Restoring the Iconic Braun SK25 Radio: A Vintage Design Masterpiece - part 1

In this video, I'll start the restoration of a Braun SK25 tube radio, the third iteration of Braun’s groundbreaking series of 'miniature' tube radios, a collection that paved the way for modern design even before the legendary Dieter Rams joined the company. These models, which broke away from traditional aesthetics with their sleek, minimalist designs, were a bold statement at the time and continue to influence design today. Though Rams would later become synonymous with Braun’s design ethos, these early radios set the stage for his work and are now celebrated in the Museum of Modern Art (MOMA). Join me as I restore this iconic piece of design history, bringing back to life a radio that helped shape the future of product design.
Interestingly, these pioneering designs, which later inspired the design philosophy behind Apple products, are experiencing a resurgence in popularity among young adults today. In their pursuit of a ‘vintage’ aesthetic, this new generation is drawn to the clean lines and timeless appeal of these classic radios. As the demand for authentic, retro pieces grows, these models are becoming highly sought-after collectibles, blending the past with the present in a way that resonates with modern sensibilities. Watch as I bring this vintage gem back to its former glory, making it a perfect addition to any stylish, contemporary space.
🎥 Back to basics Series
Many tasks in this video are covered briefly, without deep explanations of the theory-especially for those new to tube radio restoration. For a more detailed understanding, I’ve created a tutorial series called “Back to Basics,” which goes into the concepts in depth. I recommend watching those videos to better follow the steps I don’t fully explain here. The playlist is:
• Back to Basics
🎥 Other videos mentioned in this video:
■ Braun SK2b - • Braun SK 2/2 tube radi...
■ Braun SK2/2 - • Braun SK 2b tube radio...
■ Braun T1000CD - • Braun T1000CD repair -...
■ Braun Atelier 3 - • Braun Atelier 3 Stereo...
■ Back to Basics 1 - • The Parts That Make Up...
■ Back to Basics 2 - • Checking the Power Sup...
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[@daveys]

1:47 - Beautiful design on all these type of units, but that combi system is just perfect! Dieter Rams is a genius.

[@electronicsoldandnew]

I completely agree. This model, however, precedes his entry into the Braun team, so they were already looking to move away from the old look.

[@petersvan7880]

Was a pleasure watching this video. This Braun is such a well engineered radio, definitely worth giving it your love and attention!

[@electronicsoldandnew]

👍

[@DavidTipton101]

I thought I had missed this one but I did see it, old age 👴 It looks like the radio was stored in a cupboard it's so clean. Great idea hiding the dropper resistors in the selenium case 👍🙂

[@electronicsoldandnew]

👍

[@bofor3948]

Nice run through on that selenium changeover. You certainly didn't "stuff it up" 😃. Hopefully there are no other components that might cause a heavy current draw, which when replaced will allow the voltage to rise too high. Although like one of your other viewers I believe this radio will happily run nearer 200v on B1+. All those tubes are able to run at higher anode and screen voltages. The main issue in the radio will be the current ratings of the various coil windings and transformers.

[@electronicsoldandnew]

I agree that all the parts would be quite happy with those voltages, but I’ll see how it goes. It’s quite easy to alter if I decide to.

[@robharley9838]

Great video as usual Manuel! I re-fitted a very similar Siemens rectifier in a Blaupunkt radio a while back. The Selenium plates seemed to look fine, but curiosity as to what all the fuss was about with Selenium Rectifiers had me placing them under the microscope to look a little closer. Sure enough, minute deterioration was everywhere on the plates, even exposing small patches of the metal substrate. Yep, like old caps, they gotta' go. Very nice job replacing them! Thanks for all the great content. - JRH

[@electronicsoldandnew]

👍

[@techobaz55]

Great video as usual Manuel. Nice to see a really nice little radio for restoration, thank you.

[@electronicsoldandnew]

My pleasure 😊

[@BobAndersson]

What a little beauty! A huge smile on my face at about 27 minutes as I was really hoping you'd decide to "restuff" the selenium diode to maintain the original look. 👏👏👏

[@electronicsoldandnew]

👍

[@michvod]

Luckily it was just one diode there, so the resistors fit nicely. I restuffed a full bridge rectifier selenium, but was unable to put any resistors in there - but I did put 4 diodes and 4 1nF capacitors across them

[@electronicsoldandnew]

That would definitely not fit 😊 this one was a breeze.

[@JonRig]

Stuffed up by EO&N! 😂 That made me chuckle!!! Great video, as always!!

[@electronicsoldandnew]

😊

[@tonec3663]

@@electronicsoldandnew I like how Manual wrote down what was in the stuffed rectifier. The next technician will appreciate the information and not have to deal with a surprise.

