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Old 15th Feb 2019, 12:57 am   #1
Boulevardier
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Default Heathkit IG-18 Sine-Square Audio Generator bought at RWB

I bought this from a stall at the RWB event on 9 December. It’s a nice piece of kit, and just what I needed. But there’s a problem with it – the output voltage is unstable, fluctuating randomly by about 2-5% of the output meter scale (this is confirmed by my ‘scope reading of the output voltage). The fluctuations are random rather than periodic at about 1–2 per second, and clearly visible on the meter and scope. The nice chap who sold it to me had put a label on it saying that he had replaced Q’s 1,2,3, 4 and 10, and cleaned switches and pots. The fluctuations aren’t altered by any setting of the switches/pots.
I haven’t had time to get into it and take any readings yet, but my initial instinct is – power supply circuit – which is a simple affair. The zener diode in that is only described as "VR43" in the manual (attached), so don't know what its voltage would be.
Am I on the right track? I will get on to this as soon as I can find the time, but it’d be useful to know if I’m starting in the most obvious place.

EDIT - sorry, manual file is too big to attach. I will try and copy the schematic alone tomorrow.

Last edited by Boulevardier; 15th Feb 2019 at 1:03 am.
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Old 15th Feb 2019, 7:47 am   #2
MrBungle
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Default Re: Heathkit IG-18 Sine-Square Audio Generator bought at RWB

I would start at the output side of the power supply and check the voltage and see if it is oscillating with your scope. It’s possible there’s a bad capacitor or two in there for sure.

Also it’s worth looking at the calibration steps in the manual as the overall stability of the output depends on the feedback setting. If there’s too much it can drive it into oscillation.
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Old 15th Feb 2019, 11:37 am   #3
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Default Re: Heathkit IG-18 Sine-Square Audio Generator bought at RWB

I rebuilt a non-working IG-18 a couple of years back - that certainly had at least one of the original PS caps leaking. A lot of the carbon comp. resistors had also drifted high, well beyond the labelled tolerances, so probably worth checking a few as well while you're in there.

In the end, I pretty much rebuilt all of it - new Pana FC's (with increased uF in the PS section, C1 & C2 to 1000uF, C3 to 470uF), a lot of new metal film R's, new Q10 in the PS (I used a 2N3035 with heatsink), and lots of contact cleaner.... I indeed followed the manual set-up procedure once it was working, and had no problems, it's been rock-solid ever since. I did also subsequently fit one of the d8apro.com THD & Meter kits, which was very straightforward as well. There's lots of info re. the Heathkit IG-18 & IG-5218 on the net if you need it, they're still nice little signal generators, that I find very convenient for audio work especially.

VR43 is listed as a 43V Zener https://www.qsl.net/w6ovp/heathkitparts.htm
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Old 15th Feb 2019, 1:03 pm   #4
Boulevardier
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Default Re: Heathkit IG-18 Sine-Square Audio Generator bought at RWB

Thanks for those tips Mr B and Goldie. I will do as suggested when I have the time free to follow them - it's always a battle between competing demands on time! Will update on progress when I have some results.
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Old 15th Feb 2019, 2:58 pm   #5
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Default Re: Heathkit IG-18 Sine-Square Audio Generator bought at RWB

Here is a good page on the IG-18:
http://www.angelfire.com/electronic/...IG-18_Mod.html

Regards, Peter
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Old 22nd Feb 2019, 11:26 pm   #6
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Default Re: Heathkit IG-18 Sine-Square Audio Generator bought at RWB

Update and closure - I have, at last, got a round tuit. Having opened it up and tested the dc output of the power supply stage. It was steady as a rock on both a DMM and an Avo 8. I had been told that the decade switches had been cleaned before I bought the generator, but since this was an obvious possible cause, I investigated a bit.
Sure enough, I found that physical disturbance of a couple of the connecting wires where they connected to one of the switch wafers produced the fluctuation of the output measured on the meter. Remaking the soldered joints didn't fix this, but a quick squirt of contact cleaner on the switch contacts did.

Thanks for all the earlier advice, but all's well that end well!

Mike
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Old 23rd Feb 2019, 2:24 pm   #7
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Default Re: Heathkit IG-18 Sine-Square Audio Generator bought at RWB

Good to hear of a good outcome. Had similar problems on my VTVM with the contacts.
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