Good Gravy.........It Works!

 I got back on the Heathkit IM-5258 which was on the back burner for over a week while waiting for parts. The parts arrived while I was doing the Timebase Project, so they sat for a while.

After replacing "The Lone Photocell" and adjusting it, I was staggered to see it within the tolerance range, and almost at the target value when I turned the analyzer back on. I spent about 15 minutes adjusting it, then put a dab of sealer on the wire leads so they wouldn't bounce around, and put the cap on it. I did the same with the four photocells in the other housing, and it took a couple of hours. If you bumped one while adjusting it's neighbor, you had to go back and at least check that it didn't get moved off of it's target value. Tedious, but required.

Final tally is all of them are within the allowable range, and they're all within 5% of the target value. Getting them any closer would be an exercise in futility, so I'm going with what they are. Once the photocells were adjusted properly, the rest of the alignment/calibration went by the book.

Here it is measuring the residual distortion of my little Heathkit SG-1272 audio generator:

 It's reading about .04%, within the published specs for the generator, but others have reported this generator as being better than that. Suspecting a Ground Loop, I disconnected the oscilloscope from the "Output" jack, which lets me measure any phase shift between input and output, which I don't care about.

And here it is with the 'scope disconnected:

 It's reading about .015%, a definite improvement.

And yes, with NO input, and the input jack terminated, it reads "0.0%", meaning the needle doesn't move off zero.

Are these readings believable? Well, it's on it's most sensitive range, and the meter is barely off the zero. The rated accuracy is listed as "5% of Full Scale", so that number of ".015%" is open to some interpretation. It's literally at the limits of this instrument, but it does show *something* is there. It also shows how careful you have to be making these measurements. Everything has to be shielded, you should be running on an isolation transformer, and a little experience and luck don't hurt, either.

SO.......was it worth it? In terms of Educational Value, absolutely. Repairing this piece of test equipment and the companion Intermodulation Analyzer has helped me understand the measurements they make by understanding how the measurements are made. Understanding how a measurement is made leads to insights about what the measurement represents.

In terms of "Bang for the Buck" and usability of this instrument? A big "MEH"....There are far better instruments to use in this day and age to make these measurements with. This one is a 1970's hobbyist-grade device, made with decent quality parts from a proven design, but it sure ain't no HP instrument! Things like the knobs, switches, and controls are consumer-grade and are a bit sloppy.

This thing fought me tooth-and-nail. The chassis was bent, a rack handle was bent, the plastic coupling for the tuning shaft was broken, one of the power supply regulators had failed, The Lone Photocell had gone out-of-tolerance, and it was (and is) a beat-up, filthy mess...

Sometimes I feel like this when I finish one of these:

BUT........ it keeps me out of bars at night.

Looks like I can finally get back to my Pioneer SX-980, which started this trip down the rabbit hole......