Build an IM Distortion Analyzer.

Chapter 7   In Case of Trouble.

 7.1   Review of Operating Instructions.
7.2   Shooting Trouble.
7.2.1   General.
7.2.2   The Power Supply.
7.2.3   The Signal Source.
7.2.4   The Measuring Circuit.
7.2.5   The Input Attenuator.

Chapter 7 In Case of Trouble.

Back to Fun With Tubes.
Back to Fun With Transistors.
Back to Table of Contents.
Back to Top.

7.1 Review of Operating Instructions.

In order to tell when you have found the problem and succeeded in fixing it you must know how to operate the instrument. As with any new piece of test gear operating it is not likely to be second nature to you as yet. The operating instructions are given here for easy reference. In these instructions the term "HDA" (harmonic distortion analyzer) will also stand for the AC voltmeter if that is what you are using for the readout device on your IM analyzer.

  1. Connect the DUT (device under test) between the "TEST SIGNAL" and "TEST INPUT" jacks.
  2. Set the range switch on your HDA to the 100% range.
  3. Set the "SIGNAL SELECT" to "LF".
  4. Set the "METER SELECT" to "MON SIG".
  5. Adjust the "SIG LEVEL" for a meter reading of 0.3 volts.
  6. Change the "SIGNAL SELECT" to "HF".
  7. Adjust the "HF LEVEL" for a meter reading of 0.075 volts.
  8. Change the range switch on the HDA to a range that will read the voltage output of the DUT.
  9. Change the "METER SELECT" to "INPUT".
  10. Adjust the "SIG LEVEL" to obtain the desired output from the DUT.
  11. For example if you are testing a power amplifier terminated in an 8 ohm load and
    you desire a power output of 8 watts set the voltage to 8 volts. P = V2 / R = 82 / 8 = 8.
  12. Change the "METER SELECT" to "CAL".
  13. Change the range switch on the HDA back to 0.3 volts (100%).
  14. Set the "INPUT ATTENUATOR" to "X1" or "X10".
  15. Adjust the "VERNIER" for a meter reading of 100%.
  16. Change the "METER SELECT" to "READ".
  17. Change the range switch on the HDA until you get a reading in the upper 2/3 of the scale but not off scale.
  18. Take the reading.

Back to Fun With Tubes.
Back to Fun With Transistors.
Back to Table of Contents.
Back to Top.

7.2 Shooting Trouble.

Back to Fun With Tubes.
Back to Fun With Transistors.
Back to Table of Contents.
Back to Top.

7.2.1 General.

If the power supply checks above are correct but all or part of the instrument seems not to be working make the following checks. Trace the red and violet wires from TS 1 to each module. Be sure there are no errors and all connections are well soldered.

Check the inter module connections for correctness. Be sure that the connections you were instructed to leave bare are not touching a module box. At this stage all connections and terminals should have been soldered. If not solder them.

Back to Fun With Tubes.
Back to Fun With Transistors.
Back to Table of Contents.
Back to Top.

7.2.2 The Power Supply.

Measure the DC voltage between TS 1 lug 5 and chassis. It should be in the neighborhood of 19 to 20 volts.

Measure the DC voltage between TS 1 lug 1 and chassis. It should be in the neighborhood of -19 to -20 volts.

I got 19.6 on both points but your mileage may vary.

Possible Troubles and Cures.

Troubleshooting the Power Supply.
Symptom Likely cause
One voltage is approximately twice the
expected value, other is zero.		 TURN OFF THE POWER AS SOON AS YOU CAN! One of the capacitors is operating over
its rated voltage and will soon explode. I do mean explode with a bang!
There is a major wiring error on TS 1. Examine it carefully and compare it to
the schematic and photograph.
You used a terminal strip that has the ground on a lug other than 3.
Voltages are low and transformer is humming
and is getting warm to the touch.		 One or more diodes installed backward or shorted. Capacitors installed backward.

Back to Fun With Tubes.
Back to Fun With Transistors.
Back to Table of Contents.
Back to Top.

7.2.3 The Signal Source.

  1. Connect one end of a BNC (M) to BNC (M) cable to the "TEST SIGNAL" connector on the analyzer.
  2. Connect the other end to the input of your oscilloscope.
  3. Set your scope to 1 Volt per division and the sweep to 2 ms per division.
  4. Set the "HF LEVEL" control to approximately the 12 o'clock position.
  5. plug the analyzer in and turn it on, rotate the "SIG LEVEL" fully counter clockwise.
  6. Set the "SIGNAL SELECT" to "LF". You should see a 60 Hz sinewave on your scope that has an amplitude of approximately 4 volts P-P.
  7. Set the "SIGNAL SELECT" to "HF". Change the sweep to see a much higher frequency. You should see a sinewave that has a frequency of 7,000 Hz and an amplitude of approximately 1 volt P-P
  8. Set the "SIGNAL SELECT" to "BOTH" and your scope's sweep back to 2 ms / div. You should see the original 60 Hz sinewave but the trace will appear to be 1 volt thick.

