Help Needed: Restoration of a Loewe Opta Model 537GW a German Radio from about 1937

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Hi Folks, Another repost as the links have not been available in the previous posts.  Thanks for any help!!

I am in the process of restoring a Loewe Opta Model 537GW.  Replacement of the capacitors is done.  It is a German radio using unfamiliar (to me) tubes.  My German is very limited.
There seems to be a problem with the rectifier tube as there is no B+ DC output from it. The filament appears intact.  (I am unable to test it with my PACO tester.)  I am unable to obtain a replacement tube.
I would like to replace the tube with solid state diodes.  I have no idea how to do so.
The rectifier tube is a 26NG described on Radiomuseum as “two independent diodes to be used as voltage doubler or for higher current. NOT intended as full wave rectifier”.
The tube goes into a different socket depending on the AC (or DC) voltage available.  I am in N. America with the radio set to 110 volts and the tube in the socket marked  “II” on one of the schematics (see below).
Would love some help with developing a circuit diagram with any necessary resistors to rectify and double the voltage.

Schematics can be found here: schematic 1

And here: schematic 2

The radiomuseum entry is here: 537GW
and tube information here:  26NG

Help would be appreciated!  It is a very nice looking radio (nicknamed “The ice skate”) and hopefully the other (12 pin!) tubes are intact.  I would love to get it working again.

Thanks! Chris

@chriscurcio Hi, that looks to be a nice radio indeed. Just a few remarks first. the fact that you have no B+ doesn’t always means that the rectifier is faulty. Having a Quick Look at both schematic variants I understand that one is a cleaner / more modern looking and a little easier to work out than the other but they are both similar. What I want to say is that in order to have your b+ you must ensure that you have measured with an ohm-meter the full pathway/continuity of the filament circuit. The schematic reveals that the rectifier Tube 26NG has its filament in series with that of the WG35 Tube and there are ballast resistors on the same filament circuit as well so  all of them that are part of the filament circuit must be measured first. Also watch out for  bad/rusty tube pins or tube sockets as they could be the culprit too. I would start slowly and take my time and highlight every single tested component on a paper copy of the schematic to avoid confusion. But a good indication that the filament circuit is sound is by looking at each of the tube filaments to see if (after a while ) you can spot their filament glowing. If not then that circuit is open somewhere, It could even be the voltage selection switches (I spotted then in the schematic as well)

Thank you for your help!  I will go through and re-check the filament circuit and its resistors.  Unfortunately I cannot see inside the tubes as they are spray-coated.  I will also go through the voltage selector switches:  things get very complicated and hard to access in the radio at that point.  Nonetheless, if you do have any ideas on what a solid state replacement rectifier circuit would look like in this context, I would appreciate your input as well.  I am now very curious on that point and would like to learn more about that topic.  Thanks again! Chris

@chriscurcio Hi, to replace that rectifier tube with solid state shouldn’t be that big of a job. Here is some ideas you can try:

As you can see you can use 2 diodes and a resistor. Now the mechanics: you’ll carefully smash the glass of that tube and gut it out but making sure the bachelite base of the tube (the part that holds all pins together) still remains intact (in one piece with no cracks) then you can literally solder each diode terminal to the corresponding pin as pictured. The diodes you can try can even be some average run of the mill 1N4007 (as their specs are 700v @ 1A max.). or use some higher specked ones as long as the voltage rating is significantly higher than 110V-120V (The mains voltage) When soldering the diodes make sure that they don’t touch the tube base or each-other !  Now onto some calculations. The original tube (according to the manufacturers datasheet) has a filament that will require 0.18 Amps (180mA) to become properly heated and will have a voltage drop across it of about 40V. Using the Ohms law one can calculate an equivalent resistor (preferably wire wound) to replace the absence of the old rectifier tube. So our resistor (R) is. R=V/I = 40V/0.18A= approx. 222 Ohms but we will round it down to 220 Ohms (as this is a standard value) Next, we will need to take the power dissipation rating into account. That resistor will need to be able to comfortably dissipate some heat without burning over a short period of time and for that we will use the ohms law for power (this time) to find out what is the amount of power (P) that will have to be safely and quickly dissipated so: P= V x I = 40V x 0.18A = 7.2W. So it is always wise and good practice to always choose a higher power rated resistor to have a prolonged longevity and we will choose 10Watts rating to be safe. Now the bottom line is that we need to acquire and use a 220 Ohms @ 10Watts. wire-wound style resistor.  Now depending on the spacing inside the tube base after installing the diodes you can either try to fit/solder the resistor inside the tube base somehow (providing that the resistor has enough room clearance so it won’t touch any diodes or the tubes base perimeter or, have it soldered directly on the existing tube socket to the corresponding pins.

