Heathkit IG-18 mods - issues!... - diyAudio
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Old 29th July 2014, 02:05 PM   #1
Rax is offline Rax  United States
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Join Date: Jan 2010
Default Heathkit IG-18 mods - issues!...

Hi all,
I know this is a topic discussed over and over here and in many other places, but I seem to experience some specific issues which I am not sure how to handle.

Here's what I've done to the IG-18 (based on all of the literature I could find out there):
  • PS upgraded to using LM317T
  • PS caps replaced with Nichi/PW 1000uF (all other PS caps upgraded also)
  • meter buffer based on 2N3904 per Dick Moore
  • R12 replaced with cascode JFET CCS (PN4393/2N5486) adjusted for 3.5mA
  • placed a switch on/off for the square wave in the back of the chassis
  • R3 = 100k
  • C5 = 100uF
  • 220uF cap over the meter directly on its legs
  • tried cascode JFET CCS (J202/2N5486) for R4 (adjusted for about 1.1mA), but experienced instability (the sine is pulsating); I assume some caps strategically placed would cure this, but I'm not sure how to do it
Here are some issues I've been running into (and associated thoughts):
  • I just acquired my first distortion analyzer - an HP 334a - but I don't seem to be under 0.6% distortion with this after the mods... (to be noted, before any work on this the sine was pulsating like nuts which I assumed was due to issues with the PS. A check of the PS caps showed they were in surprising good shape, though). This is magnitudes over where I thought I am, obviously. The HP is either mis-calibrated (though I'm learning this is unlikely), and though I measured a signal generator app I have on my iPhone and got 0.3% distortion which sounds pretty reasonable;
  • I could see the waveform cleaning up on my scope as I was doing the mods steps... (though the square wave has never been pretty). But there may be some recent degradation of the lower leg of the sine which may be giving these high distortion results... I will try to get pictures of the scope depicting this and post here;
  • When I started the distortion measurements I inadvertently shorted the output of the generator (repeatedly, as "the sleep of reason produces monsters"). The output went back every time to the same AC voltage, but I am thinking that maybe I pushed the output transistor pair over their jagged edge... If yes, I was planning to get a pair of MPSA05/55 in there as replacements.
  • Switching off the square wave stage degrades the sine wave noticeably (!). This is extremely strange as all sources seem to indicate this should dramatically clean it up. I suspect a thorough readjustment is needed to get the improvements in line but I am pretty thrown off by this. What gives?
  • Could anyone describe in plain language how the grounding mods are supposed to be performed? I couldn't understand this from any of the articles I read. I guess a quick description of how to check whether the frequency adjustment vernier zeros out would also be nice...
Thanks!
Radu.
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Old 29th July 2014, 04:03 PM   #2
Rax is offline Rax  United States
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small addition - my lamp is dimly lit... From other reads I understand this is not by design. So maybe I have some stealth auto-oscillation, not enough to show on the scope during most other conditions?...
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Old Yesterday, 10:14 PM   #3
richiem is offline richiem  United States
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Location: Grapeview, WA
Lamp should not be lit -- if it is, the feedback pot is misadjusted. You don't say how you are turning the square-wave circuit off. If you are breaking the supply line, then some other part of the circxuitry could be cut off as well -- just check the routing. The square wave section ground trace should be cut away from the sine-wave section, with a separate ground line going to chassis ground. The sine wave ground board trace(s) should go to the chassis ground of the 0.047uF cap which is also the ground for the sine-wave output jack. Hope this help.

The "motorboating" of the sine output indicates misadjustment of the feedback pot too, plus possible misadjustment of the bias pot. When amplitude stability is poor and the lamp is dimly lit, there is not sufficient resistance change in the lamp to control the output. The capacitors in the bridged-T filter circuit should be checked with a capacitance meter -- absolute accuracy is not as important as matching, so the cap meter doesn't have to be expensive. I got a micro-based one from China for under $30 that is also a transistor/diode/fet tester -- very handy.
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