Using the 74HC240 in radio applications…

There are a number of interesting radios based upon using things like hex inverters or octal drivers as both oscillator and amplifier. In digging around, I found the most common chip used in this application is the 74HC240. Here’s a link to a page which contains many interesting links:

Testes com 74HC240

Another link to a similar design which points at a QST article which you can access if you are a registered ARRL member.

Picture 1

DC40B, Day 4, and stupidity on my part…

Well, over the weekend I completed the trial assembly of my DC40B. The voltages all measured out correct, but the oscillator seemed to not start reliably. It appears that the twin 47pf caps that provide crystal feedback may not be quite sufficient to reliably start the oscillator on power up. If I prod the base of Q5 with my oscilloscope probe, the oscillator starts right up. In discussions on the dc40kits yahoo! group, it appears that at least one other ham found that he had to add some capacitance to his dc40b kit, so I’ll give that a try.

In a fit of stupidity though, I went blindly forward, and promptly installed all the ICs. Like an idiot though, I wasn’t watching what I was doing. No, I didn’t install anything backwards, I installed the 8 pin op amp in place of the 8 pin AVR and vice versa. Guess what? The Atmel AVR gets pretty hot when you install it in the wrong place! IDIOT. Oh well. While no magic smoke was released, it appears that I have zorched the chip, so I’ve got an email into KI6DS to order some replacements (maybe I should get more than one…)

Anyway, that’s my update.

dc40b