[Jimradio]

Telefunken Opus 6, just finished recapping this radio. It has a selenium rectifier in a black tube under the chassis. Playing this radio for about 10 minutes the rectifier was quite warm but not "hot" how hot does it have to be to replace it. If it needs replacing, how should it be done, what method.

[mikethedruid]

Selenium rectifiers are either good or bad. If it works, it works. They usually have fins on them for air circulation cooling. I have never seen one in a tube, can't imagine it, unless that tube had a fan with it to direct an air flow over the selenium rectifier. Pictures of this set up might help. Oh, and, in case you don't have a manual, I found one free on the net : https://elektrotanya.com/telefunken_opus...nload.html

[BrendaAnnD]

1) If you do replace it, simply do so with four identical silicon rectifiers or a single silicon full wave bridge.

2) I've dealt with those canned seleniums many times. They do fail, often open but on occasionally they will short with catastrophic consequences. .

[mikethedruid]

Never have seen one in a can myself. This is what I am familiar with when it comes to selenium rectifiers:
[Image: http://4.bp.blogspot.com/-0dGNno5UUQs/UY...2b5eb1.jpg]
or
[Image: https://external-content.duckduckgo.com/...%3DApi&f=1]
but nothing with any kind of tube. As you can see from the pictures, they have fins for air cooling. I looked at the schematic, and it looks like a pretty bog standard bridge rectifier.

[BrendaAnnD]

   

This is the beastie. They are known for failure.

[mikethedruid]

I'm not surprised they are known for failure. Without the fins to cool them they are just asking to overheat and fail. I would think that a modern diode bridge which can mount to a proper heat sink would be a much improved replacement for that disaster waiting to happen. One of the small, square ones with the hole through the middle for chassis or hear sink mounting would do the trick.

[BrendaAnnD]

Wouldn't even need a heat sink. Just use a 1A 600PIV bridge, it will never get warm.

[Jimradio]

BrendaAnnD,
https://www.mouser.com/datasheet/2/308/1...316001.pdf

Is this what you are talking about.
Would I need a voltage reducing resistor.

[BrendaAnnD]

More like this. The one you found is an SMD, which is not ideal for this application.

https://www.mouser.com/ProductDetail/Tai...uu0g%3D%3D

I actually have several, and would be happy to send one.

As for the resistor, I've never used one. The voltage difference with a full wave bridge is only about 20 volts tops, which will not harm the radio.

[RodB]

I had to replace the selenium rectifier in a Grundig and the B+ went up about 20 volts. But had to add a dropping resistor to make the FM rf amplifier work. It was very touchy. When the voltage was brought down to what the schematic showed, everything worked. BTW, the Grundig selenium rectifiers are encased in a flat metal case attached to the chassis.

[morzh]

the bridge I had in my Grundig works perfectly fine, so I left it there.
It is very small flat pack one.

If you check the voltage against the sch and it is where it should be, you can leave it in place.

[Jimradio]

Brenda PM sent.

[Jimradio]

Hi everybody, the selenium rectifier has been replaced with a new bridge rectifier. The voltage reads 280v at the + coming off the bridge, the schematic shows it should be 255v it is 25v higher than the schematic. Should this be dropped with a resistor and if so what resistor would be needed.
Thanks

[BrendaAnnD]

Measure the current draw between the positive terminal on the bridge and the place it connects into the radio. Multiply the voltage (25) by the current you get in amps (ex. 50mA = 0.05A), this would give you a result indicating a 500 ohm resistor rated at at least 3 watts (actually 1.25W, but always go up a bit for safety and longevity.)

Your calculations will vary of course, since 50mA was just an example in this case.

[morzh]

>>Multiply the voltage (25) by the current you get in amps

I am sure you meant "divide".
Then you indeed get 500 ohm.