[@carlcoleman6156]

Great video, such a joy, just watching your videos ❤

[@electronicsoldandnew]

Thanks 👍

[@mackfisher4487]

Manuel, I thought you demonstrate your homebrew resistor substitution box, but I'm guessing you wanted to use this as a teaching moment. Brilliant video as usual.

[@electronicsoldandnew]

Thanks Mack.

[@hooshangyadegar1250]

Great video

[@electronicsoldandnew]

Thanks

[@ralphj4012]

Making good progress. Just a suggestion (only my opinion, feel free to disregard or mock) is to check the temperature of the diode / resistor arrangement after the radio has been on for a while. Total dissipation around 3W, 1.6W ish in the 150 Ohm, generally not an issue but when enclosed inside a can!

[@electronicsoldandnew]

No mockery due 😊 The enclosure actually acts as a heatsink as it’s made of aluminium, and bolted to the chassis

[@DJDavo013]

Nice one! EL95 is btw nice sounding tube though:D A while ago I worked a similar looking 50’s radio, i thought it was a Philips.. It was a “U” tube device… I still don’t like to work on such devices haha

[@electronicsoldandnew]

The U tubes are series filament, used in transformer-less hot chassis sets … I don’t like them either 😊

[@edwardhannigan6324]

Hey Manuel, Great video and info..Fab radio..Love your work..Ed UK..☺

[@electronicsoldandnew]

Thanks Ed 👍

[@BJcanal270]

What a Beauty

[@electronicsoldandnew]

👍

[@maurolimaok]

Nice channel!

[@electronicsoldandnew]

Thanks

[@andrewmcintyre8491]

The style of production, the el95 and the speaker is very reminiscent of the philips star maker portable reel to reel. It appears almost as if they sourced the parts from the same supplier.

[@electronicsoldandnew]

May well have done 😊

[@xsc1000]

EL95 was used in later Philips Philetta with mains tranformer too (B2D13). But there was standard bridge rectifier used and B+ was higher.

[@maartenschapendonk6345]

Have one on the shelves to do also. Curious to see the cabinet and the backplate done.

[@electronicsoldandnew]

👍

[@TheEmbeddedHobbyist]

Nice little hide of the diode and dropper resistors.

[@electronicsoldandnew]

👍

[@andygozzo72]

hmm reminds me of some philips made in the 50s to early 60s, especially the B2G05U , that octagonal speaker is definitely philips!

[@electronicsoldandnew]

Could well be

[@andygozzo72]

@@electronicsoldandnew i posted a link to a radiomuseum page that confirms it but you've deleted it google 'philips AD2500X'

[@randomsteve4288]

13:03 Your approach in finding the right series resistor is a good, hands on approach, and it does its job very well in almost every case. But for the sake of discussion (!!! And not as criticism !!!)) let me point out, that your approach of using a series resistor only has two shortcomings (which are mostly of no concern IRL); first one is that with the tubes still cold(!) the filter voltage rises much higher than with a selenium rectifier, for selenium has a higher static forward voltage drop even without load. Second issue is that because your series resistor has to compensate for both the static forward drop and the drop under load, you create a slightly more "squishy" supply, meaning that the B+ voltage fluctuations versus load current changes will be higher on your design than with the selenium. Some more extreme schools of thought therefor model the selenium rectifier's behaviour with a series MOSFET circuitry, which is overkill. While other schools of thought try to strike a balance and use a series combination of Zener and resistor. The thought is to approximate the higher static forward drop of the selenium stack in the form of a constant zener voltage (I would assume approx 10-12V for a E250Cxxx) and then use the resistor to only model the load dependency of the output voltage. Downside to this approach is that the repeated avalancing of the zener more introduces RF noise which may present itself as a hum or buzz. A 10uF cap ACROSS the zener may help alleviate this problem by keeping the zener in the avalance region. And as with any "siliconization" of a selenium rectifier one should always include a 5-10nF cap across each diode junction to model the "softer" reverse recovery of the selenium. But again, like I said; your approach of adding a dropper resistor only is the most common approach taken and works 99% of the time without a problem.

[@electronicsoldandnew]

Great input. Thanks. I agree fully, and am actually looking at a mosfet alternative provided it can be properly heat sunk without changing the entire layout of the radio. A zener is great but again probably will need a heatsink. I’ve looked at a “power zener” idea that R G Keen uses in guitar amps that works quite well. It uses a low power zener and a mosfet, but again heat may become an issue.

[@peterhollander6392]

I own the same model Braun radio, but it is a SK2. The difference is the dial layout.

