Heathkit IM-12 / IM-58 Distortion Analyzer Repair and restoration - Page 1
Forum section: electronics / testgear

author: jcrubin
author_rank: Frequent Contributor
timestamp: February 26, 2018, 12:58:07 pm
content: New project kickoff!!! This one is the repair and restoration of Heathkit's Harmonic Distortion Meter Model IM-12 (or IM58) going through the usual idiosyncrasies of Heathkit as I move along on this adventure to see if such a device can stand up to the likes of an HP 331A in the end. Enjoy!!Part 1 of 3

author: jcrubin
author_rank: Frequent Contributor
timestamp: March 05, 2018, 01:45:24 pm
content: 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.....

author: jcrubin
author_rank: Frequent Contributor
timestamp: March 19, 2018, 12:19:31 pm
content: In the final video, 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 one of the forums as a resource 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.

author: jcrubin
author_rank: Frequent Contributor
timestamp: May 28, 2018, 02:13:50 pm
content: I've come across another mint condition all original factory built Heathkit IM-12, and ii've decided to do an express turn-up and test video for what is required to get the device running on all of the original parts as quick and as safe as possible. I say safe as possible in the context that using 70 year old capacitors on a device with no fuse might not be very safe, this is a method if keeping all original parts including the reforming of the original caps on the IT-11 capacitor checker, Something I almost never do.As luck would have it the original caps pass muster, and all tubes are checked, they pass as well. The device is turned up on the Variac on an isolation transformer and full power shows the wattage within spec.A full calibration is conducted on this usit as per the original book, except for the use of RMS voltage instead of peak to peak, a known typo.I felt like a good way to end this video was to compare this unit to my other IM-12 with a split signal from the Feeltech FY3200S and see how close both of them arrived.