telefunken: 8; Concertino

Hello I am beginning restoring a concertino 8 and have never worked on german radios before,only Australian ones. the sound is distorted and "blotchy" when connected to an external line in source suggesting problem is in audio amplifier.Does anyone know of a common cause for this before I go replacing all the capacitors in the audio amp section? From studying the circuit it appears that the triode bit of the EABC80 is part of the audio amp and the rest of that valve is for something completely different,is that correct? the voltages are about 20% low,is that enough to make a difference? They are high on switch on but drop with warm up and the rectifier gets warm suggesting there is some power dissipation in the rectifier,I thought of changing for new silicon rectifier diodes,is that a good idea?

If anyone can advise me it would be appreciated, I have some but limited knowledge of radio cicuits.

thanks

Stuart Matousek  

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Dear Stuart,

The problem you describe seems related to a positive grid bias of the output tube, due to a leaky coupling capacitor between the plate of the triode section of the EABC80 and the control grid of the AF power tube. If you have a leaky capacitor, the grid goes positive and the output tube drains a heavy current, so causing a drop in the anode voltage as soon as the cathode temperature goes to its normal emission value.

Try to measure the control grid voltage of the output stage.

Emilio

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Thanks very much emilio for the advice,as it turns out just as I got your reply I had just replaced that coupling capacitor anyway,and it has improved a lot,however still not quite right(im fussy about the sound quality) The control grid (thats pin 2 i think?) now has a positive potential of nearly 1 volt,should it be zero?

The rest of the voltages are now closer to what they should be but not quite.should I replace the main rectifier with new silicon diodes?and the em80 is very dim,would that be the tube itself?

Stuart

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The voltage on the control grid, pin 2, must be zero. You can also check the cathode voltage. When all is working fine, the voltage should be around 7 to 8 Volts: any higher voltage could be due to an improper bias.

Another way to fix the problem is to open one of the two connections of the coupling capacitor itself. Even if in this case the amplifier cannot work, the DC bias conditions should return to their proper operating conditions. You should then find normal voltages on the cathode (about 7 to 8 volts), on the control and the screen grid and on the plate of the EL84. The control grid must go to zero volt.

If a positive voltage is still present on the control grid with the coupling capacitor disconnected, the EL84 may be gassy and should be replaced.

Emilio

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Hi Emilio,I tried that,disconnected the coupling capacitor the voltage on pin 2 was still about 0.8v,so I replaced the el84 with another(old one from other radio) and a bit better but still 0.2v,though connecting the coupling capacitor does not effect this,so probably I should get a new el84.Any brand ok? the cathode voltage is about 6 so thats OK I think.was wondering out of curiosity why the csathode is not directly earthed therefor 0v but connected via 120ohm reststor and capacitor?

Thanks for your advice,its helping me learn more about the circuitry,if only Id known that years ago it would have helped with many other things Ive tried with some but limitted sucess to fix!

another question,originally the main chassis is not connected to mains earth,maybe because there was not mains earth in those days,should I connect it to mains earth?

thanks Stuart

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Hi Stuart

First of all, the 120 ohm cathode resistor sets the proper grid bias working point. As soon as the emission starts, the current in the tube is limited by the sole cathode emission, since no negative bias is applied to the control grid. The cathode current, flowing in the 120 ohm resistor, causes a voltage drop on it. The cathode then becomes positive with respect to the chassis ground. The control grid, which is at the chassis zero potential, becomes negative with respect to the cathode. By design, in this case, the equilibrium is reached for a cathode voltage of about 6.2V (the voltage drop given by the sum of anode and screen grid current, about 51mA, flowing in the 120 ohm resistor).

The voltage you find on the cathode may then be considered normal, although close to the lower limit. The cathode electrolytic capacitor acts as a shunt to ground for AC signals: it does not affect the DC operating conditions of the tube, unless for heavy leaks or shorts.

The little positive voltage you still find on the control grid with the 22nF capacitor disconnected may indicate that the EL84 should be replaced again: I do not see on the diagram any other path from where this bias voltage can reach the grid.

About the latest question, a connection of the chassis to the mains earth can be advisable for safety reasons.

Regards, Emilio

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