united: American Bosch Model 360: R15 Values

I have some further information about the 2007 thread concerning the values of voltage dividing resistor R15 in the American Bosch model 360. A voltage dividing calculator can be found on this page:

www ohmslawcalculator.com/voltage-divider-calculator

This is a good tool especially for guys like me with no formal electrical engineering training. To determine the voltage at any tap point (to ground) in the voltage divider, you plug in the starting voltage (the voltage at the previous tap to the one you want to measure), the value of the resistor just upstream from the tap point, and the value of the resistor just after the tap point.

The R15 divider starts with a voltage of 240 which supplies the plate of the second I.F. 58 tube, as well as the screen and plate of the 2A5, among other circuits. We first need to step that down to 220 volts to supply the plates of the 58 first detector and 58 first I.F. tubes. According to the calculator, a 1.8k resistor followed by a 20k resistor, steps the 240 down to 220 almost perfectly at the second tap. (On my model 360, a repair tech had replaced this first divider with a 2k. All original R15 dividers are open on my set.)

Next, we need to drop the 220 volts down to 95 volts to supply the screens of both 58 I.F. tubes. Using trial and error, with 220 now as the starting voltage and the 20k as the upstream resistor, a value of 15k for the third resistor drops the voltage at tap 3 almost perfectly to 95 volts.

Now, we need to drop the 95 volts down to 40 volts to supply the screen of the first detector 58 tube. With 95 as the new starting voltage, and 15k as the upstream resistor, trial and error determines a value of 12k for resistor 4 produces a voltage of 40 at tap number 4 within a couple of volts.

Finally, we need to step the 40 volts down to as close to 0 as possible to ground the end of the circuit. Using 40 as the starting voltage, 12k as the upstream resistor, and "1 ohm" as a mock grounding value (the calculator won't let me use 0), you get a result of well under 1 volt, or for all intents and purposes, 0 volts.

So, according to this calculator, the values of R15 in the schematic should look like this:

- 12k - 15k - 20k - 1.8k - (current flow from right to left)

voltages at each tap point (represented by the dashes)

0v, 40v, 95v, 220v, 240v

I am still a ways off in the restoration of my set (my second model 360 restoration) but if you don't get a followup from me after several months, assume that these voltages worked out perfectly. I am planning to use 2 watt resistors for all the values just to be safe.

 

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Well, my Model 360W has been rebuilt and restored, and as I suppose can be expected, real life is a little bit different than theory. Using actual voltage readings with my digital volt meter, here are the values that produced the results closest to schematic specifications:

Ground - 6.8k - 8.2k - 15k - 1.8k - speaker field/ output transformer.

This configuration got the power down to 42 volts at the final tap before ground (between the 6.8k and 8.2k resistors). Spec is 40 volts at that point. Can't do much better than that.

It should be noted that the power from the speaker field was about 20 volts too high on my set (but still acceptable). The voltage coming out of the recifier was about 358 volts (360 is spec), so it must be a factor of the speaker field, which is a replacement and not original. If your set has an original speaker field, and your voltage before the 1.8k resistor is right around 240 volts, perhaps you might have to reduce either the 1.8k to 1k or the 15k to 12k for the best results. 

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