grundig: Voltage Calculation

I'm trying to calculate some voltages from the schematics in order to test my capacitor replacements.  On the schematics, the three capacitor group C83, C86 and C88 are all electrolytic caps.  C88 has a 285V value, but I want to calculate C86 and C83 for testing purposes.

It may not be this easy, but C86 looks like it should have a 1000 Ohm * 0.98 Amp = 98 Volt drop from C88, resulting in a value of 187V.  Is that too simplistic because there are other things going on in the circuit?  And how would one calculate the voltage at C83 since there isn't a clear one-to-one calculation like for C86?

And how long after turning on a console such as this should I wait for the voltage to stabilize to accurate readings?  I'm guessing 10-15 seconds based upon how long the sound takes to start up.

Thanks for any guidance!

craig.

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Hi Craig,

the calculation of the voltage in a given node could be very difficult and even useless in some circumnstances.

It is difficult, because you need to know the exact voltage of the generator and the exact values of all the resistors and of the loads in the circuit. When talking of resistors in the circuit, I also refer to hidden ones, as the DC resistance of windings. In the case of the drop across R97, you have to know not only the load given by the four screen grids of the power amplifier tubes, but even the currents drawn by other RF, IF and audio preamp stages. These are variable, depending upon the voltage itself and even on the configuration of the switches. You can get a rough idea of the drop across R97 or R91, adding the typical values of Ia and Ig2 for all the tubes whose supply is derived from that resistor. However remember that sometimes a tube can be used by design well outside its typical operating conditions.

Anyway for this model you find a complete voltage/current chart on the diagram. If you look at the top of the diagram, you see the bases of the corresponding tubes below, with voltage and current values that should be read at each pin. The designer gave a set of three vales for each reading, the minimum, the design center and the maximum ones. For practical purposes these values can be measured after the cathode warm-up and take account of line and load variations within the limits specified for the set.

Emilio

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