DIY 8038 Function Generator Problem

[Rob11966]

OK folks, problem solved. Unfortunately not without significant embarrassment on my behalf.

So the first thing that I did was to replace the 4.7uf cap with a 470uf cap - this led to a significant improvement in the noise, almost perfect but not quite.

I then revisited the 79L12 (IC4) which had the ripple on the out pin. I decided to replace this IC - Eureka, perfect!! I then put the 4.7uf cap back in, once again perfect. Problem solved 100% with a sharp beautiful trace.

So what went wrong? Here's the embarrassing part. I presumed it was just a dud IC but then I looked closely at the little bugger. It was labeled BC548C (tiny writing). The BC584C is an amplifier as far as I can tell. So, I figure I was taking the small ripple from the smoothing caps and amplifying it onto pin 11 of the 8038.

Sorry for wasting everyone's time and thanks for all the responses. To my defense, the BC548C did come out of a fresh pack from Farnell labeled 79L12. All the others are correct. The other problem of course is this solid state stuff - you could never mistake a 845 for and EL34 .

Anyhow, I learned a lot and despite (or because of) the frustration had a lot of fun.

Cheers,

Rob

PS Post above should read pin 11 not 8

[Conrad Hoffman]

Fantastic- ten points for finding it! I always figure if nothing ever goes wrong, I'm not doing enough.

Best,
Conrad

[PB2]

There was an app note about how to get the most out of that chip and as I recall it was fussy about bypassing or ... something. That is a very old chip. Anyone have the app note handy?

Pete B.

[EUVL]

I know it is not the topic, but why use the ICL8038 ?
It is obsolete, difficult to get, has only 1% THD, ....
I am sure a 24-bit sound card will do better.

Or if you only need sine wave, then I would recommend the func gen in Bob Cordell's Distortion Analyser.

My implementation of the Cordell Distortion Analyser


Patrick

[Rob11966]

Originally Posted by EUVL

I know it is not the topic, but why use the ICL8038 ?
It is obsolete, difficult to get, has only 1% THD, ....
I am sure a 24-bit sound card will do better.

Patrick

Hi Patrick,

Why the 8038 -

1. There was a nice simple little circuit for it on the web with a PCB template.

2. I wanted a stand alone function generator.

3. I could not find a simple option using a more modern chip.

3. Being obsolete has it's advantages - They are no longer needed where a friend of mine works and he was able to put his hand on a couple!

Cheers,

Rob

[jawih]

because it is obsolete I have made a functiongenerator using "normal" components such as opamps and so on, converting a triangle to a sine wave.
if you are interested I can look for the schema, somewhere in my pc