I hope it helps and if anyone else has some other (perhaps better) ideas or somethings to add or subtract I’m ready to hear it. This way we all learn from each-others experiences. All the best.

Thank you so much for your excellent explanation and diagram.  I will put my praise as simply as I can:  I understood it from beginning to end!  I am grateful for your efforts.
I plan to go through the radio and filament circuit as you suggested in your first reply.  You have given confidence to my suspicion that there is something else going on rather than a failed rectifier tube.  I will go slowly as you suggest and report back at some time in the future.

By the way, there is an interesting essay by a gentleman who restored 2 of these 537GW radios.  It can be found on the model page on radio museum.  If you scroll down to the bottom and click on the forum the essay and some nice photos will be revealed.  It is in German, so I used Google to translate it.

Thanks again for your help!

Best

Chris

I would also add, you may not have to break the tube if you go that route. There should be lead out wires from the glass that are soldered to the pins. To save the tube, you should be able to just desolder the wires from the bottom of the pins and pull the base off. Note there is likely some glue holding it as well you would have to loosen.

I just wanted to provide an update on the ice skate radio.  Following Ovi’s suggestion I reviewed the wiring in the power supply and corrected a major error.  Unfortunately this did not correct the B+ problem.  Following Ovi’s schematic, I replaced the rectifier tube with diodes and a resistor and voila! the B+ DC voltage appeared in the doubler circuit!  Unfortunately there was still silence.  I began on the audio section and found there was no resistance at the volume pot wiper.  Cleaning did not help.  So I replaced it with the closest pot I had on hand.  This is a shame as the original pot had a very cool switching mechanism! With the pot replaced I was able to send a signal through the aux / phono input and a tone was heard!  Unfortunately when switched to any of the AM bands there is still silence apart from the slight hum at the speaker.  So I am off to troubleshoot the radio receiver section.  So far I have determined only that the oscillator appears to be working.  On the other hand, I have not had much luck tracing a signal through the antenna.  I think this is mainly due to my ignorance as well as the fairly complicated circuit – some parts a nearly unreachable!  Nonetheless, excelsior!  Again, thanks to Ovi and RadTek!!

Here are a couple of “close ups” of the circuits in the radio I am working on

Closeup 1

Closeup 2

Just and update on the Ice Skate Radio. 

Photo 1

Photo 2

Photo 3

It is alive, receiving well and looking very art deco.  It has the solid state rectifier in place, a new volume pot along with the capacitor replacements.  Most of the original resistors were fine with few completely open that had to be replaced.  The speaker fabric was replaced with the closest thing I could find.  I think it is very close to the original.  The finish was restored and the chrome or nickel polished.  I had to fabricate the metal rings on the knobs as the originals were long gone.  There are still a few problems and quirks.  There is still some hum, the broadcast band works well with the stations spatially aligned on the dial from L to R, but the numbers don’t seem to match anything.  I think this must have been aligned for the North American broadcast band standard at some time in its 88 years.  I have to come up with suitable antenna as I don’t have an outdoor one.  

This radio really speaks to me in other ways, particularly on this day.  I wonder how it found its way to North America. The founder of Loewe Radio escaped Nazi Germany in 1938, a year after this model was introduced.  He came to the USA and the wikipedia page says he was friends with Albert Einstein.  That indicates he may have been here, in the same state where I live now, at some point in the thirties and forties.  Apparently he was able to return to Germany after the war and back to his electronics business.  Now of course it is highly unlikely that this radio is connected to him directly.  Nonetheless, it is a symbol of hope for me.  And luckily it has a phono input so I can listen to something other than today’s radio voices.

Thanks again to those of you who helped with this project! It is not perfect, but it is amazing to me nonetheless!

I’m glad you have finally got it working and yes it looks truly amazing. Nice find and it goes without saying nice restauration. I really like  the looks of this radio. Well done.  And thank you for staring.

That is a really cool one! 

I’ve seen it on a few, and I’ve always liked how they list the towns (or cities) on the dial like that. 

When they take a little more work, it’s all the more satisfying when you get ’em running. 

Good work, Chris, and you’ve got a very nice piece there.  

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