Heathkit Harmonic Distortion Meter Restoration IM-12/IM-58

Discussion in 'Videos and Podcasts' started by KJ4TLB, Mar 5, 2018.

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  1. KJ4TLB

    KJ4TLB Ham Member QRZ Page

    Part 1

    This video starts the project for the repair and restoration if my Heathkit IM12 harmonic distortion meter, functionally equivalent to the IM58. It begins with the usual introduction, inspection of the unit, which is in near mint condition.

    The first order of business is the repair of the bent connectors on the input which is a simple process, Followed by the removal of electrolytic capacitors from the unit. Old ones are tested none the less on the IT-11 capacitor tester.

    Following this the schematic is explored to understand the power supply, bringing the next task to the 300uf capacitor after the voltage regulator. This is a very clean and stable power supply design for its time. However, tube testing time comes first on the Century tube tester, which also meant Deoxit on the tube pins as well as sockets.

    Since the Deoxit was out, all of the pots switches, and connectors saw Deoxit as well. Observations are made with regard to the open wire would pots as it can clearly be seen where the oxide is on these units. One pot was kind of iffy, it was separately tested with the IM-11 VTVM to see if the sweep was ok.

    Back to the 300uf cap replacement, The old cap was left in place for show, its terminal removed, the new ones installed on the bottom of the unit using a standoff. Several are used as a temporary fix until a permanent cap can be procured.

    Following this is the initial turn-up with voltage measurements with extra attention paid to the output of the voltage regulator. Resulting ripple on the filtered output shows good results as well.

    The diagram doesn't show the voltage at A it had to be figured out with common sense. Following that the complete voltage chart form the book was filled out. It shows that things look good, but improvements can be made.

    Part 2
    The next video continues with measurements of the resistors that require 5% and 1% tolerance. It is found that the range switch is really bad, and new resistors will need to be ordered. Work can continue but the output cant be trusted. After this all of the other resistors are checked as well, and they didn't do too bad.

    The other capacitors in the unit are evaluated and the values are good and the IT-11 checks the leak-down, it turns out the the two .5's are actually reading really good. the 2uf one did not look so hot though.

    In following the manual for initial turn-up, the pots are turned to the locations as specified in the book and the calibration procedures are started. I point out that the voltmeter calibration in the book specifies peak to peak and it should be RMS for the first calibration procedure. The other calibration procedures are conducted thereafter. This also includes setting the fake frequency knob/indicator on the top right that is loosened and tightened into the correct position.

    After calibration comes a practical demonstration with the tektronix 2235 as an output device as we go through the procedure of testing a signal at a given frequency from the Feeltech. In contract to an ideal condition a sawtooth waveform is also used under test.

    The 2uf capacitor was revisited in the meter portion which did raise the voltage in the system slightly. Temporary caps were put in its place though only 1.5uf. Retesting of all voltages on the matrix confirmed a slight voltage climb. This of course completely changed the calibration of the voltmeter as shown.

    And the video stops here because we need the precision resistors to continue in the next video.....

  2. KJ4TLB

    KJ4TLB Ham Member QRZ Page

    Part 3
    With the components having arrived in the mail the swap-out work can begin. Also the temporary components can be removed and replaced. Most importantly the precision resistors will all be replaced at this time. Calibration was re-done using the correct RMS method as mentioned in the previous video with corrections to the calibration technique..

    Further testing as shown is compared against the HP 331A. This starts with comparing meter voltage and then a THD test off of the Feeltech FY3200S.

    Another complication is that the Heathkit was not designed to observe the harmonics found in the modern Feeltech and becomes deaf at the high end. I had to switch the the Heathkit IG-72 audio generator as the signal source. But issues were evident.

    Sad to say, something broke, or I broke something at this point and made reading initially taken somewhat inaccurate as shown. This lead to a marathon troubleshooting session to find the culprit. A nasty troll on the forum cut my efforts short there and I ended up going solo again.

    Through the course of troubleshooting several components were replaced, though not the cause of failure, were on the border of being out of spec, and one was iffy. Sure enough though , a component ultimately failure invalidated the testing done. A bad resistor..... However, it would seem that in comparative testing, the more distortion a signal has, the more deaf the device is as compared to the HP 331A.

    Just as testing continued, the IG-72 dropped dead, so work had to stop to see what was going on with the audio generator. Such is life with 70 year old test equipment.


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