SABA Stereo Decoder E16 for FM tube radios - Detailed scope probing and alignment
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- Опубликовано: 4 мар 2023
- Stereo FM decoders on tube radios were usually plug-in modules that could be purchased as an option. I have never worked on one of these before, so having a SABA E16 module in my hands was an opportunity that I simply could not pass up. I wanted to do a full restoration on it, as it does include a few electrolytic capacitors and poor quality resistors, but I also wanted to go further into its workings to learn more about the device.
In this video I probe the various signals with a scope to see what they actually look like. I also do an alignment, which serves to improve performance, and also to further de-mystify what is actually happening in the module.
Here is the link to the full service manual:
nvhrbiblio.nl/schema/Saba_Dec...
Enjoy the video.
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#tuberadios #antiqueradio #radiorestoration
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Наука
Curiosity killed the cat.
Satisfaction brought him back.
That’s why a cat has nine lives.
Nicely done, Mr. Cat! 😎
Thanks 😊
Wooooooowwwwwwwwww and many many years after.... FINALLY!!!!! Yes, I'm THAT one!!!!! Ciao Maestro e grazie!!!
😊
It is great to have the service information to understand exactly what you are looking at, then it becomes simpler, rather than that intimidating shielded metal box with several unknown coil and trimmer adjustments on it!
True
You managed to release this just in time for Sunday morning coffee in the US, it makes for an enjoyable start for the day for us electronics repair junkies. @ 16:50, it may be that, the shielding box is more for keeping the 19 and 38K signals inside of it, than keeping other frequency interference from getting in, especially from the high P-P voltages you measured. Also, since you are dealing with oscillator circuit frequencies that low, making the adjustments with the board remotely connected like you did would probably not affect your adjustments as well, as they are almost at audio frequencies.
True. These are indeed low frequencies. Some slight alignment influence may occur, but negligible.
I have to thank you. I totally overlooked the small electrolytics and the resistors on that board. All are done for and changing them really transformed the sound of the receiver. It's fun now and was a bit dull beforehand.
👍
This is a great video. I have a Saba decoder on a Saba 300 which needed some TLC. It came with two telefunken 12ax7’s that test nos and all the parts and layout etc. had real promise. So after watching this vid I worked up the courage to pull the cover off of mine and see if I could coax it toward better performance. Well I followed the English step by step instructions from the pdf I downloaded and tuned it up. Finding the Lissajous curve and getting the circuit stable at the fm stereo 19k/38k frequencies was quite beautiful really. One of best learning/experience I’ve had in this hobby. Thanks for the help!
My pleasure 👍
Manuel, Great restoration, I think your publishing of the FM decoder will help future repairs tech's a big plus publishing the decoder service manual. Thanks for all the work while hugging that camera.
Working around the camera can be challenging 😊
Great video! I've been waiting for your dive into the stereo decoder.
What you describe (the fear of opening something up when you have no idea how it's working) is very recognizable. Thanks to this video I now already feel a bit more comfortable should I ever need to work on a stereo decoder from this era. Thanks a lot!
My pleasure
Good deal! Nice work with the demonstration. Thanks!
👍
Great video -- indeed, it's one of the few attempts recorded on video of adjusting one of these. It certainly helps lift the psychological barrier to see what's inside one of these. Additionally, it's good to know the components inside are of such good quality that bar a few resistors that have drifted and electrolytic caps that are getting a higher ESR, all else works as expected and that the circuit is a standard circuit.
I'm not sure why the crosstalk cannot be reduced -- I'm not sure it's due entirely to the two output channels being housed together under one roof, so closely to one another. I would guess that it would be impossible to bring down beyond a certain limit due to the design itself but this is just a guess.
👍
It's amazing to see how early stereo receivers used discrete circuit design to seperate the L & R channels. Would be interesting to see how well its performance compares to modern FM radios that has the whole thing on an IC
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Stereo FCC multiplex was introduced in 1960-61 in US. So first decoders used tubes instead of transistors, but the circuit was quite the same.
👍
Brilliant. 😎 Not sure I'll ever do one but it's very reassuring to see that this is not a “sufficiently advanced technology that is indistinguishable from magic”, to slightly misquote Arthur C. Clarke. 🤣 And thanks for the link to the manual.
Thanks 😊
Bravo👍
😊 thanks
Yes, however I've got a TFK 2550 that has a tube stereo decoder. Will have to do some reverse engineering/reengineering to align that as well
Should be fun 😊
In fact you can use this scheme as a block diagram. Tube decoders used the same principle, only instead of transistors there were triodes.
Early discrete stereo decoder, interesting. I'll be interested to see what you decide to do with the cabinet.....
Still thinking 🤔
From what I can see, the original was a teak veneer case with a cosmetic strip at the base under the controls which bore their names. I guess it depends what you find underneath if you strip it.....
👍
Another advantage of it being a module is that you can swap it with another decoder.
Those first gen coil/diode decoders didn't have all that great channel separation,
but you could make a new PCB in the same size and same pinout and use
newer IC circuit to get better performance.
When it's inside that can, no one would know that it's not the original.
I thought of that. Do you know of anyone who’s done that, or published a working circuit?
@@electronicsoldandnew No, not for Saba's, but I made a few for the Tandberg radios that I restore.
