The T/R switching for the Beach 40 is pretty straightforward.A single DPDT relay is controlled directly through the PTT switch on the mic. Half of the relay switches the low pass filter connection between the transmit and receive sections. The other half routes power to the audio amplifier on receive and to the mic amp and RF amps on transmit.
Or it would be, it would have been straightforward if I hadn’t soldered the connections incorrectly twice. The first time, I turned the relay 180-degrees before soldering it down, so that the control voltage across the Normally Open pins, which obviously didn’t do anything. The second time, I reversed the coil and normally closed (receive) sections, so while I was able to test the receiver, the transmitter section would never get power.
But at long last, this section is complete. It’s a pretty adorable little relay, an NTE R40-11D2-12. It’s rated at 2A, which is way overkill for this project, but it’s what was available at Microcenter. The 12V coil voltage is driven directly from the battery input voltage.
It’s very satisfying to hold a mic in my hand for the first time on this rig, and hear the audio click in and out in the headphones when pressing the push-to-talk button. After a couple weeks away from this project, it feels like I’m closing in on completion.
Moving on to the transmit-only portions of the Beach 40, today I completed the microphone amplifier. Powered on transmit only, it’s responsible for both boosting the microphone input level to inject it into the mixer, and for switching the microphone out of the circuit on receive, as necessary.
One difference that I’m going to introduce into the original VK3YE design is that I’m going to use a dynamic mic instead of an electret mic, specifically one of the mics I picked up at the SMCC Hamfest back in June. This likely means I’ll need a bit more gain in the pre-amp than if I was using an electret element.
Peter mentions the possibility of using a dynamic microphone on his page on the Beach 40. He says:
“The circuit is suitable for an electret microphone. If using a dynamic unit leave out the 22k [bias] resistor and possibly raise the 100n [mic] coupling capacitor value if insufficient or thin audio.”
Leaving the bias resistor out of the original circuit, and replacing a couple of the electrolytic caps with ones from my stock, I ended up with this:
A quick spice simulation shows about 15 dB of gain through the audio spectrum with a slight high-pass characteristic. Note that I’m making a couple of approximations here. I don’t know exactly what the impedance of the dynamic mic is, but varying its input impedance between 100 and 500 ohms shows only a 2dB variation in gain, so I’m not too concerned. Similarly, I’m measuring power into a 50-ohm resistor as a representative of the mixer, instead of modeling the mixer directly.
This circuit came together very quickly on a piece of copper-clad, just five pads and a few minutes of soldering. The next step will be attaching the mic amp to the mixer and determining whether I do in fact need more gain. I’ll also need to experiment with the microphone input cap to see if I need more low-end response from the mic. But this is a start!
This past Sunday morning, bright and early, I made my way out to my second Chicagoland hamfest, the Six Meter Club of Chicago‘s annual hamfest in Wheaton, IL. It was quite different in feel and size to the De Kalb Hamfest I attended a couple months ago – not better or worse necessarily, but definitely different.
Where the De Kalb hamfest was spread out over several long winding paths and four or five buildings, all of the tailgaters in Wheaton were compacted in one central parking lot, which was already filled and thrumming when I arrived at 8:02. With the thermometer and humidity spiking by 9am, it was nothing like the foggy March morning of the last Hamfest.
In terms of sellers, I would say the Wheaton hamfest had perhaps a third as many actual vendors (including The RF Adapter Guy and all his wears and a few others) but three times as many tailgaters. Many tube radios to be found in the parking lot, same a last time, and maybe five or six folks with a healthy collection of CB gear and some antique television sets.
I picked up a few little things early in the day – some more trimmer capacitors, a couple used hand mics and panel-mount connectors for the same, but nothing was really catching my eye. There were more parts-dealers at Wheaton then out in De Kalb, but no one really had anything special that caught my eye. I did find a couple twins to the air-variable capacitor that I picked up, with and without casings, but no split-stator types, which is what I’m hunting for now. Not many enclosures or antenna parts either.
Just as I was ready to pack in for the day, a gentleman announced that the Antique Radio Club of Illinois auction was about to start. This proved to be the most bountiful part of my morning, and had a fascinating structure. Lots of equipment, lots of it Boat Anchors, was laid out in rows along the side of the main exhibition building. After allowing a few minutes for folks to wander through and poke at things, we began the action proper, which they called “bidders’ choice.” Basically, if you wanted to bid on something, you held it up for the gentleman in charge to see, and he’d throw out a starting bid, say $5. Anyone else who wanted that item could volunteer a higher bid, much like a regular auction, but there wasn’t much bidding happening. No item went above $20. It was during this period that I snagged an antenna analyzer on the cheap (see below)
Then, once there were no items left that anyone wanted to bid on, the gentleman in charge announced, “Everything left is $5. Grab what you want, and get your $5 to Rudy.” Rudy did well for himself at this point, and maybe half of the items vanished. Once the dust had settled, the gentleman in change once again proclaimed, “Alright, now everything left is a dollar!” Well, for just a dollar…. and once that was concluded, anything left on the ground was set loose for free! All in all, I snagged an MFJ-207, a big analog multimeter, an antique transistor radio, and a hustler resonator for under $20. Not too shabby! I must remember to stick around for Auctions in the future.
After that, a quick trip to Fry’s and Menards capped my morning, and I was home by 2pm. Another great, friendly, fruitful hamfest.
So, here’s the final haul:
An MFJ 207 Antenna Analyzer. The big score of the day! I’d be eyeing an identical unit in the parking lot marked at $70, but to score this one, working, for $15 at the auction really made my day. All it needed was a new battery.
A Micronta Analog Multimeter. With settings for AC and DC voltage and current measurement, as well as resistance, it’s a neat unit with a six-inch analog meter movement. It’s in pretty good shape, but I’m sure it’ll need new batteries.
An Arvin 9562 Transistor Radio: Apparently made in the late 50’s, it’s got an attractive wood case and a big ole internal six inch speaker. I was planning on gutting it and using the case, but it turns out the thing actually works – it turned on while I was carrying it in from the car and scared the bejeazus out of me.
A Hustler 40m Resonator: I took a flyer on this, since it was free. It’s designed to go on a mobile vehicle mast and turn it into a 40m antenna that’s only 6′ long. We’ll see what it gets gutted and turned into.
Two hand-mics, both with 5-pin connectors
Two 5-Pin Panel Mount Connectors to go with the above microphones
Two baggies of Trimmer Caps of a couple different sets of values.
A Pack of Binding Posts, probably will end up used as grounding logs
A Small Metal Box – can never have two many enclosures!
A two-pack of NTE110 germanium diodes (from Fry’s electronics, on my way back home).
Another swell time hanging out with hams. And with ARRL Field Day coming up this weekend, I’m sure I’ll have more ham stories to tell. In the two days I’ve had it, I’ve already put the antenna analyzer to good use out by the lake, but that’s a story for another post.