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Old 10th October 2010, 10:53 PM   #1
richiem is offline richiem  United States
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Default Re-greening the Heathkit IG-18

I've just finished updating Reg Williamson's greening of the IG-18. With just a handful of parts, I'm happy to say that performance is significantly better than stock and along the way I found that some changes for the better are slightly counter-intuitive and different from Reg's experience.

While a major replacement of the guts can yield an outstanding oscillator (see IG-18 #2 -- simple changes can give very good performance.

I've got 0.007% THD at 1kHz with feedback set for a 5V output, and about twice that for 10V out, with good stability at all frequencies. It's here for those interested:
IG-18 #1
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Old 11th October 2010, 04:01 PM   #2
JimT is online now JimT  Canada
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Thanks Dick,

Re-greening has now become one of my projects for this winter.

PS: If you get time, a final schematic would be nice.

Thanks
Jim
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Old 11th October 2010, 04:31 PM   #3
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Would you like to take a crack at Heathkit's lesser-known audio generator, the IG-1272? Uses a JFET along with three incandescent lamps, and has a bipolar supply, but seems generally similar to the IG-18. My measurements suggested distortion is down around .02%.

(Waves atcha from the east side of Puget Sound)

Last edited by Damon Hill; 11th October 2010 at 04:50 PM. Reason: corrected faulty memory after reading schematic
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Old 11th October 2010, 06:05 PM   #4
richiem is offline richiem  United States
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@ Damon Hill -- I've never seen one. Do you have a pdf of the circuit? 0.02% seems a bit high for an elaborate AGC circuit. I'd love to see the circuit if you have it or can scan it.

I'm about to post some general observations about RC oscillators to my site which rounds up stuff I've discovered trolling around for info. Stay tuned.
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Old 16th October 2010, 03:21 AM   #5
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At one time there was a schematic online, at Bama I think; some company bought the rights to the manuals and it was taken offline, as I understand.

So, I'll have to see if my old scanner is still working and get something posted this weekend.

It's quite different from the IG-18; all pushbutton and no square wave output, but there is a sync output on the back.
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Old 16th October 2010, 05:56 AM   #6
richiem is offline richiem  United States
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Thx, Damon -- if you can do it, that would be great. I've seen pix of it, but know nothing about it.
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Old 20th October 2010, 03:53 AM   #7
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This thread reminded me of one of those back-burning projects I have to try: AN43

See figure 47 for a 0.0003% wien-bridge circuit from Jim Williams.
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Old 20th October 2010, 08:10 AM   #8
richiem is offline richiem  United States
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@davygrvy -- I have one mostly breadboarded and space-wired -- all that's missing is the common-mode suppression amp. Once I get the HP334 greening done and written up, I'll go back to play with that.
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Old 20th October 2010, 08:13 AM   #9
richiem is offline richiem  United States
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@JimT -- final circuit diagram is down the list of to-dos. In the meantime, my web page has the few parts changes that are needed and is specific enough to get you going:

IG-18 #1
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Old 20th October 2010, 11:50 AM   #10
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Quote:
Originally Posted by davygrvy View Post
This thread reminded me of one of those back-burning projects I have to try: AN43

See figure 47 for a 0.0003% wien-bridge circuit from Jim Williams.
There are also some circuits replacing the integrator with Linear's true RMS converters --

Krohn Hite uses a a FET AGC in their 4400 -- it's the only oscillator which comes close to the Audio Precision. They run from about $300 to $700.

Click the image to open in full size.
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