Good news on the BITX. I think I have solved the low frequency oscillation problem in the power amplifier. I tried running the PA off a separate power supply -- the oscillations disappeared leading me to conclude that the feedback was taking place via the power supply lines. I put a 22 uF cap to ground from the top of the PA's RF choke. With this cap in place I can dispense with the separate supply kludge. The amp is stable.
This morning I also resolved a different stability problem. I'm using a relay for T/R switching. One set of contacts controls the DC power, the other set was supposed to disconnect the receiver input from the antenna/lowpass filter when on receive. But I had positioned the relay too far from the LP filter and RX input, so I ended up with this long piece of coax that was hanging off the PA output terminal and carrying lots of RF on transmit. Yes, this led to oscillations. I thought about repositioning the relay, but I think part of the problem is that even if optimally positioned, the long unshielded contact levers inside this relay would be radiating a lot of RF and causing stability problems. So instead I put a small reed relay very close to the PA out and RX in connections. It carries the signal to the receiver when in R mode, and disconnects the RX when in T mode, without any long coax lines or unshielded relay levers. It works.
I still have a few things to do: -- I need to build a proper speaker amplifier so that I can dispense with the amplified computer speakers KLUDGE. -- I need to get a 3 pole double throw switch and some crystal sockets so that I can switch crystals from the front panel. Three rocks will cover all of 17 meter phone. -- I have to build a case for this rig. I'm thinking wood, with tin or aluminum sheeting glued to the inside (for shielding).
In response to popular demand, "SolderSmoke -- Global Adventures in Wireless Electronics" is now available as an e-book for Amazon's Kindle.
Here's the site:
For the print version:
For shipping from a printer in the U.S. (probably better for N. American buyers) Click here: SolderSmoke USA Version
For shipping from a printer in the UK, Spain, or the USA (probably better for UK and other European buyers)
Click here: SolderSmoke EU Version
The two versions are identical, except for a minor difference in the paper used. That's why the prices are a bit different.
Bill's OTHER Book (Warning: Not About Radio)
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W4HBK's QRSS Grabber: The Amazing Pensacola Snapper (Live!)