dumont-la: Dumont 1120 clock radio Couplet failure

Martin Renz Ernst Erb Vincent de Franco Martin Bösch Mark Hippenstiel Bernhard Nagel Dietmar Rudolph † 6.1.22 Otmar Jung Heribert Jung Eilert Menke 
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Forum » Radios and other type of sets (Physics) etc. » MODELS DISPLAYED » dumont-la: Dumont 1120 clock radio Couplet failure
Jay Graven
USA  Articles: 5
Pict.: 62
01.Oct.21 04:53
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I'm preparing to restore the circuit of a Dumont 1120.  I noticed that the measured resistances for couplet were far off.  The 470K resistor between pin 5 and pin7 measured 1.8M ohms.  The other 470K resistor between pin 6 and pin 4 measured 5.5M ohms.  And the 10Meg resistor between Pin 1 and Pin4 measured 16.5M ohms.

Is it common for these to fail?  I just had to rebuild one on a Sylvania R518 couplet.

Schematic included by DR









This article was edited 01.Oct.21 09:32 by Dietmar Rudolph † 6.1.22 .

Torbjörn Forsman
S  Articles: 101
Schem.: 26
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01.Oct.21 08:59
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Yes, it is very common that cheap carbon composition resistors become unreliable when aging. This is particularly true for resistors that have been subjected to DC current for a long time (for example, screen grid resistors and anode loading resistors in tube technology).

It is also common that a carbon composition resistor that has been used for DC for a long time, becomes unlinear which can cause strange phenomena (for instance, distortion in an AF amplifier). A simple way of checking this is to measure the resistance twice and change polarity of the meter between measurements. If the resistor does not show the same resistance with both polarities, it should be regarded as suspect even if both measurements are within tolerance.

Jay Graven
USA  Articles: 5
Pict.: 62
01.Oct.21 19:08
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Further review of the schematic from Rider, I now believe the Dumont Clock radio is a model 1140 or 1150 and not the 1120.