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Old 11th Sep 2017, 9:04 pm   #1
Tim
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Default Heathkit IG42 Sig Gen

Hi Gang,
Just bringing one of these back to life.
It's working quite well at the moment, with good output on all ranges. Specs say accuracy is +/- 3% but mine is out by more than that. I have checked it with a known to be accurate frequency counter and it's about 5-6% low on all ranges( In other words to get 470KHz I have to set the frequency knob at a whisker below 500KHz.) In my reckoning +/- 3% of 470KHz is about 14 KHz either way. The frequency knob goes to each end of the scale fully, the same distance at both ends.
I'm exploring oscillators at the moment, and am I right in thinking I need to DECREASE the capacitance across the oscillator coils to increase the frequency slightly? These are variable/tuning capacitors, which presumably don't change in value much.
Would it be worth adding some series capacitance to each tuning capacitor, and then modify the parallel trimmer to get it about right? No values in the parts list or circuit diagrams I'm afraid.

Actually how accurate does a sig gen have to be just for doing radios? I'd just like it to be REASONABLY accurate.
I only asked as I am also doing a Bush TR106 that is WAY out on LW(Radio Four appears at about 1300m) so will probably need aligning and it's a faff to have the frequency counter connected up all the time.


I suppose I could take a few turns off the inductors but that might end up upsetting the calibration.

Thanks
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Old 11th Sep 2017, 10:06 pm   #2
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Default Re: Heathkit IG42 Sig Gen

Quote:
could take a few turns off the inductors but that might end up upsetting the calibration.
I mean linearity
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Old 13th Sep 2017, 6:06 pm   #3
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Default Re: Heathkit IG42 Sig Gen

Heathkit signal generator calibration.
Am I just stupid, but why not shift the dial pointer.
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Old 13th Sep 2017, 6:12 pm   #4
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Default Re: Heathkit IG42 Sig Gen

Hi Bill.
I had considered that, as the error seems to be more or less the same on all ranges; but I'll have to confirm this. The pointer doesn't want to move relative to the shaft at the moment.
It's a vernier thing so I'll have to take the knob off and have a look. I can't work out how to shift the pointer at present though as the generator is elsewhere.

I might have a fiddle with the oscillator circuitry anyway, for curiosity sake.
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