Heathkit IG-18 repair question

[Othello]

I have an old Heathkit Sine-Square wave generator which seems to oscillate.
I know its supposed to do this but this oscillates on top of its regular wiggles, on the scope you see several sine curves originating from the same starting point.
First I thought there was a power supply hum on the signal, so I disconnected the heathkit power supply and connected it to the bench power supply, but the unwanted oscillation persists.

Does anybody have a decent circuit diagram?? I have one so washed out I can't read it.

Uwe

 

[dsands54]

Heathkit IG-18

Try **broken link removed**
They have the schematics for the IG-18 there.

 

[Othello]

You are right, thank you they are a bit more readable.
That leaves the questions why I experience these additional oscillations, guesses anyone

Uwe

 

[Othello]

The signal I get is actually an AM signal, superimposed on the regular oscillator output I find a lower frequency component which I first thought to be a power supply problem but that seems not to be the case.
Hopefully the upload of the schematic worked, so people can see the circuit. The hand written entries are not mine, I assume to have the original, unmodified circuit.

I disconnected the square wave generator at R15, 4.7K, and the oscillations persist in the sine wave, so the devil sits somewhere in the let part of the diagram, Q1 to Q5 I suppose. I take my scope signal of between Q4 and Q5, so none of the output switches etc. should be at fault, it is the oscillator.

Insights into the workings would be appreciated.

Uwe

 

[Dean Huster]

The Audio Amateur had a multi-part article back in the 1970's entitled, "The Greening of the IG-18". That article and the complete manual for the IG-18 used to be available on the Tom's Heathkit web site, but that site has been taken off the air in the past couple of years for unknown reasons. I have an IG-18 manual and the AA article(s) and can scan whatever you need and sent it your way. Just let me know.

Dean

 

[Othello]

Dean, thank you for your offer.
I did find the article with mods for the unit, but that would be the second step wouldn't it. First I would need to fix what I got. I would most appreciate the info in the manual if it would help troubleshoot the unit I got.
Is it much to scan?

Uwe

 

[audioguru]

I think C5 is open or is shorted. Replace it then re-adjust the bias pot and the feedback pot.

The sine-wave oscillator will have much less distortion if the Q6 circuit is added to replace R12.

 

[Dean Huster]

Uwe, I made this offer a bit too quickly. I used to have this stuff out and collected together for quick access at the school where I taught, Xeroxing the material and sending it out via snail mail. I since have left that school and all my stuff, including the technical manual and the "Greening" material is all packed away in storage, not even at my house! It was shameful to hold such a carrot out to you like that, but it was not intensional. When I looked back in my "IG-18" file folder, I found that the material was gone and knew immediately where it was. Sorry.

Dean

 

[Sig239]

Insights into the workings would be appreciated.

Uwe

The complete circuit description is available in the manual that is available for download at the previously mentioned site. The site is slow and only one item at a time may be downloaded. Since I already downloaded it I will post it here. Page 66 may also be usefull but it doesn't look like I downloaded it. Let us know how it turns out.

 

[Othello]

Thank you, I was finally able to download some of these pages myself.

I changed the capacitor audioguru mentioned with one I found in my supply box. There was a slight change in performance.
I do not have a brand new electrolytic cap handy and I always wonder how to best test them, to see for sure if what I got now is better than the one I removed.

The often violent vibration of the meter needle stopped for most part. The scope shows at times sine waves of identical frequency but varying amplitude, the images are hard to trigger. But the phenomena is intermittent and quitens down at times. The higher frequency range seems to be more prone to have these effects.

Uwe

 

[wltomlinson]

hello.
newbie here. sorry if this is a 'hijack'
just picked up a Heathkit IG-18 signal generator at a flea market.
can anybody give me a number for a replacement light bulb in the feed-back circut?
smashed to bits.

thanks

bill

 

[hotwaterwizard]

I Would love to have the complete manual The site mentioned is empty of manuals they can not be download **broken link removed**

 

[mvs sarma]

I Would love to have the complete manual The site mentioned is password protected and I can not download **broken link removed**

You may download one here for every 4 days. but if you contribute to the site, you become VIP and the download permission is unlimited. Hope it serves.
btw, I have a IG-5218, an improved version of the IG-18, and attached.
mods.dk – manuals

 

[hotwaterwizard]

Got it Thanks

I tried to Register at that site and it said I have been a member since 2006.

LOL forgot all about that site.

 

[madisonhack]

This oscillator is supposed to generate harmonics. It's possible that you're triggering on them and fooling yourself into thinking that you have a problem when you may not. Sorry if this isn't helpful.