Monday, December 10, 2012

Progress on the Beach 40 txcvr

Spent a couple of hours Sunday on the Beach 40. ( The new commonly accepted name for VK3YE's 40M DSB transmitter/ receiver.)

I got the Receive audio section installed, and a few immediate problems debugged. I had absolutely no audio out, except for when I touched my finger or signal tracer to the input of the audio chain.

Further probing revealed:

1) The transistor in the VFO buffer amplifier was bad. Replaced it.

2) A 500 ohm bournes trim pot is not a satisfactory substitution for the required 200 Ohm part. I tacked in to 100 ohm resistors for now, need to get out to All Electronics soon.

3.) "Trying to be cool" and using surface mount diodes in the mixer ring failed. ;)  There was poor gain, and things "looked funny" on the scope. Tore a few jumpers out, but left the SMD diodes as a lesson to myself, and then installed a new diode ring with proper through-hole diodes. MUCH better.

Once it was working in hard-wired receive mode:

4.) For my tastes (listening with a speaker, not headphones) this thing needs an additional audio amplifier stage. Using the good old Radio Shack amplified speaker cranked all the way up, I found that my perceived MDS was down below -110 dBm on my HP sig gen. Yeah, that won't occur in real life, but its a usability enhancement worth adding. I'm not excited about using the ubiquitous LM386, instead I'm going to try a TDA7052, (perhaps like SPRAT 142) which I have on hand but have never played with.

5.) I started building it "Drilled Island Manhattan" style, then for my attempted SMD mixer stage, I went crazy with the dremel. Made some mistakes. At that point, I found it much easier to switch to "Super Glued Island Manhattan."  That's how I think I'll finish it.

More updates when I have them!

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