You gotta love the name, right? It sounds like some sort of psychological condition. Perhaps someone who just doesn't care if his girlfriend breaks up with him, or is indifferent to getting fired.
But no, we know that that's not what it means.
I recently had to make three sets of TIA amps for my new 15-10 rig. First, I decided not to use the boards provided by Todd of Mostly DIY RF. You can see one of these boards above the tin shears in the picture above. Note how compact it is. Even though you would need two of these (one for receive, one for transmit) to get what you need for a bilateral rig like the BITX, using these boards (as I did on my 17-12 rig) saves you a lot of space. But this time I wanted to build a rig that is TOTALLY DIY RF. So out came the tin shears and the super glue.
First I decided on the Manhattan pad placement. I used a pattern that had worked on previous rigs. Pete is right -- I use a lot of TIAs (but no, I have no tattoo of the circuit anywhere on my anatomy!). I scaled it down a bit, thinking that I could come a bit closer to Todd's compactness.
Knowing that I would need three sets, I first made three boards. Then it was just a matter of soldering in the components: I did resistors, then capacitors, then, finally, the transistors. In this way I was able to put together the three dual TIA amps in a couple of morning build sessions. It was kind of tight and I had to be careful to avoid wiring errors or unintentional shorts. But it worked.
I like the TIA circuit. It lets me select the gain of a stage by simply using two resistor values from W7ZOI and K3NHI's chart. And with this circuit I know that it will look like 50 ohms both ways. This is really important on bilateral rigs like this.
I don't think there is anything wrong with using BITX-like bilateral circuits or TIAs or LC VFOs. This is all for fun right? I just like using these circuits.
My friend Pete is right in pointing out that the bilateral concept did not originate with the BITX -- it has been around for a long time, going back at least as far as the Cosmophone. But I think Farhan's BITX definitely moved it into the solid-state homebrew mainstream. Three cheers for Farhan for doing that.
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I *totally* think this is a good back-to-basics approach. I've actually been thinking of putting out a few more versions of the TIA boards. One version I'd call the "large-print edition": a regular through-hole PCB but with components spread out a bit, and with larger silk-screened nomenclature--for those with FFS and declining eyesight. Another idea is to make the TIA as a Chuck-Adams-style "muppet" board. Essentially a Manhattan board without the loose pads and glue. This would make it easier to swap out the gain-setting resistors if needed. Finally, I think I'll get around to offering PCBs for the TIA in the bi-directional, two-amp form. Right now I'm up to my you-know-what in kit packing.
ReplyDeleteI use .2mm pcb for my manhattan pads, so much easier to cut. I use a paper guillotine to cut strips that I snip to pass or use as tracks
ReplyDeleteThe Cyanoacrylate (CA) glues do set quickly and are ok on clean surfaces. But when exposed to solder heat from soldering the top pad- be careful.
ReplyDeleteI have used Loctite Epoxy Marine with faultless results in this application. Just be sure it is mixed and cured properly. Consider the pads not simply "glued" but laminated at that point. Not coming apart!