Many of them was delivered with only a dummy decoder as the real stereo decoder was an option.
I just copied the latest rev. of the original schematics with KiCad and had JLCPCB make a stack of them.
Most stereo decoder IC's has a example schematics in the datasheet that one could just copy from.
Thanks. 👍
Did you make the decoders to size for the slot on the mainboard? An did you base your schematic on the TB 4-2?
@@scienide2k Yes, it's the Dekoder 4-2 and I made the board identical to the original so it fits all models that takes the 4-2 decoder.
The only difference is that I rearranged the components a bit. I might do another revision of the board, maybe with a different IC
as the MC1310P is difficult to find, and also use multiturn pots and put them and the test points at the top of the board for easy
adjustments.
I've aligned a lot of decoders similar to this one, and I'm sure you can get it a lot better than this.
You should go over the three coils L2,3,5 that maximize the DC at point IV repeatedly until you get a maximum. One pass isn't sufficient. Probe the 38kHz signal at the indicated point to see what is really going on. The more voltage here the better.
You should have adjusted L3 _before_ P2 as per the instructions, not afterwards. P2 is only fine tuning: L3 does most of the work. You don't seem to have done steps 5-9 at all.
The channel you aren't modulating should be nearly flat on the scope, consisting practically only of pilot. You should be able to get -36dB of separation out of a decoder like this, especially if it has decent pilot filtering at the output, or if you can bandpass-filter the output to remove the pilot before measuring dB. It makes a difference not only to the measurement but also to the actual optimum alignment point: otherwise you can just end up trying to cancel the pilot, which is both wrong and futile from the audio point of view.
NB L1 isn't a 114kHz low-pass in the unit you aligned, it is a bandstop, as your experiment with varying the frequency showed.
The L-R imbalance is largely down to the accuracy of R18,19,21,22, and the resistors around T4 and T5, and the adjustment of P1, the balance pot. You should be able to adjust it so that either channel is larger, and find a happy medium.
You should also check R42 and C28, and the same in the R channel, for accurate 50uS de-emphasis.
Fun stuff.
Fun stuff indeed. Thanks for your input. As I mentioned in the video, the alignments were repeated off-camera and the result improved considerably. However, I will have another look at the components you mention.
Nice description of this "LC filter" decoder. I have learnt about it once, but the IC MPX decoders were/are much more usual. Sadly, here in Norway the national FM transmitters were switched off some 5 years ago and replaced with DAB.. Some locals still use FM. How is the situation in Madeira, is DAB used much?
We still have FM going strong in Madeira. DAB did not catch on.
Yes, been doing a lot at my end with both AM stereo and FM stereo decoding, where wanted to design a common decoder for both formats, as they both use R-L & R+L encoding process. With AM stereo I used two SA612 mixers using sine (R+L) & cosine (R-L) offset, on the FM stereo decoder I use a CD4066 to do the converting to left and right from R+L & R-L. The both use a common logic gates to synchronize to the AM IF at 450kHz & FM base band at 38kHz, as well as out putting the pilot tones of 25Hz, 19kHz and the carries at 450kHz, 38kHz as logic levels. I have circuits if you are interested in how I did all this?
I would definitely be interested in the FM decoder 😊
@@electronicsoldandnew I will send you the circuits and a few pictures of the prototype.
👍
@@electronicsoldandnew sent with details on how it works
👍
My Tandberg Huldra 8 also has an optional stereo decoder model 3, it has almost no stereo separation, your description gave me a clue to find out the reason.
In addition, the stereo decoder of my Nordmende 6/634 only uses one ECC81, which can produce amazing stereo separation and sound quality, but recently the stereo indicator light of its EMM308 is off, but the stereo effect is still normal. Do you have any suggestions for this issue? I hope you will have more videos about this in the future, thank you!
I’m afraid I can’t advise you on that as my experience with this issue is limited to this particular decoder.
Great video Thank you
I have an old Stereo FM Tuner Kit (maybe 40 years old) I had change IF transforms settings with screwdriver while a young lol
FM Tuner board have:
TBA 120S (14 Pin DIP Case IC) and SN76115N(14 Pin DIP Case IC anyway have not SMD cases 40 years ago as you know )
Also have red colour and green color IF transformers also have BF198 transistors( 2pcs)
For FM Tunning have 4-5 cm long sensitive trimmer potantiometer.
Coud you advice a video about correct FM adjust principles (or a pdf link) I have 2 channel 50 mHz Oscilloscope and to 60 Mhz Function Generator (also have sweep feature too)
Can I make adjust this tuner ? If possiable I want listen to this tuner for remember old memories
I think you was understand me with this bad english
Regards
I’m afraid I have no knowledge about that tuner.
Where do find the service info on all these European radios?
This is one of the main sites:
www.nvhr.nl/frameset.htm?&ContentFrame
Senhor Manoel boa tarde como está o senhor tudo bem?? Estou de volta aqui em Londres passei um tempo descansando tenho um novo projeto quem sabe o senhor possa fazer uma revisão e alinhamento em um Ar-88Dum rádio pesado 45 kilos abraço aguardo resposta…
Try Mr Carlson's Lab. He is restoring an RCA CR-88. Almost identical.