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Old 16th June 2010, 07:52 PM   #1
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Default Heath IG-18 IG-5218

Decided that in fairness to the THD, s/n thread the IG-18 should be discussed on its own.

I modified this IG-5218 a long time ago -- looks like it's in need of modding again --

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Old 16th June 2010, 08:01 PM   #2
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Have you looked at the classic "Greening of the IG-18" article by Ed Dell (1971)...I've been picking away at the items but have no distortion analyzer to judge the outcome.
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Old 17th June 2010, 10:25 PM   #3
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Disconnecting the Schmitt Trigger:

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Old 17th June 2010, 11:29 PM   #4
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Just for fun, I once dropped in a TO-5 version LM317 for the power supply's regulator transistor, with appropriate changes and substitutions of parts on the little PCB; works great.

The meter circuit significantly loads the output and benefits from a buffer amp; a precision meter rectifier/buffer from an opamp that mounts right on the meter terminals is one of those projects I never got around to designing...
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Old 21st June 2010, 01:38 PM   #5
TheGimp is offline TheGimp  United States
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I have an IG-18 (1969?) that I just finished doing most of the mods per the Williamson article. I have not done the driver transistor pair yet as I am waiting on parts. I have a pair of 2SC2235/2SA965 on order (ebay) to try.

I also have an HP333A that shows almost identical distortion numbers, so I believe the plots I'm getting with AudioTester V3 thorugh my M-Audio M-192.

I get less distortion (0.008%thd) and higher harmonic/interference with the IG-18 than I do if I use the computer sound card output (M-192). This is at 1Vrms.

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A LM317 may be next after the driver transistors.
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Old 1st July 2010, 11:39 PM   #6
JimT is offline JimT  Canada
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TheGimp,

Please list your substitutes for the Greening of the IG-18. I am planning on working on my IG-5218 and could use a leg up on the substitutes.

Thanks
Jim
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Old 2nd July 2010, 01:10 PM   #7
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The Kruger modification in The Audio Amateur 3/84 is probably the best -- the author ditches the transistor oscillator and substitutes an NE5534. The light bulb AVC is replaced with a photo-dependent resistor AND amplitude compensation is added via a new wafer on the range switch. He also provides for switching the square wave circuitry on or off. As mentioned before, the square wave circuitry adds a lot of noise.
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Old 2nd July 2010, 04:33 PM   #8
JimT is offline JimT  Canada
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Thanks Jack

I have ordered the 1984 back issues. It should be interesting to compare the three methods of improving the IG-18/5218.

Jim
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Old 2nd July 2010, 05:07 PM   #9
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I did a bunch of mods based on several articles, but I was never able to go below 0.01% ( HP 334A). I mounted the power transformer outside, used +/- 20 volt 317/337 regulators, used a LMEXXXXX 22 volt op amps, for the oscillator and a LME XXX for output, bypassed the output cap, output offset is near 0 volts, used switches to enable square wave and meter, meter is now buffered and with 1000uf across the meter it is basically flat from 1 hz to 110khz slow but flat, used a led for power lamp.
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Old 25th August 2010, 01:58 PM   #10
jez is offline jez  United Kingdom
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I made a clone of the ig18 from the audio amateur article. it has all the mods from the article plus some of my own and has given excellent service for many years now.
Distortion is very low. my home made distortion meter has a minimum reading (noise floor) of 0.0016% with the input shorted. It shows the same figure from the "IG18" so it must be considerably better than this figure!
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