Monday, April 17, 2017

Tube curve tracer board working and calibrating


I resolved the screen voltage problem - turned out to be two hard to spot solder joint issues. The solder welled up on the component lead in a beautiful shiny cone, but had not flowed onto the pad. Looked great until I went to extreme magnification with the stereo microscope, where I could see only 1 or 2 joints at a time. 

Repaired and the hangup of the main logic is gone, as I expected. It goes through a measurement interval, applying anode and screen voltage to the tube under test (but I had no tube connected) while maintaining the voltage reservoirs at their target level.

In addition to the screen circuit problems, now fixed, the grid bias circuit is not working. It should develop a bias from 0 to -50V during the measurement phase of the curve tracer. The microcontroller produces pulse width modulation on a pin which drives a low noise operational amplifier acting as an low pass filter (or integrator). This yields a smoothed voltage between 0 and 5V, which is fed to a second op amp, LM741, wired as a 10X inverter, to produce the intended 0 to -50V.

I zoomed in to this section of the board and looked quite carefully. There was one solder joint that looked as if it would benefit from reflowing the solder to improve pad bonding, but it is for the unconnected pin on the LM741 op amp package thus it doesn't matter. Time to debug the circuit, measuring and observing, until I figure out what is wrong.

The observation steps to make are:

  1. scope on the microcontroller signal that drives this, which is a 19.5KHz signal whose on time is varied (pulse width modulation) to swing the first op amp output between 0 and 5 volts.
  2. if the pulses are good, look at the first op amp to verify it is filtering/integrating and generating the 0 to 5 volt range expected
  3. if the first op amp is working properly, observe the behavior of the second op amp which should be generating 0 to -50 based on the 0 to 5 of the first device.
  4. if the second op amp is working properly, observe the current mirror transistors that produce the actual grid voltage when driven by the second op amp.
  5. If the mirror transistors work, debug the continuity problem from here to the grid connection.
The microcontroller pulses were behaving exactly as they should, varying the pulse width as the target value changed from 0 to 5. We passed test one above. The second test involved watching the output of the NE5535 low noise op amp to see if it did produce the 0 to 5 level. 

The second test failed! The op amp circuit is not integrating nor acting as a low pass filter to produce the target value. I saw a constant output on pin 6, the output pin. There are only a few components in this portion of the circuit, two capacitors and a couple of resistors. If they are good and the connections and solder joints are correct, it points the finger at the chip.

The sequence of tests to perform for this are:

  1. verify that V+ and V- are making it to the chip
  2. verify the proper resistance for the two resistors and two capacitors on the input side
  3. verify that the input pulses are making it to the non-inverting input
  4. monitor the waveforms on the inverting inputs
  5. check for shorts of the output pin
Test 1 failed - no V+ to the IC, nor am I getting +15V to the other three ICs it feeds. Digging further, I found that my 7815 voltage regulator was bad. I had a replacement in stock, put it into the board, and now everything is working properly! Filament, anode, screen, grid and all the measurement logic doing fine.

Now I have a bit of calibration and then have to build it up with tube sockets, switches and all to make a full tester. Calibration is done easily, with a GUI screen to adjust values to match measured results with the DMM. I worked through this tonight as I collect tube sockets and order any remaining ones I will need to install in my project box/chassis. Every is calibrated and ready to install in a box and begin using.

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