Heathkit TC-1 Tube Checker – Theory of Operation – Filament Check

The TC-1 uses a neon bulb to sense relatively higher current flow in the plate circuit of the tube under test.  It uses this function in order to detect short circuits between the various elements of the tube.

It uses this same circuitry to test whether a filament has continuity or is open.  To set up the test, the tube tester is set to connect the plate and grids of the tube under test together and connect them to a 100vac source.  The cathode and one side of the active filament are connected to the low side of the source transformer (o volt of the filament winding).  The other side of the filament under test is connected to the appropriate filament voltage winding (i.e. 6.3 volts).  This is the “normal” starting settings for testing the tube. The neon lamp does not light, because the current through the vacuum of the tube is not high enough to cause it to light.

When one is ready to test the filament, the filament voltage switch is set to its lowest setting (0.75v), in order to provide a low impedance return path for the test.  Then the normally low side of the filament is switched in parallel with the plate circuit. This connects the 100 volt winding to the filament (through a very high value resistor – 270K ohm across the neon bulb, which limits the current to prevent burning out the filament).  The low impedance of the filament causes a much higher current to flow through the resistor, creating a much higher voltage across the resistor.  This higher voltage is high enough to cause the neon bulb to light, indicating that there is current flow through the filament, so it is  presumably good.

In the diagram below, you can see that the normal path for this circuit is in the faded red color.  When the filament is added to the circuit, the bold red path indicates the higher current flow portion.

TC-1 Filament Check Circuit

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Heathkit TC-1 Tube Checker – Theory of Operation – Line Set Mode

Theory of Operation: Line Set Mode

In Line Set Mode, the TC-1 is set up to measure the 115 volt winding of the transformer, and set it to 115 volts, regardless of a variation in the line input voltage.  This allows the checker to give valid go/no-go checks of tube emissions even if the line voltage is above or below the nominal 115 volt line voltage.

TC-1 Line Set Circuit

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Heathkit TC-1 Tube Tester Restoration Part 10

Part 10: Operational Testing

With the TC-1 Tube Checker electrical restoration complete, it was time to run through a few tubes to see how it works…

Step 1: Set all controls for startup.

  • Set Line – Center Range
  • Type – 1
  • Filament – 0.75
  • Plate 0

Initial Settings

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Heathkit TC-1 Tube Tester Restoration Part 9

Part 9: Cabinet Restoration

The cabinet was a little beat up, but not in too bad of condition mechanically.

Cabinet start condition

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Heathkit TC-1 Tube Tester Restoration Part 8

Part 8: Initial Testing

The moment of truth has arrived.  Will it thrive, or go up in smoke??

I plugged the newly reassembled TC-1 into my variac to bring up the voltage slowly.  Before powering on the unit, I put the controls in the following positions:

  • Set Line: Full CCW
  • Filament Voltage: 0.75v
  • Lever switches: All in center position
  • Type: 1
  • Plate: 0 %

Then, I turned on the tube checker.  As I raised the voltage, there were no signs of obvious electrical problems, and the Line Voltage indicator slowly crept up.

I found that I could center the Line Level on the meter at any voltage between 107 and 130 vac line voltage.

I checked various filament voltages.  With the Line Level centered in the meter, I found that they were all around 10% higher that what was marked.  This is not too far off from the state it had arrived in, where filament voltages were high at that time, too.  But I figure that if some higher power tubes are tested, they may cause the voltage to drop a bit, so I can live with that.

Line Level Test OK

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