Saturday, 22 February 2014

Adding a Tx/modulation indicator to PSK-20

The older version of PSK-20 was somewhat different to mine. For a start, it had a 9-pin D socket to allow control of the PTT by a computers RS232 serial port (RS232 was mentioned in a recent training course at work, and I, rather embarrassingly, was able to real the protocol specs off!). The newer version like mine has 'tone controlled PTT', meaning that the rig detects a control tone on the sound card right channel to control the transmitter, eliminating the need for a serial port, and making connecting up one less cable.

Something however that the older version had, which the newer one doesnt, is a Tx/modulation indicator LED. Now, although the computer will say when its in transmit mode, I rather like having a hardware confirmation! So as previously mentioned I will be adding this Tx LED. The additional circuitry is very simple and will easily fit on the underside of the PCB

The schematic above is the recommended circuit taken from the old PSK-20 manual. Its not a great diagram, as I recreated it in MS Paint, which is frankly rubbish, but it gets across just how easy this mod is.

This will need building as close as possible to the antenna connection. Luckily, the antenna socket is an insulated PCB connecting but panel mounting BNC, which means it has easily accessible pins. The simple RF sampling circuit will fit directly to the antenna connections, I might even be able to build it from SMT parts for the absolute minimum of adverse insertion effects. I dont really know why they didnt keep this circuit in the kit in the newer versions.

Heres what the manual has to say about this circuit under 'Transmitter Operational Adjustment -

If no wattmeter:
Install a T1-3/4 LED at TP2. The 'flat' edge of the
LED body should face to the left. When transmitting,
this diode should glow steadily when idling and will
flicker noticeably brighter when you're typing. Adjust
the master volume slider as needed to achieve this
indication. Note that this diode may be installed
remotely on a front panel to serve as a rough power

Im not exactly sure what a T1-3/4 LED is, but I suspect its a common or garden 5mm LED!

UPDATE - Ive just looked this up and it seems T1-3/4 is just the package style for a typical 5mm LED. So thats that sorted. Im off to bed now as its Sams karate grading in the morning and ive already had three glasses of wine...

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