Troubleshooting the Signal Source.
Symptom Likely cause
No output shown on scope in initial test Wiring error on S 2, S3, between modules
M 1 and M 3, wiring error between M 2 and M3.
Wiring error or bad solder joint on P 2 or P 3.
No low frequency output. Check green wire from TS 1 to M 1.
No high frequency output. Wiring error between M 2, P 2, and M 3.
Both signals on all the time. Terminal A of switch S 3 not grounded.

Back to Fun With Tubes.
Back to Fun With Transistors.
Back to Table of Contents.
Back to Top.

7.2.4 The Measuring Circuit.

The best way of testing the measuring circuit is to attempt to calibrate it. Since this is part of testing the instrument an accurate calibration is not necessary. Therefore the least accurate method will be described here. For the most accurate calibration you can achieve see "Final Calibration in chapter 6.

If at any time you do not get the described results jump to the troubleshooting chart below.

  • Equipment Required.

  • IM analyzer.
  • The AC voltmeter you intend to use with it. I will assume an HDA (harmonic distortion analyzer).
  • DC voltmeter.

Calibration Procedure.

  1. Connect the "METER OUTPUT" of the IM analyzer to the input of your HD analyzer.
  2. Use a short BNC to BNC cable to connect "TEST SIGNAL" to "TEST INPUT".
  3. Set the range switch on your HDA (Harmonic Distortion Analyzer) to the 0.3 volt / 100% range and the "FUNCTION" to "VOLTMETER".
  4. Make sure both instruments are turned on.
  5. Set the "METER SELECT" on the IMA (Inter Modulation Analyzer) to "MON SIG".
  6. Set the "SIGNAL SELECT" on the IMA to "LF".
  7. Advance the "SIG LEVEL" clockwise while watching the meter in your HDA.
  8. You should be able to run the meter up to and beyond full scale.
  9. Adjust the "SIG LEVEL" for a meter reading of 3 on the 0 to 3 scale. This represents 0.3 volts.
  10. Change the "SIGNAL SELECT" to "HF".
  11. Adjust the "HF LEVEL" for a reading of 0.075 volts.
  12. Set the "SIGNAL SELECT" to "BOTH".
  13. Change the "METER SELECT" on the IMA to "INPUT". The meter should read exactly the same as when in the "MON SIG" position.
  14. Change the "METER SELECT" to "CAL".
  15. Set the "INPUT ATTENUATOR" to "HI Z" and the Vernier fully clockwise.
  16. Adjust the trimming potentiometer in M 4 to obtain a reading on the meter of 1 on the 0 to 1 scale. This represents 100% IMD.
  17. Change the "METER SELECT" to "READ". Your DC meter may begin to indicate.
  18. Adjust the trimming potentiometer in M 5 for a reading of 0.447 volts.

Troubleshooting the Measuring Circuit.
Symptom Likely cause
Failure to get meter deflection in step 8 above. Wiring error or bad solder joint on switch S 2,
or bad solder joint on "TEST SIGNAL" connector.
Failure to get deflection in step 11. wiring error on S 2 or between M 2, P 3, and M3.
Failure to get identical reading in step 13. Wiring error on S2 or bad solder joint on "TEST INPUT" connector.
Failure to get deflection in step 16. Wiring error or bad solder joint on S 2, S 1, or P 1.
No DC reading in step 18. Wiring error or bad solder joint on S 2.

Back to Fun With Tubes.
Back to Fun With Transistors.
Back to Table of Contents.
Back to Top.

7.2.5 The Input Attenuator.

Problems with the input attenuator might not be noticed especially by an inexperienced operator. The following checks will reveal any subtle problems in this area.

  1. Connect the "METER OUTPUT" of the IMA to the input of your HDA.
  2. Connect the "TEST SIG" output to the "TEST INPUT" input on the IMA.
  3. Set your HDA to the 0.3 volt range and voltmeter mode.
  4. Set the "INPUT ATTENUATOR" to the "HI Z" position and the "VERNIER" to half rotation, 12 o'clock position.
  5. Advance the "SIG LEVEL" until the meter reads 0.3.
  6. Change the range switch on the HDA to 1 volt.
  7. Turn the Vernier on the IMA to the fully clockwise position. The meter should read approximately 0.6 volts.
  8. Turn the Vernier back to the 12 o'clock position and the range switch back to the 0.3 volt position.
  9. Change the "INPUT ATTENUATOR" to "X1". The meter should fall to a reading of 0.084 volts. Change the range switch on the HDA to get the most accurate reading.
  10. Change the "INPUT ATTENUATOR" to "X10". Change the range switch on the HDA. The reading should now be 0.0084 volts.

Depending on the tolerance of the potentiometer and the accuracy of the center of resistance your measurements may vary by as much as +/- 20%.

Troubleshooting the Input Attenuator.
Symptom Likely cause
Voltage does not change in step 6 or goes to zero. Wiring error on S 1 or P 1.
Voltage does not change in step 7 or goes to zero. Wiring error on S1 or P 1.