loewe-opta: High voltage on the anode of the EABC80 valve - pin2

Dear collegues,

I did a restoration work on a Loewe Opta Kantate 3721w radio which involved replacing the power transformer, taken from a Philips BCH462A radio, replacing the levelling capacitors and replacing the selenium rectifier with two diodes.

Due to these changes the anode voltage increased to about 305 V, also due to the higher mains voltage, now of about 240 Vac. To reduce this anode voltage I inserted a load resistor which reduced the voltage to about 260 V. In addition to these interventions on the power supply circuit, I had to replace the coupling capacitor between the EABC80 and the EL84 and on the latter I also replaced the capacitor and the cathode resistor: at this point the radio has reached a good level of operation.

That said, I come to the point for which I ask for your opinion. 

Measuring all the voltages these are aligned with those indicated in the schematic, but I was surprised by the voltage I measured on the anode of the diode of the EABC80 pin2 which turns out to be about -46 V, with a good incoming signal, which frankly, in my little experience, I had never seen so high, so I ask if this represents a situation that can be considered normal or if instead it derives from some anomaly generated I don't know where and from what.

Thank you very much for the help you can give me.

Roberto
 

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Dear Roberto,

The observed high (positive to ground) voltage at pin 2 of the EABC80 is absolutely normal when a strong FM station is received. This is the directional voltage generated during demodulation (ratio detector). Therefore the used electrolytic capacitor 4µF was dimensioned accordingly, usual here are 60-70V as working voltage.

Tune to a weak FM transmitter, or tune to a reception-free place. The voltage at pin 2 (= voltage at the ratio electrolytic capacitor) should then be much lower or go to zero.

Via a voltage divider 2x 16kΩ the ratio voltage is also used to drive the tuning indicator tube EM84. Since the light fields already close completely at -22 V, half the ratio voltage is completely sufficient here.

So here everything is in best order.

Basic information about the function of the ratiodetector can be found here (in German): Schaltungstechnik der FM-Demodulation

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I would say that it is quite normal for a FM reciever with a ratio detector. The electrolytic capacitor in the ratio detector seldom has lower rated voltage than 50 V, and of course the manufacturers would have specified cheaper capacitors with lower rating if possible.

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