[@electronicsoldandnew]

👍

[@jonvincentmusic]

Saw the switch on the back and immediately figured it’s to turn the internal speaker off and on because there’s a two pin DIN speaker socket right next to it.

[@electronicsoldandnew]

👍should probably have been a double throw switch to avoid both speakers being in circuit. The extra low impedance could over stress the transformer/tube.

[@johnstone7697]

I have to wonder whether or not your conclusion on the B+ being similar to the earlier Braun model is correct. One clue to me is the fact that replacing the selenium with a silicon requires such a large value series resistor. i looked at the schematic of a similar set, the SABA 90 Triberg, which has the same tube complement (except for also having an EM84). It also uses a half wave rectifier..the same exact one as the Braun.If you look at the voltages on the SABA, you see 225 at the rectifier cathode for AM and 210 for FM...which I think is fairly close to what you got on the Braun without the resistor in line at full mains voltage. One thing you might check is the hv secondary winding ac voltage which in the SABA is shown at 210. With the resistor out of circuit, if the secondary voltage on the Braun is the same, then you can assume that the running voltages in this set should be higher than the other Braun. As you know, most German radios do run at higher DC voltages than that older Braun. Just my thoughts.

[@electronicsoldandnew]

👍

[@randomsteve4288]

11:58 The first letter E demarks Einweggleichrichter - single diode . If it was a B it would demark Brückengleichrichter - bridge rectifier. The next number demarks the AC working voltage RMS. Now the following letter C does not stand for current, but it indicates that a smoothing cap can be connected directly to the output of the rectifier without the need of current limiting resistors. And finally the last digit is the continous operating current in mA.

[@electronicsoldandnew]

Thanks for the info. We never stop learning new things 👍

[@tubeDude48]

👍 Have wonder about these resistor's getting smaller, but handling more wattage! In some of my experiments, I find they don't always hold-up to what they say the rating should be. I wonder about the material they are using, and the source they are coming from. Any thought's Manuel?

[@electronicsoldandnew]

I don’t actually know. Just know how to recognise the bad ones 😊

[@tubeDude48]

@@electronicsoldandnew - I know, it's a mystery. Have a good weekend!

[@electronicsoldandnew]

👍

[@daveturner5305]

I was a little concerned about the restuffed rectifier. It appeared to me that where the leads were soldered to the external connectors that the solder barely came halfway up the leads. If so is this good for the long term?

[@electronicsoldandnew]

It’s well soaked, never fear. I made sure of that.

[@daveturner5305]

@@electronicsoldandnew Thanks. Just an illusion on the video then.

[@skyemac8]

They had Applesque look to them.

[@electronicsoldandnew]

They sure did 👍

[@p_mouse8676]

EL95 are great little tubes I wish they would still produce them today next to the EL84 etc They only need 200mA heater current and have a higher output power to heater power ratio compared to many tubes and are also quite linear.

[@electronicsoldandnew]

👍 I don’t see many of them around these days. As you say, a pity.

[@p_mouse8676]

@@electronicsoldandnew btw. It's actually a bit nicer to put the resistors before the diode. Much less strain on the power (valve wizard has a little chapter this on his website/book).

[@randomsteve4288]

The EL95 (6DL5) was a physically miniaturized audio output tube, designed for those cases where a EL84 was not desireable sizewise but a similar output power was aimed at. The latter led to the problem that a EL95 was almost always pushed to the (plate dissipation) limits, which along with the fact that the cathode was quite small (hence only 200mA heater current) made it one of the more short lived tubes. Alas not many more NOS EL95 are around. A school of thought is using the slightly more robust 6BM5 as replacement. It is a drop in replacement, yet without changing the cathode resistor to suit, the 6BM5 will run at lower plate current. But even if tuned to the plate current the EL95 is run at, a 6BM5 will work happily and still have some overhead. Only downside of the 6BM5 is the heater current demand of 450mA versus 200mA.

[@electronicsoldandnew]

True. Ac power is less strenuous on the parts.

[@electronicsoldandnew]

Great info. Thanks

[@fotoralf]

I don' tthink the mains plug is original. It's a plug with a PE contact (Schuko-Stecker) but the mains cable only has two wires, i.e. no PE. This configuration doesn't make sense and it has been illegal in Germany for as long as I can think. The reason for someone fitting the Schuko plug might be that the older German non-Schuko plugs had thinner prongs and wouldn't give good contact in a modern Schuko outlet. Still, it's not allowed. But I suppose on Madeira they wouldn't care too much about this. 🙂

[@electronicsoldandnew]

They won’t, but … I bought this radio from Germany. It arrived two weeks ago 😊 I guess there are naughty radio owners everywhere 😊