braun: SK25 (SK 25); Kleinsuper

Hello !

I am restoring one of this set for a friend who likes the design and was looking for a tube radio.

She want to listen to French FIP which is at 103.5 MHz in her town. I said to her that it should pose no problem as the dial is marked up to 104 MHz. I said that I will re-align the set as part of the renovation procedure and that she will get the FIP sound clear and loud... I was wrong. Badly wrong. This set goes from 88.5 up to 102 MHz maximum. Of course I was able to tinker with the oscillator coil and tune FIP on 103.5 but doing so I loose the lower frequencies (and France Inter which is at 88.1 and used for emergencies announcement. So I would normally refrain  to deliver the set in this state. So here are my questions :

This set was unmodified from factory (as said the previous owner) was made in Feb. 1959 (stamped on the transformer). How come it goes only to 102 MHz ? With a dial up to 104 MHz ?

How could I modify it to get 104 MHz or, better, 108 MHz ??

Thanks a lot in advance for your help and answers !

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Hi, Georges,

the Circuit Diagram shows a Trimming capacitor in Parallel to the Variable Capacitor C9, It is shown with dottet lines, i would think, that this means, that this is a modification done during the Production period.

To raise the Oscillator Frequency, You schould first see, if C9 goes really to the mechanical End in both Directions. If this is the case, then normally an L-Alignment of L4 will be done at apx. 90 or 91 Mhz, and a C-Alignment at about 3 Mhz below Maximum of the UKW Range.

If the Frequency Range doesn't go up to 104,0 , this means that the total Oscillator Capacity is too high. Reason can be a ECC85 Tube with slightly higher inner Capacity ( You can try different ones, if You have ), but also that somebody has sprayed the Tube Socket with Contact cleaner.

If the trimming Capacitor for Oscillator is not there, then You can try to reduce C15. You need to use the same Type of Capacitor, to avoid Thermal Drift !

Please be shure not to bend any wires or Components inside the shielded box. I recommend to make good pictures before changing anything

Hope this helps,

Henning

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Hi Henning !

Thank you for your help.

No there is no trimmer capacitor fitted on my set. And the tuning capacitor is in ristine order. I first checked it for mechanical damage (and thought I could spread the blades a little to reduce the capacity but refrained myself...)

Your answer makes me blush. I tried to reduce the total capacity of the tuning circuit, but I was mistaken and reduced C14 .... You know what ? It didn't work ... Well it worked but the set tuned at maximum at 34 MHz. quite far from the FM in France.

What capacity should I try for C15 ? Because I've to buy mica capacitors so I would go for the exact value needed plus 2 or 3 above and below the correct one to allow for stray capacity during soldering.

As the case need an heavy restoration and that the set will be on constant use, I would prefer to get up to 108 MHz, my friend willing to make the new dial...

Thanks again for your help and advice.. I should pay more attention to the circuit before grabbing the soldering iron !

I forgot to ask with what I should clean the ECC85 socket to remove all previous contaminants ?

 

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Hi Georges,

before changing C15 please try another ECC85, or text this ECC85 in another Radio, to see, if the tube causes any Frequency offsets.

 

I would try to replace C15, which is shown in the Diagram with 17pF, with 15pF, or 12 pF. You need to do this step by step, close the shielded Box, align L4 to have the lower End of UKW correct, and see, where the last Station is at the upper end. If let's say You get to 103 Mhz, you can make another Test with lower Capacity.

You need to play the whole game again. open the Box, remove C15, put in the next lower Value, close the box, and realign L4 for the lower end frequency. Then check again where the upper limit is.

please make shure to mount the Caps with short wires, and in the exact position.

I recommend to make notes, what exactly You are doing, what the Values are, and what the Frequcies You can tune are. Make shure, not to get lost.

Regards, Henning

 

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Thanks for the help.

I'll try a couple of ECC85, the original one has the same behavior, the one in the set now is a new one.

I'll will fish for the caps tomorrow and do my homework. In my drawer of spares I have a 10 pF which I can try tomorow. I will see if it is too low or not. If too low, it will narrow the value to put in.

Your help was invalluable as I was vastly mistaken !  I had the theory right but was completelly wrong in putting it in use !

Thanks for that.

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So, I was not able to buy capacitors today as the shop was already closed when I get out of work.

I've put the 10 pF I had instead of the original 17 pF. No so easy to get to the solder joints without burning something in the VHF head ...

With the 10 pF in place, the set tunes up to 110 ~ 112 MHz (I do not own a counter and my generator dial ends at 110 and is not perfectly calibrated) So 10 pF is a bit too low. Tomorrow, I'll buy all values I can find between 10 and 17 pF and try to get  87.5 to 105 or 106 MHz. This way, I hope I won't be too wide for the other parts of the VHF head. Then I'll re-align the set and hopefully she will be happy with it's new toy !

