The IG/SG-18 was a very popular product for the Heath company. The IG-18 was the kit version and the SG-18 was the assembled version which was targeted primarily to schools and colleges. Both models were identical, differing only in the paint job.
They, it from now on, is a lamp stabilized bridged-T oscillator with a schmitt trigger to provide a square wave output. A lamp stabilized oscillator has the potential to have very low distortion but this one, for a number of reasons, did not live up to those expectations. It has been about 10 years since I performed the modifications described here so I don't remember exactly what the distortion was but I think it was about 0.5%. After a series of modifications I was able to get it down to 0.015%.
Of course if you don't have a harmonic distortion analyzer you really have no need for an audio oscillator with a distortion of 0.015%. Many older vacuum tube analyzers such as those made by HP are now turning up on the benches of hobbyist and experimenters. The lowest range on these units is 1% so an oscillator with distortion of 0.1% would be considered adequate. The steps to reduce the distortion in the IG/SG-18 are given from the simplest to the hardest so when the distortion is low enough for use with your analyzer you can stop.
At the time I modified my IG-18 I didn't do it in the logical order presented here. Actually, turning down the voltmeter calibration pot was the last thing I did. That took away the last few hundredths of a percent. Some of the mods may not even be necessary. I honestly don't know
There are a number of causes for the high distortion in this unit and they are as follows.
To clean up the signal the following things must be done.
Fear not, I will elaborate further on each item. The order has been very carefully chosen so you can do the simplest things first. Now we need to see the schematic diagram.
For a verbal description click here.
The day that the Heath company closed its doors
was a sad day indeed for electronics home brewers.
Before it existed there was nothing in the world like it,
It was never equaled by its wood-be competition,
And there will never again be anything like it.
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This page last updated April 15, 2006.