Sunday, July 23, 2017

Failed part discovered in C-3 Condenser Checker, improvements made


The main power is set by a voltage divider, with ground at the center node. There is a 47K resistor up to the side which feeds B+ to the 1629 magic eye tube. On the other side, there is a series chain of resistors which total 98K. This gives roughly 1/3 of the voltage to the B+ and 2/3 to the resistor chain that are on the selector switch which determines what negative voltage is presented across the capacitor under test.

The first 22K resistor in the series chain was wide open, which meant that the minus side of the divider was disconnected. This explains the values I measured and is consistent with the design flaw where the divider string uses mainly 22K resistors to drop about 100V per step. This is because the chain of resistors has about 4.5ma running through it.

That puts 1/2W on the resistor, its rated value, but if a capacitor is tested with substantial leakage current then the power on the resistor will temporarily be well above its rated power. Over time, the abused resistors change value or open up like mine did. The replacement components I bought are rated at 2W which gives a lot more margin.

The prior owner of this tester had substituted a different magic eye tube for the 1629 I used. It was a 6V type while the original has 12V filaments. He or she put in some large low value resistors to divide the filament voltage in half. I bought the proper magic eye tube therefore I removed this hack.

The circuit should be back to original design when I finish the restoration, other than any changes I need to introduce to deal with excess HV from the transformer. It should produce no more than 500VAC which when rectified and filtered puts about 450V on the low side filter capacitor, within its margin. Thus, I don't strictly need to introduce by pair of series capacitors to provide a higher voltage handling capability.

Instead, I added resistors in series from the cathode of the rectifier so that they drop the excess voltage. I measured the transformer output at 600V, about 1.2x the intended voltage. The appropriate resistance appears to be 29K which will dissipate about 0.6W. I chose to build this as a series combination of 1/2W resistors, values of 11K and 18K, to lower the power on the two resistors since they now split the total voltage drop.

I prepped the unit for the new parts, removing the old series resistor chain and 47K resistor that all had drifted out of spec or failed open. I also reversed the wiring changes introduced by the prior owner for the substitute magic eye tube, since I will have the proper one to insert.

The two filter capacitors I have are 500V rated units, adequate to the voltages I will see now that I am dropping some of the voltage from the transformer across the new 29K of resistance. These are compact modern electrolytics, which I relocated so that I made use of the ground tabs around the rectifier tube socket. The capacitors sit below the tube socket, as does the 11K + 18K pair of resistors.

I made use of a spare pin on the tube socket that is not connected inside the tube, spanning the 29K of resistance from the cathode pin 1 to the spare pin. The filter capacitor and wiring for the B+ line to the magic eye were moved to the spare pin to effect the voltage drop.

When I get my replacement components, I have to install the series resistance chain of 22K and 11K resistors onto the selector switch. I also need to put on the new magic eye tube grid and bias resistors and wire up that tube socket properly for a 1629. The last step will be to put the 47K resistor that completes the voltage divider, completing the divider that apportions my 650VDC as 210V for B+ and -440V for driving the capacitor under test. When I pop in the new tubes it will be ready to go.

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