Beach 40 Build: Part 6 – T/R Switching

This post is part of a series: Building the VK3YE Beach 40 DSB Transceiver.

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.

Hear you on the air!

73

T/R Switching

A quick post from the day’s experiments: the transmit/receive power switching arrangement for my next project. The scheme is very much like that in the KN-Q7; I wouldn’t have stumbled across this (very simple) setup without an excellent write-up by Andrew Woodfield, Zl2PD.

Here’s the simple schematic:

The circuit itself is straightforward – when the “key” pad is left floating, current can flow through the 2k2 resistor attached to the 3904, providing a small base current and driving the 3904 in conduction and powering anything connected to the +12R pad. At the same time, there is nowhere for base current to flow in the 3906, so no current is provided to the +12T pad.

When the key pad is grounded (by a morse key, or other TR switching method), the base of the 3904 is pulled to ground through the small signal diode, and very little current will flow through the 3904 and into the +12R section, effectively killing receive functions. At the same time, a small current will flow through the 2k2 resistor attached to the 3906, allowing it to conduct and powering anything attached to the +12T circuit.

In short: when the key line is floating/disconnected, the circuit is powering receiver functions. When it is grounded, the circuit powers transmitter functions.

I added the little 7808 regulator to this power board, which will be powering some NE602 mixers on receive only. I put both circuits together on a little piece of copper-clad, in something like Island-Pads-meets-manhattan-construction:

I like how the layouts ends up showing off the inherent symmetry between PNP and NPN transistors. The whole thing looks quite nifty on the bench. I left the input and output pads deliberately large, to accommodate however many connections end up being made.

I did begin construction and testing on the next project, but was stymied by my SLA-battery power source running down (it was down to 7.4 out of 12V by the time I thought to put a meter on it). Rather than switch to the bench power supply (an old computer power supply with the 12V and 5V rails brought out), I took it as a sign to call it a night.

Hear you over the sizzle of solder!

74