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Tandberg Huldra 10

Tasos, SV8YM, has written about “The Mysterious Case of the Withering Filters“. This seems to affect not only ham radio transceivers, but FM stereo receivers as well.

Tandberg from the 70’s are collectors items and since I actually worked one summer at Tandberg in the early 70’s they bring back good memories for me. The latest generation of receivers (2nd version of Huldra 10, Huldra 11, and Huldra 12) had ceramic filters for the 10.7 MHz intermediate frequency for FM. It is also known that these filters deteriorate leading to reduced sensitivity over time.

SV8YM has pointed out that ceramic filters deteriorate due to DC on the terminals, especially the output terminal and that this leads to electromigration. In the Huldra 10, both filters have 7.1 V DC on the input. Filter F1 has 0 V DC on the output, while F2 has 2.1 V on the output.

Old filters (left) and new filters

As I got ready to replace mine with new Murata filters (SFELF10M7GA00-B0, 230 kHz bandwidth), I read the same warning in their specifications, which says: “For safety purposes, connect the output of filters to the IF amplifier through a D.C. blocking capacitor. Avoid applying a direct current to the output of ceramic filters.“

I also noticed that the filters had been replaced before. To be sure, I added three coupling capacitors (10 nF – 0.01 uF). This value has a reactance of 1/(2 pi 10.7e6 0.01 e-6) = 1.5 ohms which is negligible compared to the 330 ohm termination impedance. They were surface mount capacitors which is quite some upgrade as they perhaps were not even invented when this receiver was designed. They were fitted under the PCB by cutting the appropriate trace. Whether this has any long-term effect I don’t know, as the Huldra 10 at the age of 40 years is beyond its design life anyway.

My somewhat inaccurate oscilloscope measurements indicated that the passband attenuation in F1 was reduced from 7 to 4 dB and in F2 from 9 to 2 dB, in total maybe as much as 10 dB gain. FM sensitivity seems to have been restored to normal value after this replacement, so it was well worth the effort.

The three new 10 nF coupling capacitors, circled in red

The two filters, circled in red

0.33 uF X2 capacitors which measured only 0.097, 0.1, and 0.118 uF.

Many devices now use a capacitor power supply saving the space that a mains transformer occupies. The principle is that a series capacitor from the mains supply is used to drop the voltage and reduce the current. Provided that the circuit is completely isolated from human touch, this is an economical way to provide DC power.

The image shows three such capacitors as I were measuring them. They came from three malfunctioning devices in my home: two wall-mounted thermostats for floor heating and a remote controlled mains switch.

Their power supplies were designed with a capacitor of 330 nF in series with a bridge rectifier which supplies the low voltage DC. This value is typical, it seems for 230 Vac, 50 Hz circuits that are designed for about 20 mA. The value will be higher for an equivalent 115 Vac, 60 Hz circuit.

The malfunctioning happened because the value of the capacitor in my cases was reduced to 1/3 and less of the nominal value. These capacitors are all marked X2 and a voltage of 275 Vac.

The X2 means that they are safety capacitors which will not fail by short-circuiting as this would be a fire hazard in this circuit. They have self-healing properties and that means that they fail by “burning away” on their own foil, leading to a reduction in capacitance and eventually failure of the circuit as the power supply cannot supply the required current any more. They should never be replaced by anything but X2 capacitors with the same or higher voltage rating.

Go to the Wikipedia page Capacitive power supply for more description of this circuit.

By the way, the devices which these capacitor came from were 15 year old Microtemp MTN-1991 thermostats and a 20 years old Nobø System 500 RCE512 remote receiver.

0.33 uF X2 capacitors which measured only 0.097, 0.1, and 0.118 uF.

Many devices now use a capacitor power supply thus saving the space that a mains transformer occupies. The principle is that a series capacitor from the mains supply is used to drop the voltage and reduce the current. Provided that the circuit is completely isolated from human touch, this is an economical way to provide DC power.

The image shows three such capacitors as I were measuring them. They came from three malfunctioning devices in my home: two wall-mounted thermostats for floor heating and a remote controlled mains switch.

Their power supplies were designed with a capacitor of 330 nF in series with a bridge rectifier which supplies the low voltage DC. This value is typical, it seems, for 230 Vac, 50 Hz circuits that are designed for about 20 mA. The value will be higher for an equivalent 115 Vac, 60 Hz circuit.

The malfunctioning happened because the value of the capacitor in my cases was reduced to 1/3 and less of the nominal value. These capacitors are all marked X2 and a voltage of 275 Vac.

The X2 means that they are safety capacitors which will not fail by short-circuiting as this would be a fire hazard in this circuit. They have self-healing properties and that means that they fail by “burning away” on their own foil, leading to a reduction in capacitance and eventually failure of the circuit as the power supply cannot supply the required current any more. They should never be replaced by anything but X2 capacitors with the same or higher voltage rating.

Go to the Wikipedia page Capacitive power supply for more description of this circuit.

By the way, the devices which these capacitor came from were 15 year old Microtemp MTN-1991 thermostats and a 20 years old Nobø System 500 RCE 512 remote receiver. They now all work again thanks to the fitting of new 0.33 uF capacitors. And all of them are safety capacitors of type X2 of course – no gambling with safety here.