IT-12
Harmonic Distortion Meter
Heathkit Repair - IM-12 IM-58 Part 1/3
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.
Heathkit Repair - IM-12 IM-58 Part 2/3
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.....
Heathkit Repair - IM-12 IM-58 Part 3/3
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.
DYNA ST-70 Harmonic Distortion Testing using all 1960's Heathkit Tube Technology
In this video I will demonstrate harmonic distortion testing of my Dynaco ST 70 amplifier using all Heathkit tube gear from the 50's and 60's. This demonstration will show the THD produced from this amplifier at 2KHZ using three different power inputs.
The equipment used includes:
Heathkit IG-72 Audio Generator
Dynakit ST70 Power Amplifier
Heathkit IM-12 Harmonic Distortion Meter
Heathkit O-12 Oscilloscope
The entire process is shown in the video over several iterations, some have been shortened
Heathkit Repair - IM-12 IM-58 Harmonic Distortion Meter - Turn-up and test with and demo
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.
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