I've a dead SK2 I used for parts and wonder if the VC is same values or not ?  There are no values stamped on it, and none on the schematics. It is difficult to measure such low capacities with my capacitor meter, the wires themselves havig a big capacity compared to the one I try to measure, so I do hope changing C15 will be good and enough !

Thanks again for your help and very usefull advice !

P.S. : Do you have any clues or idea why the set was tuning from above 88 to 102 MHz when I get it ? It is way out of specs, and provided it was in use just before I get it, and that all valves where original Valvo from factory, it seems not to have been tampered with. (the schematics was still inside the case and the wave change switch was seized in FM as if it has never been used in MW)

 

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Hi !

I finally got a 15 pF silvered mica capacitor suitable for the VHF head. It is actually 15.5 pF.

With it in place I can get a little under 88 MHz on one end and a little above 103.5 MHz on the other end.

Still out of specs (87.5 to 104) but close.

As I've dismantled a dead  SK2, fishing for wavechange switch, volume control and because this set is quite un restorable ( dead capacitors (smoothing and many others), dead rectifier, mains transformer not delivering rated voltage) I can get a trimmer capacitor to put into the set. Add to this the 10pF cap I got from it and I should be able to do something, I guess.

Do you think I will be able to get the 87.5 to 108 MHz with good sensitivity and selectivity ? Or should I refrain to modify the set ? (as the dial is far from rpistine, making a new one won't be overkill restoration....)

Thanks a lot for your advice and help !

 

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Hi Georges,

the now reached frequency range 88-103.5 MHz could probably increased by a greater variation range of the ganged variable condenser C9. Five MHz more should be achievable by bending the FM oscillator and rf moving vanes slightly towards the middle fixed plate without making a short. The leaving air gap between the vanes should not become too small...

Here comes a step-by-step guide, the actual pictures comes from my Braun SK2, a former model:

First, tune the radio to the high frequency end by totally swing out the variable condenser.

Ganged variable condenser of the Braun SK2 at lowest "C".

After this, select a appropriate small parallel capacitor (the 10 pF from post #6 should be fine) for the oscillator circuit. If the receiving frequency is too high, please carefully adjust the oscillator coil L4 to the demanded limit of 108 MHz. Now comes the second step:

With red arrows marked vanes for FM reception.

The two FM sections are marked with red arrows and RF for the radio frequency and OSC for the oscillator. First, bend the two OSC moving vanes slightly towards each other in the direction the arrows shows. Do this in the middle and the end section (turn the condenser 1/2 way in), and check the now obtained lowest reception frequency if the variable condenser is fully swinged in. If the result ist still above 87.5 MHz, try to bend a little more. When you've reached finally 87.5 MHz, the oscillator variation is now ok.

Tuning condenser turned to max capacity.

After this, you should treat the RF part the same way. Start with reception setting of 105...106 MHz, and adjust coil L3 for best sensivity (a weak station makes it easier to perform it). Now bend the RF vanes in the middle and the end (1/2 way turned in) as described for the OSC section to obtain the best sensivity over the whole reception range.

Do this very carefully in small steps and avoid to bend the vanes with sharp metal tools. And please do not touch or alter the much finer vanes for the AM section... Good luck!

Hope, this will help.

Bernhard

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So here is the status :

1) with a reduced C15 of 15.5 pF instead of the 17 pF original, I get 87.8 to 103.9~104 MHz. Quite good. It took me a complete set adjustment and a lot of patience to achieve this. The set only suffer a little tuning drift during the first 5 minutes, so I suspect that the new silvered capacitor I used is not of the same temperature coeficient that the original one. I'll live with that.

2) with a 10 pF C15 and a trimmer in parallel, nothing more than above. But a PITA to get there. Murata adjustables caps are difficult to adjust when buried into a VHF head ...

3) I was unable to increase the maximal capacity of the variable capacitor because I think it has already been done. The blades are so close I won't be able to get them closer than they are. I looked at the capacitor from the SK2, and the blades are a lot more spaced than in the SK25. I think that Braun had difficulties to get the specs right from factory ;-)

I ran the set for hours with solution 1 applied and it performs very well, has a quite good sensitivity using an indoor FM antenna glued to the wall. I hope my friend will be happy with it. It is a pity that the SK2 is so badly worn out because it would have been good to see how it behaves with it's EF89 instead of the ECH81. Maybe one day I would get another one or the guts to rebuild it.

Now the more difficult part : getting the case to the car painter to have it re-painted properlly in less than a year ;-)

Thank you all for your help and support.

Cheers !

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