philips: BX700A (BX 700 A);

For some time I tried to obtain a reasonable FM reception on this set.

I replaced all the tubes of the FM part,I adjusted everything according to the sevice manual but the

reception is pretty poor,on many stations there is a annoying background noise like there is a problem with

the tuning of the hf stage.

Henk Kramer

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Dear Henk,

I guess that you have already performed the tuning of RF and IF stages. In this case, the poor sensitivity may derive from an improper grid bias of one or more EF42s. In this radio I see a fixed negative grid bias, generated across R3. I do not know its nominal value, but it should be around -2 to -2.5V, with power supply current drain in the order of 100mA. A positive voltage, from the cathode of B14, is summed to the said voltage, in order to bias the grid of B13, operating as limiter. The same fixed bias is also summed to a negative AGC voltage coming from the left anode of B14 and R69. The resulting voltage biases the grids of B12 (IF amplifier) and a fraction (about three quarters) biases the grid of the RF amplifier B10.

EF42 is a quite sharp cutoff pentode. It operates with about –2V grid bias, but at –4V it is close to the cut-off. Many causes can change the operating point of the tubes, from a change in the current drain of the radio itself, to an oscillation somewhere as in the limiter stage, to a drift in the value of one summing resistor. More, we do not know the design values of the biasing voltages.

Try to measure the static bias voltages (no signal values) across R3 and on the grids of the three controlled tubes, B10, B12 and B13. Check for values appreciably far from the typical –2V. Try to measure what happens when receiving local stations.

Regards, Emilio

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Dear Henk
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Dear Henk,

I well understand your hesitation to plan the service of your radio starting from the long shot I had proposed. I would have done the same. Unfortunately you gave no details about the actual behavior of the set. Why did you realigned the FM section or replaced the tubes? Was the sound poor and distorted? If you noted some distortion in the sound, I can propose a quick shortcut. Try this simple approach.

Replace the grid coupling capacitors of the output amplifier, C48 and C49, 33 nF each. Use two plastic film or film/foil capacitors selected for matched values.

I may be wrong and the cause can be elsewhere, but this is the most common fault in an old radio. The probable defect is in leaky capacitors from the inverter stage, B4, to the output power tubes B5 and B6. Leakage currents move the DC bias of the EL41s, causing a huge increase in their plate current drain.

This abnormal current, flowing in R3, increases the negative bias voltage that controls the EF41s of the FM section, driving them close to cut-off.
 

Emilio

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First of all, I do not know how, but this note should be extended to other similar Philips models. I am referring to BI700A and DI700A, from Philips Italia, and probably to BD700A, Germany, all having the same chassis. I guess that Henk is no longer interested in it, nevertheless other people could have troubles in the FM reception with these quite complicated radio sets.

I have no direct experience on the BX700A: I serviced the BI700A and the console DI700A. The major difference is in the part list, having Philips Italia qualified some Italian-made alternate components, such as capacitors from Siemens-Olap (Milano), Mial and Microfarad, and resistors. By the way, I found many capacitors still in properly working conditions: aluminum capacitors still retaining 150% of their nominal value; or paper capacitors with over than 100Mohm insulation at 500Vdc. Other types of different brands were faulty at all. However, my experience about reliable or faulty components is only applicable to the radios produced in Italy. I suggest to check every aluminum capacitors for its actual capacitance, every paper ones for leakage at high voltage and every self-biasing or voltage divider resistors for actual resistance value.

Here I want to give an additional note on the troubleshooting in the FM tuner section. Its improper operation can derive from faults in other sections, as in the audio power stage. The no signal grid bias of the RF and IF amplifiers, all EF42, is supplied by the voltage drop caused by the total B+ current flowing in R3. A relevant part of the B+ current is given by the no signal current of the class AB audio output stage. One of the most common faults is a leakage in the grid coupling capacitors, C48 and C49, as elsewhere said. In this case, the sound is more or less distorted, but also the FM sensitivity can be greatly impaired.

I measured the voltage values on the cathodes and on R3 in the two properly operating sets I own. With R3 and R37, cathode resistor, within 2% of their nominal value, the voltage across R37 (EL41 cathodes) was 7.0 to 7.1 Vdc; the bias voltage on R3 was – 2.4 Vdc. These values indicate about 70 mA in the audio output stage and about 110 mA overall current drain.

When measuring voltages different from the values given above, it can be advisable to restore the proper bias values before looking for faults inside the FM tuner.

Regards, Emilio

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