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Code Practice Oscillator based on the Heathkit HD-1416

PostPosted: Thu Nov 03, 2011 5:14 am
by ys1rs
Guys, thanks to the members that helped me out with the circuit diagram, I was able to build this little CPO based on the Heathkit HD-1416 design.

Heathkit introduced the HD-1416 Code Practice Oscillator in 1975.
By using this very same circuit design, I assembled this circuit you see here.
Sound is better than the one you get from the modern 555 timer IC based circuits on the market today.
The 555 sounds "chirpy" to my taste.

Being a little retro decided to go for the Heathkit design from 1975.
Not perfect as a full sine wave sound, but better than the 555 based square wave sound.
I spent less than US$12. The original (used) HD-1416s you find on eBay will cost around US$60+
In my case, I still have to add international shipping costs. So decided to assemble my own CPO, relax while doing it and have fun at same time.

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Here is the video. Sorry for the Latin American accent:
http://youtu.be/f4a8wxvakqM

I tried to embed the video as you do in other forums but this one does not accept it.... Does anyone knows why?

PostPosted: Thu Nov 03, 2011 5:43 am
by N6EV
Great job Rob! That CPO sounds good. About the only thing I would have suggested to make the video better would be to flash up the schematic diagram of the final product. Perhaps you could provide a link to it here so other members could do this same build.

PostPosted: Thu Nov 03, 2011 5:57 am
by ys1rs
Hi Paul.

It is exactly the very same Heathkit circuit. All components exactly same value.

Only changes are:
-size of the headphones plug. I am using a small modern stereo jack instead of the big mono 1/4 jack used on the original.
-added a jack for the key (instead of using the red and black connector posts) so I could take the key I have in my rig, unplug it and use it with the oscillator at anytime.
-installed the Tone pot at the front. I used one that accepts a knob instead of the original one, that was manipulated by using a screwdriver.
-using the NTE123AP as a replacement for the 3 NPN transistors. If I could have found the exact ones, I would have used them. The NTE123AP is the exact replacement for those.

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Of course, you can not compare the cheap metallic case I am using to the original Heathkit plastic case. The original Heath case is so nice :oops:

One thing I consider important to mention is the fact that I also tried the Heathkit CO-1 design. The result was also a poor sound. It is based on one single transistor 2N407. The nice feature about the CO-1 and the subsequent HD-16 (both predecessors of the HD-1416) is that these included also a bulb for you to practice sending Morse by light instead of sound.

Of all three Heath CPOs, the best one, is the HD-1416.

PostPosted: Thu Nov 03, 2011 9:05 am
by PA1FOX
Nice job, Rob.

It looks quite well on the nice wooden base together with the key.

Since the schematic is a standard multivibrator at audio frequencies, any standard low power NPN transistor would do the job. Here in EU the BC547 series are well known and could be used.

It' just what you would have available in the junk box.

73,
Alex
PA1FOX

SKCC no. #8265