
PostPosted: May Mon 06, 2019 10:22 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 717
Location: Merrick,NY,USA
N4NYY wrote:
Michael Simos wrote:
If you're going to play the radio regularly, I would change out the Selenium Bridge. You can make your own bridge out of 4 diodes or buy a silicon bridge rectifier such a KBU810 8A @ 1000V–you can buy 10 of these for about 6 bucks on eBay.

If you go the discrete diode route use 1N4007s. Opinions differ about using a dropper resistor, but personally I do. I use 100 ohm @10 watts between the + terminal of the bridge and the first filter cap and dress it against the chassis to dissipate the heat.



I have 1N4007s in stock. However, the bridge would be likely much more easily to mount. So I think I may do that. Thanks.

I found wirewound 10W resistors fairly cheap at Mouser. I assume those would be OK since it is the power supply section.



Yes, a bridge is easy to mount. Most have a hole that will take a screw and nut and mount directly to chassis. All wires can be directly soldered to the four legs. I restore a lot of German radios so I just stocked up on KBU810 bridges. Cheap, reliable and easy to find from Chinese Ebay suppliers. 10 Watt wirewound resistors will be just fine to use for the dropper.


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 Post subject: Re: Selenium Bridge replacement on Nordmende chassis 1/613?
PostPosted: May Mon 06, 2019 10:26 pm 
Member

Joined: Nov Tue 15, 2011 11:19 pm
Posts: 441
Location: Woolwich Twp (Swedesboro), NJ
Michael Simos wrote:
N4NYY wrote:
Michael Simos wrote:
If you're going to play the radio regularly, I would change out the Selenium Bridge. You can make your own bridge out of 4 diodes or buy a silicon bridge rectifier such a KBU810 8A @ 1000V–you can buy 10 of these for about 6 bucks on eBay.

If you go the discrete diode route use 1N4007s. Opinions differ about using a dropper resistor, but personally I do. I use 100 ohm @10 watts between the + terminal of the bridge and the first filter cap and dress it against the chassis to dissipate the heat.



I have 1N4007s in stock. However, the bridge would be likely much more easily to mount. So I think I may do that. Thanks.

I found wirewound 10W resistors fairly cheap at Mouser. I assume those would be OK since it is the power supply section.



Yes, a bridge is easy to mount. Most have a hole that will take a screw and nut and mount directly to chassis. All wires can be directly soldered to the four legs. I restore a lot of German radios so I just stocked up on KBU810 bridges. Cheap, reliable and easy to find from Chinese Ebay suppliers. 10 Watt wirewound resistors will be just fine to use for the dropper.


Superb. Thanks, Mike!


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 Post subject: Re: Selenium Bridge replacement on Nordmende chassis 1/613?
PostPosted: May Tue 07, 2019 4:00 am 
Member

Joined: Nov Tue 15, 2011 11:19 pm
Posts: 441
Location: Woolwich Twp (Swedesboro), NJ
Mike,

Does your version of the bridge have a mounting hole? I went to Mouser in case I had to order from there with all the caps I am going to need, and their data sheet did not have a bridge with a mounting hole.


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 Post subject: Re: Selenium Bridge replacement on Nordmende chassis 1/613?
PostPosted: May Tue 07, 2019 4:20 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 717
Location: Merrick,NY,USA
N4NYY wrote:
Mike,

Does your version of the bridge have a mounting hole? I went to Mouser in case I had to order from there with all the caps I am going to need, and their data sheet did not have a bridge with a mounting hole.



Yes it does. You can get this bridge from many different sources on eBay. Here is a link to just one example that ships from the US:
https://www.ebay.com/itm/10Pcs-Kbu810-8 ... 23bbb0246e


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