Saturday, 22 February 2014

DC Power input mod on SWL's PSK-20

 I have mentioned the Small Wonder Labs PSK-20 a few times, and it occurred to me earlier today that I really ought to explain these things a bit more when I mention them, before going on and on about what im doing to them!

Well, the PSK-20 is a small single conversion superhet transceiver from Small Wonder Labs. It is fixed tuned to the digital sub-band on 14MHz for operation of BPSK (binary phase shift keying) mode transmissions. Its a very simple rig in terms of use, you simply connect a 12v power supply, a 50 ohm antenna, and a pair of audio cables from and to a PC sound card. Then, running suitable datamode software such as FLDIGI, HRD, MultiPSK etc etc, this simple set should give about 2W of output, ample for plenty of fun making PSK contacts on 20m. It was supplied as a kit, but sadly by the time I wanted one the owner of the company had retired, and they were no longer available, so I bought mine from someone else, meaning I didnt have the fun of building it.

My only concern with the  transceiver was the lack of any protection on the power input. DC at 12v is fed via a DC barrel connector to the unit, via only a series diode. This means, apart from the Vcc being 0.6v lower than the incoming supply due to the drop across the diode, that the only protection is against reverse polarity. I doubt however, that the diode would really hold up in that situation for very long!

The unit is relatively simple, and so I was more worried about a  fault in the transceiver knackering an expensive power supply! Or, and probably of greater real concern, of causing a huge current draw and melting the power cable, a definite fire risk. As someone who has but one level of OCD, which is an obsession with fire extinguishers, this is what needed sorting.

The solution was, obviously, to fit a fuse holder! This needed a surprising amount of thought, as not only must it be in keeping with the layout of the rest of the rear panel, but I also had to consider the depth of the devices penetration into the housing. Too deep, or in the wrong place, and I run the risk of damage to the transceiver components. The photo shows the new fuse holder fitted in place. This holder had a little locating stud, so as well as drilling a 12mm hole, I had to file a little extra cutout for the locating stud. A small piece of heat shrink tubing covers the end terminal as further assurance against any contact with the board components.

As the PSK-20 has no power switch, I decided to add one at the same time. The connection to the fuse holder for the incoming power comes from the reverse polarity protection diode. The cathode leg of this was cut and the wire removed, the holder and switch then being wired in series.

The two other issues with the transceiver design were the lack of any indications of when the unit is powered, and when it is transmitting. To solve this, a pair of 5mm LEDs have been fitted onto the front panel

Keeping things simple, ive gone for red and green. The green LED will be the power indicator, and will be lit whenever the rig is switched on. The red LED will indicate Tx, and is a bit more complex! The drive for this will come from the addition of an RF power sniffer circuit, which will sense the RF power at the output of the LPF, just prior to the antenna connector. Its a simple diode detector circuit. The addition of this will also act as a rudimentary VSWR indicator, although im likely still to make use of a proper VSWR bridge in line with the antenna.

This is the whole thing back in its case, but without the lid. I was a bit concerned about the LEDs being too close, but there is in fact plenty of space. The LEDs need their current limiting resistors fitting, and then wiring up to the board. Im almost ready to try this on air, but I also have the power cable to make up before that. The audio cables have been tested. That reminds me, I need to organise a container to keep audio patch leads in! Ive got one now for USB cables, that sits under the PC desk, so all those cables are now tidy!

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