telefunken: 8; Concertino

A new problem has developed with my Concertino. after playing at reasonable volume(not so much to distort the sound)  for a minute or two the sound begins to become very distorted. It does not happen after being switched on for a while, in which case I would assume it to be a capacitor which leaks when heated up, only after playing at volume. Does anyone know if this points to a specific area I should look for the fault?

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Dear Stuart,

you do not say if the arising distortion can be eliminated tuning again the radio. Should this be true, your problem has two aspects. The first one is the normal frequency drift during the initial warm-up, common to all the LC resonating circuits, RF, oscillator and IF. The heat coming from vacuum tubes, power resistors and other components slowly rises the temperature inside the radio. Tuning capacitors and inductors change their values according to their temperature coefficients.

This is normal for every radio and I do not have experience on this specific model to evaluate how much drift can be considered acceptable. May be a problem with the tuner itself, but in your case this phenomenon may be exaggerated by an improper IF and/or discriminator tuning. One of the causes may be a very sharp IF bandwidth, if you simply tuned the IF transformer cores for the peak.

If the problem disappears after few minutes of warm-up, you could accept it or try a better alignment of the radio.
 

Regards, Emilio

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Thanks Emilio, perhaps I should have been clearer about my question.It is when using the radio as an amplifier with an external input that this happens. (maybe it happens as radio too but i dont usually play it loudly in that case). So there is some concern about tha amplifier circuitry, as I said it is not when it is on for a while that it starts to distort, only after playing at reasonable volume for a few minutes. 

Stuart  

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Dear Stuart,

your problem could derive from two probable causes. One could be the 2.2 nF capacitor across the primary winding of the output transformer. But I would first try to replace the old EL84 with a new one.
 

Regards, Emilio

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Hi Emilio thanks for your help once again. I am fairly sure I would have replaced that capacitor when I did the initial work on the radio, meanwhile I found another new el84, i think i bought a pair initially, JJ brand from Slovakia. I replaced it and it seems to be fine now.I was just a bit surprised that the tube wore out so quickly as always in the past I have found valves to last almost indefinately and that they are usually the last things to be at fault especially after recent replacement. Anyway I'm happy again now my Telefunken if functioning properly, so thanks again! 

Stuart  

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Dear Stuart,

of course a defective tube can justify the short life you experienced. Nevertheless the failure may depend upon an improper use. Try to start the reproduction at a moderate volume level and turn the volume control full clockwise after a couple of minutes. This simple trick gives the cathode the time to reach its full operating temperature before giving full emission. When the cathode is still partially cold, it can be easily damaged if full emission is required.
 

Best regards, Emilio

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