I am trying to restore my Grandfather's old Telefunken Opus 6 (1955) to working order, and have a few questions. All the "experts" say to start by replacing all the electrolytic capacitors and the selenium rectifier. Since these are no longer available, they suggested using a silicon bridge rectifier with a dropping resistor rated at 100 ohms @10 watts. I assume (did I say that I am NOT an electronic engineer) that the resistor would be connected in series with the output of the bridge, but rather than do damage to the radio, I have chosen to appear stupid and ask someone that knows for sure. Also, there are a couple of 50MFD caps in it that I have been unable to locate so far. Would a 47MFD work as a replacement? Thank you in advance for your help with this!

Functionally it doesn't make any difference, but if you put the resistor in series with the AC terminals of the bridge rectifier; it gives the transformer a chance if the rectifier fails short circuit. A failing selenium rectifier lets you know in no uncertain terms - by the time you smell a toasting transformer, the damage is done.

Thermal fuse resistors used to be quite common in the days of valve TVs - usually square section white ceramic body with a spring loaded contact soldered to the stub of lead coming out one end of the resistor, if it gets hot enough to melt the solder, the contact springs open. If you can get one of those, its good protection - but you need a fuse of *SOME* sort in front of the rectifier.

On the subject of electrolytics, the tolerances are very wide - -50% to +100% are not completely rare! Generally speaking; you can use a higher voltage rated replacement - but don't over do it, in stored capacitors with no voltage applied for a long time; the oxide dielectric layer can decompose - this can still happen to a lesser extent if the capacitor is consistently operated too far below its rated voltage.

 

Functionally it doesn't make any difference, but if you put the resistor in series with the AC terminals of the bridge rectifier; it gives the transformer a chance if the rectifier fails short circuit. A failing selenium rectifier lets you know in no uncertain terms - by the time you smell a toasting transformer, the damage is done.

Thermal fuse resistors used to be quite common in the days of valve TVs - usually square section white ceramic body with a spring loaded contact soldered to the stub of lead coming out one end of the resistor, if it gets hot enough to melt the solder, the contact springs open. If you can get one of those, its good protection - but you need a fuse of *SOME* sort in front of the rectifier.

On the subject of electrolytics, the tolerances are very wide - -50% to +100% are not completely rare! Generally speaking; you can use a higher voltage rated replacement - but don't over do it, in stored capacitors with no voltage applied for a long time; the oxide dielectric layer can decompose - this can still happen to a lesser extent if the capacitor is consistently operated too far below its rated voltage.

There is a .8 amp fuse on the AC line in. I would think that will provide adequate protection for the transformer, but I may wire it as you suggested anyway. No such thing as too much protection. Thanks for your input.

 

There is a .8 amp fuse on the AC line in. I would think that will provide adequate protection for the transformer, but I may wire it as you suggested anyway. No such thing as too much protection. Thanks for your input.

If there's already a fuse in series between the secondary and the rectifier - adding a resistor there could make the fuse blow more slowly by a shorted rectifier.

However; the resistor is there to replace the apparent resistance of the original selenium rectifier, so it will actually be no different to the original equipment.

But it does flag up the need to avoid sweeping generalisations.

 

I recently re-capped a Telefunken Andante 5353W. Also, I converted the old selenium rectifier to a full wave bridge silicon rectifier using 1n4007 diodes with .0047 ceramic capacitors in parallel to each diode to prevent noise on the AM dial. I had to install a 350 ohm 5 watt power resistor to keep the B+ voltage at 270 volts. The new electrolytic capacitors are 50uf rated at 450volts. I mounted the new rectifier on a terminal strip - contact me if you would like a photo of it. The radio plays beautifully!!

 

I recently re-capped a Telefunken Andante 5353W. Also, I converted the old selenium rectifier to a full wave bridge silicon rectifier using 1n4007 diodes with .0047 ceramic capacitors in parallel to each diode to prevent noise on the AM dial. I had to install a 350 ohm 5 watt power resistor to keep the B+ voltage at 270 volts. The new electrolytic capacitors are 50uf rated at 450volts. I mounted the new rectifier on a terminal strip - contact me if you would like a photo of it. The radio plays beautifully!!

Welcome to the team. I know why you did each part of that and hope you will become a regular member. As a matter of general knowledge, this is good, however, RickW, the Original Poster, hasn't been here since Feb 21, 2015. He probably won't see it.

 

[Ohm it out, post: 945695, member: 309390"] Andante 5353W.
I am going to attempt to the same.Recap and replace the selenium rectifier on the same radio If you still have the picture I would love for you to post it thanks

 

I recently re-capped a Telefunken Andante 5353W. Also, I converted the old selenium rectifier to a full wave bridge silicon rectifier using 1n4007 diodes with .0047 ceramic capacitors in parallel to each diode to prevent noise on the AM dial. I had to install a 350 ohm 5 watt power resistor to keep the B+ voltage at 270 volts. The new electrolytic capacitors are 50uf rated at 450volts. I mounted the new rectifier on a terminal strip - contact me if you would like a photo of it. The radio plays beautifully!!

Many years ago I put silicon rectifiers in someone's guitar amp - he was very impressed by the blue glow around the output tube plates.

 

[Ohm it out, post: 945695, member: 309390"] Andante 5353W.
I am going to attempt to the same.Recap and replace the selenium rectifier on the same radio If you still have the picture I would love for you to post it thanks

 

Here is a few photo's of the new silicon rectifier. The two blue wires come from the power transformer secondary. Because the secondary wires were too short to reach the top terminal strip, I had to install a short strip and add the blue wires - I hope explaining everything - if not ask anything and I'll help out. Good luck!

 

schematic is on post #5

404 error - link is dead.

 

Here is a few photo's of the new silicon rectifier. The two blue wiresView attachment 103770 View attachment 103771 View attachment 103771 come from the power transformer secondary. Because the secondary wires were too short to reach the top terminal strip, I had to install a short strip and add the blue wires - I hope explaining everything - if not ask anything and I'll help out. Good luck!

Color blind? Maybe green wires? Also, the voltages you are working at can arc over very short distances. Some of those wires look very close from the perspective shown.

 

[QUOTE="Ohm it out, post: 985103, come from the power transformer secondary. Because the secondary wires were too short to reach the top terminal strip, I had to install a short strip and add the blue wires - I hope explaining everything - if not ask anything and I'll help out. Good luck!!

Thank you very much for the pictures. I recently bought this radio for my 84 year old father.. The radio worked great up until a week ago. It had great sound and pulled in all the stations . I want to get it back up running for him. Thanks again I truly appreciate it!

 

Color blind? Maybe green wires? Also, the voltages you are working at can arc over very short distances. Some of those wires look very close from the perspective shown.

My mistake - green wires. No, I am not color blind. I played the radio many hours before the owner came and picked it up. If there was any arcing, my sense of smell and hearing are still good in spite of working around jet aircraft for 35 years. By the way, I'm also a General Class Ham radio operator with morse code endorsement, so I know a little about arcing.